CHAPTER XXX

THE VIVIPAROUS MANGROVES OF FIJI

RHIZOPHORA AND BRUGUIERA

_Rhizophora._—Represented by Rhizophora mucronata, Rhizophora mangle, and the Selala, a seedless intermediate form.—Their mode of association and characters.—The relation of the Selala.—Polyembryony.—The history of the plant between the fertilisation of the ovule and the detachment of the seedling.—Absence of a rest period.—Mode of detachment of the seedling. Capacity for dispersal by the currents.—_Bruguiera._—The mode of dispersal.—Peculiar method of fertilisation.—Length of period between fertilisation and the detachment of the seedling.—Mode of detachment of the seedling.—Summary.

BETWEEN 1897 and 1899 I made numerous observations on the Fijian species of Rhizophora and Bruguiera (mostly around the coasts of Vanua Levu and in the Rewa delta); and these were supplemented in the early part of 1904 by observations on the first-named genus in Ecuador. I did not make any collections in Fiji until Prof. Schimper asked me to obtain specimens; and a fair-sized collection containing specimens dried, and preserved in spirit, was sent to him. His illness and death shortly followed, and I lost the advantage of his great experience in these matters. In a letter written to me in 1898 he expressed the hope that I would publish my notes on the mangroves of Fiji. Years have since passed by, and as I read again his words of encouragement I take up once more the interrupted task.

RHIZOPHORA

Of the three species of this genus, two of them, Rhizophora mucronata and R. conjugata, are Asiatic and are unknown in America; whilst the third, R. mangle, was until recently regarded as peculiar to the American and West African regions.

When Mr. Hemsley wrote the Report on the Botany of the Challenger Expedition he remarked (iii, 149) that the American Rhizophora (R. mangle) appeared to be restricted to that region, and he questioned its existence in the Pacific Islands as indicated by Jouan for New Caledonia. The same view was taken by Prof. Schimper in his work on the Indo-Malayan strand-flora published in 1891. There was, in fact, much to support this view, since Dr. Seemann, one of the most accomplished botanists who have explored the Pacific, describes only the Asiatic Rhizophora (R. mucronata) in Fiji, and nothing is said of any other species collected by the United States Exploring Expedition under Wilkes in Fiji and Samoa.

However, in a paper on the flora of Tonga, read before the Linnean Society in 1893, Mr. Hemsley includes the American mangrove, Rhizophora mangle, amongst the collections made there by Mr. Lister; and he refers to its occurrence also in Stewart Island (I suppose in the Solomon Group), but he suggests that it was accidentally introduced with ballast in both these localities. In 1897 I found a species of Rhizophora, to all appearances identical with the American species, in great abundance in the Rewa delta in Fiji. Subsequently the same mangrove came under my notice as the prevailing species in Vanua Levu in the same group; and on sending photographs of a branchlet in flower and fruit and of the germinating fruit to Prof. Schimper he expressed the opinion that they belonged to the typical Rhizophora mangle.

There are four typical mangroves in Fiji, namely (1) Bruguiera rheedii, the “Dongo” proper of the natives; (2) Rhizophora mangle, usually known as “Tiri-wai,” that is to say, the Tiri of the river, or rather of the estuary; (3) Rhizophora mucronata, the “Tiri-tambua” of the Fijian, signifying the Whale’s Tooth Tiri in allusion to the form of its fruit; and (4) a seedless form intermediate between the two species of Rhizophora, which the Fijians designate “Selala,” a name signifying “the tree with empty flowers.”

Bruguiera rheedii and Rhizophora mucronata were alone recorded by Dr. Seemann and his predecessors; but he significantly refers to the natives speaking of four mangroves. Mr. Horne, who spent twelve months in the group some years later, also overlooked the American Rhizophora; but it is apparent that both these botanists were naturally more interested in the vegetation of the inland regions than of the coast swamps, and we have before observed that they failed to record Scirpodendron costatum, a giant-sedge very common and conspicuous in the swamps. It is not easy to understand Dr. Seemann’s remark that “mangroves are restricted to but few parts of the larger islands.” Horne, who was in the islands eighteen years afterwards, makes frequent allusion to them. The natives whom I questioned closely on this subject scouted the idea that any of the four mangroves above named were recent arrivals. The coasts, as they said, had always been extensively fringed by mangroves; and the reader has only to refer to my remarks in the second chapter of my volume on the geology of Vanua Levu to convince himself that mangrove swamps of considerable extent existed in the time of Commodore Wilkes (1840).

_The Relative Abundance and Mode of Association of the three Fijian forms of Rhizophora._

Stated in their order of frequency, we have first Rhizophora mangle, the American species, then Rhizophora mucronata, the Asiatic species, and lastly the Selala. The first is equally at home at the sea-border and on the banks of brackish estuaries. The second is, as a rule, more exclusively at home on the sea-coasts; and the same may be said for the Selala. Usually all three kinds occur in the lower part of an estuary; but as we ascend the river and the water freshens, the Asiatic Rhizophora and the Selala disappear, and the American plant is alone found in the higher reaches, where the density of the water ranges according to the state of the tide between 1·000 and 1·010. I examined the distribution of these three forms of Rhizophora in numerous estuaries of Vanua Levu, as well as in the Rewa estuary in Viti Levu; and it was ascertained that in all cases they followed the rule above indicated. When the estuary receives but few streams and the water is mostly salt, the three Rhizophoras may extend miles inland; but when it contains a large body of fresh-water, Rhizophora mangle may be the only form observed from the mouth of the river to the head of the estuary, and it may monopolise the adjacent coasts. On the other hand, Rhizophora mucronata may occupy almost exclusively a long extent of coast; or the Selala may prevail in certain localities, as on parts of the Mathuata coast of Vanua Levu.

The manner of association of these three Rhizophoras is of interest in connection with the origin of the seedless Selala. They very rarely occur mingled together, but grow gregariously in contiguous colonies; and not uncommonly all three may occur on the same line of coast within a distance of a few hundred yards. The colonies pass into each other without a break, and there is no fixed rule of association. Whilst on the south side of Vanua Levu the Selala is generally associated with the American Rhizophora, on the north side it is usually in touch with the Asiatic species. In other localities all three occur in contiguous colonies. The Selala colony may be exposed on the line of a river-bank or along the sea-coast, or it may lie in the heart of an extensive mangrove tract. The most extensive mangrove region in Fiji, that of the Rewa delta, is in great part occupied by Rhizophora mangle; but all three forms grow together in the eastern part of the delta; and here, strangely enough, as at Daku, the Selala may grow sporadically, and all three may grow mixed together with their branches intercrossing.

_The Characters of the Selala or Seedless Rhizophora compared with those of the American Mangrove (R. mangle) and the Asiatic Mangrove (R. mucronata)._

The three kinds of Rhizophora, when seen at the same time along a tract of coast, may be readily distinguished by the different shades of green of their foliage, that of Selala being dark green, that of Rhizophora mucronata light green, and that of Rhizophora mangle intermediate in shade. The Selala is usually the tallest of the three, and attains a height of from 20 to 30 feet or even 40 feet and over, the aerial roots dropped from the higher branches giving it a characteristic aspect. Rhizophora mangle is generally the shortest, and at the coast is from 10 to 12 feet high; but where the mangrove vegetation is most luxuriant, as in the great swamps in the interior of the Rewa delta, it forms tall trees as much as 40 feet in height, displaying the aerial roots hanging from the higher branches. Rhizophora mucronata is, as a rule, intermediate in height, and is distinguished by its stout, reddish trunk and reddish aërial roots.

The trunks of Selala are often in an inclined position and supported entirely by the trestle-like aërial roots, the lower end raised some 5 or 6 feet above the ground with the rest of the trunk inclined upwards. They then look like gigantic walking-stick insects. The same habit may be sometimes observed with the larger trees of Rhizophora mucronata, and in fact all three may present at times the same habit of growth. The taller trees of Rhizophora mangle may resemble the Selala in habit, and the smaller trees of the Selala may approach the habit of Rhizophora mangle.

The distinctive characters of the Selala are given in the table opposite. It will be there seen that this form is intermediate between the other two species as regards the form and size of the petioles and peduncles; the size of the bracts and bracteoles; the colour, form, and size of the flowers; and in the length of the style. Its leaves are smaller than in the case of the other two species, but pointed and semi-aristate like those of Rhizophora mucronata. There are, however, two varieties of the Selala, both with larger foliage than that belonging to the prevailing type of the tree, and from 10 to 15 feet in height. In one the flowers are more numerous, each flowering stem branching four or five times and bearing at least twenty-four flowers, the first branch being trichotomous and the rest dichotomous. In the other, which is the prevailing form on the Mathuata coast, there is a nearer approach to Rhizophora mucronata in the rounding of the peduncles and in the length of the style. Then, again, there are divergent varieties of Rhizophora mangle which in the larger bracts and bracteoles and in the greater size, form, and paler hue of the flowers come nearer to the Selala. Taking all the characters together, the Selala, though intermediate between the Asiatic and the American species, comes in the most critical diagnostic points, as in the inflorescence, in the individual flowers, and in the form of the apex of the leaf, nearest to Rhizophora mucronata, the Asiatic species.

The seedless character of the Selala is well known to most Fijians of the coast districts, the native name signifying empty (lala) flowers (se). Now and then they aver that it produces fruit, but the numerous offers of rewards in money never resulted in their bringing me the fruits. During my residence of two years in the group I examined the Selala trees in a great number of localities and never succeeded in finding them in fruit.

With all three kinds the anthers burst in the bud before it begins to open, and we may ask why the process of self-fertilisation, which is effectual with the other two kinds, produces no result with the Selala. In all three cases the flower-buds and expanded flowers hang downwards, and the expanded flowers retain their parts for the first twenty-four hours, the pollen being caught in quantity on the hairy edges of the petals. During the next day the withering stamens fall out, and on the following day the petals fall too. With the Selala, the style soon begins to blacken and wither, and in a few days the flower becomes detached and drops off. With Rhizophora mucronata and Rhizophora mangle, the style preserves its healthy condition, and shortly evidences of fertilisation appear in the altered shape of the ovary. It is apparent, therefore, that in the case of the Selala fertilisation has not occurred, although the mechanical processes connected with it have been carried out. The cause of this is not far to seek.

+-----------------+--------------------------------------------------------+ | | FIJI | | CHARACTERS +------------------+------------------+------------------+ | | RHIZOPHORA | SELALA (a | RHIZOPHORA | | | MUCRONATA. | seedless form). | MANGLE. | +-----------------+------------------+------------------+------------------+ |Height of tree | | | | | in feet | 12-20. | 20-40. | 9-12. | | | | | | |Colour of foliage|Pale green. |Dark green. |Intermediate | | | | |shade. | | | | | | |Base of leaf |Tapering. |Sub-rounded. |Tapering. | | | | | | |Apex of leaf |Acute, and |Acute, and |Very obtuse, | | |terminating |terminating in |with no | | |in a twisted |a twisted point |twisted point. | | |point a line |less than a line | | | |(2·5 mm.) long. |(2·5 mm.) long. | | | | | | | |Leaf-stalk |Rounded, 1 - |Rather flattened |Length as in | | (petiole) |1-2/10 inch, |horizontally, |Selala, but | | |(25-30 mm.) long, |5-8/10th inch |flattening | | |about as long as |(12-20mm.) long, |very marked. | | |the peduncle. |shorter than the | | | | |peduncle. | | | | | | | |Inflorescence |Branching |Branching usually |Usually branching | | |(dichotomous) two |twice, but |only once | | |or three times |sometimes three |(trichotomous) and| | |with four to eight|times; first |bearing only three| | |flowers. |branching |flowers; but | | | |trichotomous, rest|sometimes | | | |dichotomous; six |branching again | | | |to twelve flowers;|(dichotomous) and | | | |in one variety, |bearing then six | | | |flowers as many as|flowers. | | | |twenty-four. | | | | | | | |Peduncle (lowest}|Rounded. |Flattened above. |{Flattening more | | flower-stalk) }| | |{marked than with | | | | |{Selala. | | | | | | |Pedicels |As stout as the |More slender than |As in Selala. | | |peduncle, and |the peduncle, and | | | |rounded. |angular. | | | | | | | |Bracts and |Large, 1-1/2 line |Small, 2/3 line |Very small or | | bracteoles |(4 mm.). |(2 mm.). |absent. | | | | | | |Calyx |Very pale yellow, |As in R. |Pale or bright | | |or dirty white, |mucronata. |green, angular at | | |rounded at base | |base in the bud, | | |in the bud, lobes | |lobes 3-1/2 lines | | |4-1/2 - 5 lines | |(8 mm.). | | |(11-12 mm.). | | | | | | | | |Length of style |1-1/2 lines |1 line (2·5 mm.). |1/2 line | | |(4 mm.). | |(1·5 mm.). | | | | | | |Fruit |Ovoid and usually |No fruits |Conical, somewhat | | |symmetrical, with |produced. |curved, and thus | | |large persistent | |not symmetrical; | | |bracteoles at | |bracteoles at base| | |base. | |very small or | | |(Hypocotyl 16 | |absent. | | |inches.) | |(Hypocotyl 9 or | | | | |10 inches.) | | | | | | |Colour of trunk, | | | | | rootstock and | | | | | roots |Reddish. | — | — | +-----------------+------------------+------------------+------------------+

+-----------------+-------------------------------------+ | | ECUADOR. | | CHARACTERS. +------------------+------------------+ | |RHIZOPHORA MANGLE |RHIZOPHORA MANGLE | | | (Mangle chico). | (Mangle grande). | +-----------------+------------------+------------------+ |Height of tree in| 10-15. | 50-80 and more. | | feet | | | | | | | |Colour of foliage|Pale green. |Dark green. | | | | | |Base of leaf |Tapering or |Tapering. | | |sub-rounded. | | | | | | |Apex of leaf |Very obtuse, with |Very obtuse, with | | |no twisted point. |no twisted point. | | | | | |Leaf-stalk |Flattened above |Flat above with a | | (petiole) |and below, with no|median groove, | | |median groove, |1 inch (25 mm.) | | |1/2 inch (12 mm.) |long, two-thirds | | |long; not half as |the length of the | | |long as the |peduncle. | | |peduncle. | | | | | | |Inflorescence |As described under|Branching at least| | |R. mangle of Fiji.|three times, | | | |sometimes four or | | | |five times, | | | |trichotomous or | | | |dichotomous, | | | |twelve to | | | |forty-eight | | | |flowers. | | | | | |Peduncle (lowest}| | | | flower-stalk) }|}Sub-angular. |Rounded. | | | | | |Pedicels |More slender than |More slender than | | |the peduncle, and |the peduncle, and | | |rounded. |angular. | | | | | |Bracts and |Scarcely |Well developed, | | bracteoles |developed, 1/2 |1 line (2·5 mm.). | | |line (1 mm.). | | | | | | |Calyx |As with R. mangle |As with R. | | |of Fiji. |mucronata and | | | |Selala of Fiji, | | | |but lobes 4 lines | | | |(10 mm.). | | | | | |Length of style |Less than a line |1-1/2 lines | | |(2·5 mm.). |(4 mm.). | | | | | |Fruit |As in R. mangle of|Conical, not | | |Fiji. |symmetrical, and | | |(Hypocotyl 9 or |somewhat curved; | | |10 inches.) |large persistent | | | |bracteoles at base| | | |as in R. | | | |mucronata. | | | |(Hypocotyl 12 to | | | |15 inches.) | | | | | |Colour of trunk, | | | | rootstock and | | | | roots | — | — | +-----------------+------------------+------------------+

Although the ovaries of the Selala contain four ovules, which in size and appearance do not differ from those of Rhizophora mangle and R. mucronata, its pollen when compared with that of the other two forms presents a remarkable difference. The pollen of these three mangroves was examined in five localities far apart from each other, and in all the same results were obtained. The pollen-grains of the Selala are much smaller than those of the other two, and differ much from them in form. They are irregularly oval in outline, and have a shrunken look beside the spherical symmetrical grains of the two species with which they are compared. They are from one-fourth to one-third the size of those of Rhizophora mucronata, and from one-third to one-half the size of those of Rhizophora mangle.

There is much to support the view that the Selala is a cross between the other two species, its intermediate characters and its seedless condition being especially indicative of such a derivation; but there are several difficulties in accepting this explanation.

(1) The circumstance of the anthers bursting in the flower-bud would considerably lessen the chances of cross-fertilisation; but this objection is not insurmountable, since numerous insects, such as flies, ants, and small coleoptera, visit the newly opened flowers, and they might sometimes produce a result. When I made this suggestion to Prof. Schimper he replied that insect-pollination was quite possible after the expansion of the flowers.

(2) If, as seems highly probable, the pollen of Selala is impotent and the ovules fertilisable, then its seedless condition implies not only an incapacity for self-fertilisation, but also for cross-fertilisation; and if Selala with its impotent pollen does not admit of cross-fertilisation, this would still less be expected of Rhizophora mucronata and R. mangle where the pollen is potent and where fertilisation takes place in the bud. I endeavoured to fertilise the Selala flowers with the pollen of the two other species; but there were no results, the flowers falling off in a few days. It may here be remarked that on one Selala tree I found a solitary flower with an enlarged ovary, as if through fertilisation.

(3) It is not easy to explain the gregarious growth of the Selala if it is a seedless hybrid. The colonies could not be renovated by mere intercrossing, especially in places where, as on the north coast of Vanua Levu, the dense belt of mangrove is for many miles composed in mass of Selala trees, with a few trees of the Asiatic and American Rhizophoras growing on the outskirts.

It is obvious that in order to clear the way for considering this problem the means of renovating the Selala colonies should be inquired into. In the first place, whilst seedlings occur in numbers under the trees of the other two Rhizophoras they are never to be found under the Selala trees. The mode of reproduction of the Selala is evidently vegetative, and the question arises as to what mode of vegetative reproduction occurs. The Selala trunks, as already observed, are often inclined, the trunks being supported on trestle-like aërial roots. These trunks send out branches which in their turn drop aërial roots; and when the decay of the parent trunk takes place, the branches are able to live independently. The primary branches in due time send out secondary branches which also let fall aërial roots; and thus the process is repeated indefinitely, the result being a maze of semi-prone trunks, branches, and aërial roots. The first stage of the process ends with the death of the parent trunk, and the primary branch, supported by its own aërial roots, is often all that the observer can distinguish in the centre of a colony. This is evidently the mode by which the Selala colonies are renovated in their interior. One sometimes observes in the midst of one of these colonies extensive bare mud-flats 100 to 500 yards across from which apparently the trees have died off _en masse_. The natives assert that when part of a Selala tract is cleared the trees never grow again.

But _pari passu_ with this process of vegetative reproduction of the Selala, by which the mass of the colony is preserved and renovated, there is evidently some other process of reproduction in operation amongst the trees of Rhizophora mangle and R. mucronata at the edge of the colony, as a result of which Selala seedlings are produced. Whilst no seedlings are to be observed striking into the mud under the Selala trees, numbers occur, as before observed, under the trees of the other two species. Those under the trees of R. mangle possess in nearly all the cases the distinctive leaf-characters of that mangrove, and would be recognised at once as belonging to that species. On the other hand, those beneath the trees of R. mucronata are of two kinds, some of them being readily recognised by their foliage as of the Selala type others, again, being typical seedlings of R. mucronata. Only those seedlings, or “keimlings” as we might term them, were noted that had dropped plumb from the branches above.

Such were the results of my investigations on Vanua Levu. My field of inquiry was then shifted to the Rewa delta, where, with the assistance of the Daku natives, who, like most Fijians, display a keen interest in matters relating to their plants, I spent a few days in investigating the origin of the Selala trees that grow sporadically in that locality. On pulling up some of the young trees we found that the original radicular or hypocotyledonary portion of the keimling could be still distinguished. My zealous native friends also pointed out to me that though the leaves in form and colour were those of the Selala, the rootstock was reddish like that of R. mucronata, and not white as with R. mangle. The natives averred that the Selala trees are produced in the first place from fruits of R. mucronata. When young, they said, they are Tiri-tambuas (R. mucronata), but when old, Selalas. Yet although R. mucronata may be now regarded as the source of the Selala trees, and my Vanua Levu observations pointed unmistakably in this direction, it could not be definitely settled whether this was the result of a cross with the male element of R. mangle or whether the Tiri-tambua (R. mucronata), in producing two types of seedlings, one fertile with the parent characters and the other seedless of the Selala type, brought about the same end. On the whole I am inclined to the view that the Asiatic Rhizophora presents us in the dimorphism of its seedlings the true explanation.

This inference is supported by the behaviour of Rhizophora mangle on the coast of Ecuador, a subject which is discussed in Chapter XXXII, and I have given the results of my observations on the Ecuadorian Rhizophoras side by side with those on the Fijian trees in the table before given. There are two very distinct forms of the American Rhizophora (R. mangle) in the swamps of Ecuador. There is the low coast tree, the “Mangle chico” of the Ecuadorians, ten to fifteen feet in average height, which grows on the sea-front of the swamps and has all the general appearance and the more conspicuous characters of the American Rhizophora in Fiji. There is also a tall tree, 60, 80, or even 100 feet high, that forms the great mass of the mangrove swamps. In its inflorescence, in the dark green colour of its foliage, and in other characters, it comes near the Fijian Selala; but it differs in fruiting abundantly. This is locally termed the “Mangle grande,” and its true relation to the Fijian Selala appears to be as follows. Whilst both as regards the flowers approach the Asiatic Rhizophora (R. mucronata), the Fijian Selala resembles the Asiatic tree also in its foliage, whilst the “Mangle grande” or the Ecuadorian Selala more resembles the typical American tree (R. mangle) in its leaves and also in its seedlings. Here in the Ecuadorian swamps there can be no question of crossing, since both, according to Baron von Eggers, belong to one species. Therefore I am inclined to the opinion that whilst the Asiatic Rhizophora displays dimorphism in Fiji, the American Rhizophora displays dimorphism in Ecuador. The reversion on the part of the “Mangle grande” of Ecuador to some of the characters of the Asiatic plant is remarkable, and points to the greater antiquity of the Asiatic R. mucronata as compared with the American R. mangle.

This accords with the opinion expressed by Schimper in his work on the Indo-Malayan strand flora that the American Rhizophora is either a degenerated descendant of the Asiatic R. mucronata or a sister form derived from a common ancestor. America, as we have seen, possesses only one of the three species of Rhizophora, and this is the only representative that it owns of the four Asiatic genera (Rhizophora, Kandelia, Ceriops, Bruguiera) that constitute the tribe Rhizophoreæ. The rule prevailing with current-dispersed plants that America is a distributor and not a recipient evidently does not apply to the Rhizophoreæ; and to explain their distribution we must go back to some epoch very remote from the present. That Fiji derived its representatives of Rhizophora mangle from America by the agency of the currents I do not for a moment admit. The restriction of the species and indeed also of the genus to the Western Pacific is very significant. It is far more likely that, as I have pointed out in the case of Lindenia (see page 396), the American Rhizophora was once widely distributed over the tropics of the Old and New Worlds, and that it is now on the “down grade” towards extinction. Its survival in the Western Pacific could thus be explained without our being obliged to suppose that the seedlings or keimlings have been carried uninjured across the Pacific Ocean, an ocean voyage for which, as shown in a later page, they are not well fitted.

_The Occasional Occurrence of more than one Seed in the Fruits of Rhizophora mucronata and Rhizophora mangle (Polyembryony)._

The bilocular ovary contains four ovules, one of which only as a rule becomes a seed. But it is incorrect to say that the fruits are always one-seeded, since two or even three seeds are occasionally produced, and they may all germinate. In November, 1897, I noted eight hundred fruits of Rhizophora mangle germinating on the trees in one of the creeks of the Rewa delta. Out of this number eight fruits had two germinating seeds and one had three, the protruding radicles being in all stages of growth. Just two years afterwards I counted eight hundred more fruits in the same locality, and then observed seven with two germinating seeds and none with three, the radicles protruding in all cases. On another occasion at Wailevu in Savu-Savu Bay I counted four hundred, and none had more than a single radicle protruding. The results appear to vary with the locality, but in the Rewa creek the proportion of fruits in which more than one seed germinated was fairly constant at dates two years apart, namely, about one per cent. Occasionally, however, in particular localities a greater proportion may be noticed. Thus near Daku in the Rewa delta I found that the proportion was between two and three per cent. for the same species (R. mangle), those with three germinating seeds being about half per cent.

The case of more than one seed germinating in the fruits of Rhizophora mucronata never came under my observation; but in one locality, where I examined a considerable number of fruits near the stage of germination, between ten and fifteen per cent. showed two seeds approaching maturity.

Warming thoroughly investigated the polyembryony of Rhizophora more than twenty years ago, seemingly from materials brought to him from the West Indies (Engler’s _Botanische Jahrbücher_, band iv., 1883). With the usual German thoroughness he deals with the work of earlier observers, and goes back to Piso in the middle of the 17th century. Of the four ovules, he remarks, three usually abort, and only in rare cases are two seeds developed. He quotes Baron von Eggers to the effect that only in three per thousand cases was more than one seedling observed protruding from a germinating fruit. These remarks evidently all apply to the American species. I do not find any reference in my notes to polyembryony in Ecuador, and evidently its occurrence is not so frequent there as in Fiji.

It is frequently apparent in the cases where more than one seed germinates in a fruit that on account of the difference in the length of the protruding seedlings germination does not always begin at the same time. Thus in Fiji the difference in the length varied between one and three inches, an amount representing at least from ten to twenty days’ growth, as will be subsequently pointed out. Warming in one of his figures gives a fruit where an interval of some months seems to be indicated, since one of the seedlings has fallen out and the other is protruding less than an inch. By cutting across a fruit containing two seeds one may sometimes observe one seed quiescent and the other beginning to germinate. The significance of this occasional interval between the germination of seeds in the same fruit will be referred to in a later page.

_The Seasons of Flowering and Fruiting of the Species of Rhizophora in Fiji._

The Selala flowers all the year. With the two American and Asiatic species there are considerable variations between different localities. Generally speaking, they flower and fruit all the year through; but the flowers are usually less abundant in the warm season from December to February, and the germinating fruits which are to be observed on the trees every month of the year are more numerous in that season.

_The History of the Reproductive Process in Rhizophora from the Fertilisation of the Ovule to the Falling of the Plantlet or Seedling from the Tree._

I devoted great attention to this subject in the instance of Rhizophora mangle, being desirous of determining two points, in the first place as to whether there was any period of rest between the maturation and germination of the seed, and in the second place as to the period that elapsed between the commencement of germination and the fall of the seedling.

The principal change in the ovary for the first three or four weeks after fertilisation is shown in its increased breadth. The increase in height is but slight during this period; and in fact after thirty days the ovary only added 2 millimetres to its original height of 3 millimetres. After this the growth of the fruit proceeds until the tip of the radicle pierces its summit, the fruit being then about eleven lines (2·8 cm.) long. _From the date of fertilisation to the time the radicle pierces the top of the fruit a period of about fifteen weeks elapses._ (The fruit, it should be here remarked, continues to grow in length and breadth after the radicle has protruded, attaining a length of thirteen or fourteen lines [3·5 cm.] when the seedling or “keimling” is ready to fall.)

By referring to the table below it will be observed that there is no period of rest in the growth of the fruit up to the date of the protrusion of the radicle. It will now be shown that there is normally no pause between the epoch of the maturation of the seed and the beginning of germination, or, in other words, that from the time of the fertilisation of the ovule to the onset of germination there is no cessation in the process of growth of the embryo. That period of dormant vitality which almost all seeds pass through forms no normal feature in the life-history of this species of Rhizophora.

RHIZOPHORA MANGLE AND R. MUCRONATA.

+-------------------------------------------+ | RHIZOPHORA MANGLE. | +-------------------------------------------+ | _Growth of fruit in height._ | +---------------------+---------------------+ | Lines or tenths of | Number of days | |an inch (millimetres | since | | in brackets). | fertilisation. | +---------------------+---------------------+ | | | | 2 (5) | 30 | | 3 (7·5) | 42 | | 4 (10) | 50 | | 5 (12·5) | 61 | | 6 (15) | 67 | | 7 (17·5) | 74 | | 8 (20) | 83 | | 9 (22·5) | 92 | | 10 (25) | 100 | | 11 (28) | 105 | | | { Protrusion| | | 107{ of the | | | { hypocotyl | | | | _Growth of the protruding hypocotyl._ | | | | 10 (25) | 127 | | 20 (50·5) | 141 | | 30 (76) | 151 | | 40 (101·5) | 160 | | 50 (127) | 167 | | 60 (152) | 175 | | 70 (177·5) | 185 | | 80 (203) | 202 | | 90 (228) | 222 | | | {Fall of | | | 229{the | | | {seedling | +---------------------+---------------------+

_Explanation of the Table._

We have here shown the period between fertilisation and the fall of the seedling from the tree.

This period divides itself into two parts, the first being concerned with the continuous growth of the fruit and of the inclosed embryo until the tip of the hypocotyl appears through the apex of the fruit, the second being indicated by the growth of the protruding hypocotyl until the fall of the seedling.

The height of the fruit is measured from the base of the calyx-lobes, and the length of the hypocotyl at first from the apex of the fruit and afterwards from the edge of the protruding neck of the cotyledonary body. The height of the ovary at the time of fertilisation is about 3 millimetres; and from that time onward it is to be regarded as a fruit.

[_To face page 452._

FIGURES ILLUSTRATING THE DEVELOPMENT OF THE SEED AND THE GERMINATING PROCESS OF RHIZOPHORA AND BRUGUIERA

(Natural size. Drawn for convenience of description in the erect position.)

1. Rhizophora mucronata Fruit 3-1/3 lines (8-9 mm.) high, six to seven weeks after fertilisation. The micropyle is but slightly dilated, and is occupied by a small plug of endosperm.

2. Rhizophora mucronata Seed of fruit represented in Fig. 1.

3. Rhizophora mucronata Fruit 5 lines (12 mm.) high, eight to nine weeks after fertilisation. Germination is about to begin. A large plug of endosperm now protrudes through the dilated micropyle, but still covers the lengthening hypocotyl.

{Seed of fruit represented in Fig. 3. In 4.} Rhizophora mucronata {Fig. 4 the plug of endosperm is shown on the 5.} {upper end of the seed; whilst in Fig. 5 it {has been removed, exposing the tip of the {hypocotyl.

6. Rhizophora mucronata Fruit 7-1/2 lines (18 mm.) high, eleven to twelve weeks after fertilisation.

7. Rhizophora mucronata Fruit, seventeen or eighteen weeks after fertilisation.

8. Rhizophora mucronata Full-grown fruit with upper portion of seedling just before detachment from the tree. The long tapering plumule is here inclosed in the cotyledonary body. The * indicates the point of detachment of the seedling.

9. Rhizophora mucronata The cotyledonary body of Fig. 8.

9A.} {Illustrating different stages in the 9B.}Rhizophora mucronata {development of the plumule and of the neck of 9C.} {the cotyledonary body resulting finally in 9D.} {the expulsion of the plumular end of the {seedling from the fruit cavity as in Fig. 8. {(See page 458.)

10. Rhizophora mucronata Fruit with two seeds.

FIGURES ILLUSTRATING THE DEVELOPMENT OF THE SEED AND THE GERMINATING PROCESS OF RHIZOPHORA AND BRUGUIERA—(_continued_)

(Natural size. Drawn for convenience of description in the erect position.)

11. Rhizophora mangle Fruit, six weeks after fertilisation.

12. Rhizophora mangle Seed with plug of endosperm, as shown in Fig. 11.

13. Rhizophora mangle Fruit, eight weeks after fertilisation. The tip of the hypocotyl is now piercing the plug.

14. Rhizophora mangle Embryo (enlarged) shown in Fig. 13.

15. Rhizophora mangle Fruit, ten weeks after fertilisation. The growing hypocotyl has now pierced the plug.

16. Rhizophora mangle Embryo shown in Fig. 15.

17. Rhizophora mangle Fruit, nearly sixteen weeks after fertilisation.

18. Rhizophora mangle Full-grown fruit, just before the detachment of the seedling from the tree. The long tapering plumule is inclosed in the tube of the cotyledonary body. The point of detachment of the seedling is indicated by *.

19. Rhizophora mangle The cotyledonary body of Fig. 18.

20. Rhizophora mangle Fruit with two seedlings in different stages of growth (given in the first plate).

21. Bruguiera Rheedii Fruit, about four weeks after fertilisation. (The shaded portion is the calyx-tube or cup, in the midst of which rises the style.)

22. Bruguiera Rheedii Germinating seed.

23. Bruguiera Rheedii Germinating fruit, about eight weeks after fertilisation.

24. Bruguiera Rheedii Germinating fruit, about ten weeks after fertilisation. Here the growing hypocotyl, carrying the style with it, has pushed upwards the lining membrane of the floor of the calyx-tube, which has ruptured and forms a cap on its extremity.

25. Bruguiera Rheedii Germinating fruit, thirteen or fourteen weeks after fertilisation.

26. Bruguiera Rheedii Fruit with full-grown seedling just before its detachment from the tree.

[_To face page_ 453.

RHIZOPHORA MUCRONATA.

_Growth of the first seven inches of the hypocotyl after it protrudes from the fruit._

10 lines (25 mm.) after 26 days 20 lines (50·5 mm.) after 41 days 30 lines (76 mm.) after 51 days 40 lines (101·5 mm.) after 61 days 50 lines (127 mm.) after 70 days 60 lines (152 mm.) after 78 days 70 lines (177·5 mm.) after 86 days

In my description of the germinating process of Rhizophora mangle from this particular standpoint I adopt the general views of Prof. Schimper, the observations being my own, the phraseology employed being his. It would be out of place here to deal with the biological significance of a process to which observers like Warming, Goebel, Karsten, Schimper and Haberlandt have applied their greater talents as well as their greater experience. I investigated the subject carefully from my own standpoint of inquiry, and whilst the reader will find in my rough sketches of the various stages of the process a little aid in following the argument, he is referred for detailed treatment of the subject to the memoirs of the above-named botanists as well as to those of yet more recent investigators.

After fertilisation, according to Prof. Schimper (_Ind. Mal. Strandflora_), the embryo-sac is filled with endosperm, which subsequently protrudes and forms a plug completely closing the micropyle (see my figures). As my observations showed, the seed during the first eight weeks after fertilisation increases continuously in size, and the plug of endosperm, at first inconspicuous, becomes of considerable size, the seed attaining a length of seven millimetres. The embryo meanwhile grows rapidly, and at the end of this period of eight weeks the radicular tip or the point of the hypocotyl begins to protrude from the micopyle, still covered by the plug of endosperm, the fruit being between four and five lines (10-12 mm.) in length (figures 11-14). In another week, when the fruit has grown another line in length, the tip of the radicle is on the eve of piercing the plug, and this may be termed _the commencement of germination, nine weeks after the act of fertilisation_. The next stage, after an interval of one and a half weeks, is illustrated in figure 15; and _after a period of about fifteen weeks from the date of fertilisation the tip of the radicle pierces the top of the fruit_. As shown in the figures, the fruit grows in length throughout the process.

The question as to whether the matured seed passes through a stage of quiescence before it germinates finds its answer in the statement that only nine weeks elapse between fertilisation and germination. It may, however, be urged that the maturation of the seed could be accomplished in a few weeks, and that after this a period of dormant vitality might follow. This objection can be at once disposed of and the whole matter placed beyond reasonable doubt by making, as I did, a large number of vertical sections of the fruit in all its stages. It will then be perceived that there is a fairly constant relation in all stages of growth between the seed and the fruit, whether maturating or germinating. Since the growth of the fruit is continuous (see Table) up to the time of the protrusion of the tip of the hypocotyl through its coats, it follows that there can be no appreciable pause between the completion of maturation and the commencement of germination of the seed. In other words, both fruit and seed preserve the same relation during the process, and the absence of any period of rest is to be inferred from the uninterrupted growth of the fruit.

We will take, to illustrate this point, a fruit between four and five lines long in the stage that immediately precedes germination (see figure 11). The fruit proceeds with its growth, and the seed, we will suppose, remains quiescent for a month. At the end of that time (see Table) the fruit would be eight lines long, and the seed, of course, would be unchanged. This condition of things never presented itself to me. Fruits eight lines long were always far advanced in germination (see figure 15). If the seed passes through an interval of rest before germination, it must be of a very short duration and practically _nil_.

This absence of any period of rest between the final maturation of the seed and its fertilisation had already been assumed by Prof. Schimper. Writing to me on July 14, 1898, when my observations were in progress, he says:—“I am ready to assume, according to my own experience, that there is continuous development until the falling off of the embryo. More accurate observations on the subject would be interesting, and would not present any great difficulties.” At the end of the same month he wrote the preface to his great work on Plant-Geography; and he expresses himself decidedly on this point. Speaking of Rhizophora mucronata (English edition, p. 396), he says that “the fruit ... soon after the completion of its growth is pierced at its summit by the green hypocotyl, as the embryo does not undergo any period of rest, but continues to develop without interruption.”

Though the rest-period is normally non-existent with the seeds of Rhizophora, it has already been observed that it is indicated in rare cases and under exceptional conditions. Thus I have already remarked that in Fiji about one per cent. of the germinating fruits of the American species exhibit more than one seed. These seeds usually begin to germinate about the same time, but in a few cases, say, one in ten, a marked difference in the length of the protruding hypocotyls points to the fact that one of the seeds began to germinate some weeks after the other. We at times also meet with fruits which when cut across display two seeds, of which only one is beginning to germinate. Such cases indicative of a pause between the maturation of the seed and the beginning of germination would be very rare. With Rhizophora mangle, probably one in a thousand fruits would be a generous estimate.

In passing it may be remarked that the same stages occur with Rhizophora mucronata in the development of the seed and in the subsequent germinating process. When the fruit is three lines long the micropyle is but slightly dilated (see figures 1 and 2). When it is four lines long the endosperm begins to escape from the gaping micropyle and forms a projecting plug. The growth of the embryo now becomes rapid, the endosperm escapes in greater quantity, and by the time the fruit is five lines long the tip of the radicle is on a level with the micropyle, although still covered by the plug (see figures 4, 5). After this, germination begins; and when the fruit is six lines in length the radicle is in the act of penetrating the plug. Ultimately the tip of the radicle pierces the top of the fruit when this last is nine or ten lines long. As shown in the figures there is continuous growth of the fruit during the maturation and germination of the seed, until, in fact, the plantlet drops into the water. With reference to the stage when germination begins, it should be remarked that the formation of the large plug of endosperm outside the micropyle does not necessarily indicate the beginning of germination. Germination is in progress only when the hypocotyl or radicle begins to lengthen and is on the point of piercing the plug of endosperm that fills up the gaping micropyle. This is well shown in this species in the case of fruits with two seeds. Both seeds may have large plugs of endosperm, and yet only one may show indications of germination in the lengthening hypocotyl.

We must now return to the subject of the growth of the hanging seedling of Rhizophora mangle. We have already remarked that, as shown in the Table, about fifteen weeks (107 days) is the average time elapsing between the fertilisation of the ovule and the protrusion of the tip of the radicle through the top of the fruit. A further period of seventeen and a half weeks (122 days) is occupied by the growth of the seedling on the tree, at the end of which period it drops into the water or mud according to the state of the tide. This gives a total period of nearly thirty-three weeks (229 days) as the duration of the time between fertilisation and the fall of the seedling. This may be divided, as has been already implied, in the following manner:—

(1) Period between fertilisation and germination. 9 weeks.

(2) Period between the commencement of germination 6-1/2 and the protrusion of the tip of the radicle weeks. through the top of the fruit.

(3) Period occupied by the growth of the hypocotyl 17-1/2 outside the fruit, and terminating in the fall weeks. of the seedling from the tree.

——

Total 33 weeks.

This represents the average of numerous observations, the deviations being from two to three weeks on either side. In the latter part of its growth, the lower end of the hypocotyl becomes thickened or club-like, and during the last week or ten days the increase in length is arrested altogether.

My observations on the growth of the seedling on the tree of Rhizophora mucronata were comparatively few; but, as shown in the Table on page 453 they give nearly the same rate of growth. Taking the average length attained by the hypocotyl on the tree at sixteen inches, and employing as well the data supplied by Rhizophora mangle, a period of 26-1/2 weeks would elapse from the time the hypocotyl pierces the top of the fruit until the plantlet falls from the tree. If we then add, as in the case of the other species, 15-1/2 weeks for the preceding period between fertilisation and the protrusion of the hypocotyl, we get a total of 42 weeks for the whole period from fertilisation to the fall of the seedling. In the extreme cases where a length of almost two feet is attained on the tree, the period would somewhat exceed twelve months; and in those rare instances in other regions, when, according to Schimper, the seedling is a metre in length, probably eighteen months would be required. The period for Rhizophora mucronata is thus considerably longer than for R. mangle, which is sufficiently indicated by the difference in the average length of their hypocotyls on the tree in Fiji, that for R. mucronata being sixteen inches, and that for R. mangle nine or ten inches.

The only other observations that have come under my notice relating to this subject are those made by Jacquin on Rhizophora mangle in the West Indies in the middle of the eighteenth century. The results are literally quoted by Warming; but I have referred to the original account in the work of Jacquin, entitled _Selectarum Stirpium Americanarum Historia_, Vindobonæ, 1763. According to this observer the seedling falls from the tree in the twelfth month from the fecundation of the flower. This happened in my observations on the same species in Fiji in the eighth or ninth month. Jacquin states that the tip of the radicle protrudes from the fruit in the third month, whilst my results give it as taking place in the fourth month. The difference in the length of the total period, it may be remarked, would be to a great extent determined by the varying length acquired by the seedling before it drops from the tree. In ordinary conditions it averages about ten or eleven inches, and the hypocotyl itself attains a length of nine or ten inches on the tree, both in Fiji and Ecuador; but in sheltered localities it may attain a length half as long again. I have already pointed out in the case of the fruits of Rhizophora mucronata that a year and more would be sometimes required, and the same remark would apply to unusually long fruits of R. mangle. Local conditions would often produce varying results, both in the rate of growth of the hanging seedling and in the duration of the period of its attachment to the tree; but it is probable that nine or ten months would represent for the genus the average length of the period between fertilisation of the ovule and the detachment of the seedling from the parent tree.

_The mode of separation of the seedlings of Rhizophora mangle and Rhizophora mucronata_

This is a process of expulsion almost akin to parturition, and is brought about by the outward growth of the neck of the cotyledonary body. There is much that is of great interest in this subject; and I may add that Haberlandt, in a memoir published in the _Annales du Jardin Botanique de Buitenzorg_ for 1894, gives the results of an elaborate study of the viviparous process in this and other genera of mangroves. The same analogy seems also to have presented itself to him, but only in connection with the means employed in some of the genera, as with Bruguiera, for conveying nourishment to the growing embryo. He remarks that he was involuntarily reminded by these structures of the chorion-tufts and lobes in the placenta of mammals, and that such structures in the mammal are functionally nothing more than true _haustoria_ as found in the viviparous mangroves.

When studying the germination of the American and Asiatic Rhizophoras in Fiji, I observed that the neck of the cotyledonary body did not begin to form, nor the inclosed plumular bud to show signs of differentiation, until the hypocotyl had protruded about 4-1/2 inches with R. mangle, and between 6 and 7 inches with R. mucronata. The neck of the cotyledonary body then proceeds to grow in length, pushing before it the plumular end of the embryo-seedling, which it surrounds as a sheath. This operation continues until the hypocotyl has acquired a length of about seven inches with R. mangle, and about nine inches with R. mucronata, when the neck begins to protrude outside the fruit. The cotyledonary neck proceeds with its growth, and by the time the seedling is ready to fall from the tree it protrudes about an inch from the fruit-shell, having carried the growing plumular bud with it. The plumular end of the seedling has been now more or less expelled from the fruit-cavity, and the connection between the suspended seedling and the fruit now alone depends on a slight bond between the base of the plumule and the inner margin of the cotyledonary neck, as indicated by a cross in the figures given in the plate. The union is soon broken and the seedling falls.

Whether there is anything more than an analogy between the expulsion of a Rhizophora seedling and the birth of a mammal seems most unlikely; but the process is at all events a very remarkable one.

_The means of dispersal of the genus Rhizophora_

My experiments and observations were for the most part made on the Asiatic and American species in Fiji; but I enjoyed the opportunity of confirming some important points on the coast of Ecuador. We can only look to the currents for the explanation of the capacity of the genus to cross tracts of ocean; but, given this capacity, there is much that is difficult to understand in the distribution of the genus and of a species like Rhizophora mangle; and it is probable that we shall have to look behind the means of dispersal to a distant age in the distribution of shore-plants of the mangrove type.

When Schimper published his work on the Indo-Malayan strand flora in 1891, but little was known of the duration of the floating capacity of Rhizophora seedlings (p. 166). In giving the results of my investigations I am merely describing the agencies of dispersal at present in operation. Such agencies have their limitations, and we may, perhaps, be thus able to explain why Rhizophora is restricted in the Pacific islands to the archipelagoes of the Western Pacific; but many serious objections would at once present themselves if we regarded the occurrence of the genus in America, as well as in Asia and Africa, as a matter depending on capacities and means of dispersal.

The fruits of Rhizophora, as they display themselves before the protrusion of the germinating seed, have no buoyancy, and the germinating fruits until the hypocotyl has protruded for some inches (6 inches in the case of R. mangle) also sink in sea-water. With a further increase in the length of the hypocotyl, the germinating fruit acquires buoyancy; and when the seedling, usually 10 or 11 inches in length, becomes detached from the fruit on the tree and falls into the sea, it floats readily in 95 per cent. of the cases. Such seedlings occur very commonly in the floating drift of the estuaries and out at sea both in Fiji and in Ecuador.

Out of five seedlings of the Asiatic species, Rhizophora mucronata, that had fallen naturally from the tree, three were afloat and healthy after eighty-seven days’ immersion in sea-water. Out of twenty seedlings of the American mangrove, Rhizophora mangle, sixteen floated after ninety days and four were afloat and healthy after one hundred and twenty days, the greater number sinking during the fourth month. These results indicate considerable powers of buoyancy, and go to show that extensive tracts of ocean could be traversed by the floating seedling.

It should, however, be observed that not all the full-sized seedlings float. With Rhizophora mangle about 5 per cent. sink in sea-water and from 20 to 50 per cent. sink in fresh-water; whilst with R. mucronata the proportion of non-buoyant seedlings is rather greater. There would thus appear to be a rather nice adjustment of the specific weight of the seedlings to the density of sea-water. Generally speaking, they may be seen floating vertically or steeply inclined in the fresh-water of estuaries and horizontally in the sea. With the buoyant seedlings of Rhizophora mucronata, as a rule, about 90 per cent. float horizontally in sea-water, and about 70 per cent. float vertically or steeply inclined in fresh-water. The same general rule applies to R. mangle, whether in the rivers and seas of Fiji or in those of Ecuador. In those cases where the seedling drops prematurely on account either of storms and floods or of the depredations of a grub that frequently attacks the fruit, this rule would not apply. One may frequently notice in Fiji after heavy weather that seedlings detached prematurely, and often carrying the fruit, are floating in numbers horizontally in the rivers. In a few days, as a rule, the fruit-case becomes detached and sinks.

It may be remarked that the horizontal position is much better adapted for the safety of the seedling in transport than the vertical position. In the last case the plumule, which protrudes above the water, would be unable, as indicated in my experiments, to withstand the scorching rays of the sun in a smooth sea; whereas in the horizontal position, which the seedlings assume in sea-water, the plumule is more or less completely submerged, and the risk of withering in the sun is very much less. The Rhizophora seedlings would certainly have little chance of crossing in safety a large tract of sea, if they floated, as they do in river-water, with the plumule exposed above the surface. It is not unlikely that the comparatively restricted area occupied by Rhizophora conjugata may be due to the attitude its seedlings assume when floating in sea-water.

The stranded seedlings of Rhizophora readily establish themselves for a while in very different situations; and it is by no means necessary that they should be washed ashore on a muddy coast. When half-buried amongst the heap of vegetable drift piled up on a sandy beach they are frequently to be found striking into the sand and showing their first leaves. Here they ultimately perish in the great majority of cases; but when protected long enough to reach the moist sand four or five inches below, they may give rise to a little mangrove colony. When caught in a fissure in the bare reef-flats these plantlets are sometimes able to establish themselves. Rhizophora seedlings would, however, require a coast prepared by them by the work of ages before they could form extensive swamps. It is, therefore, not surprising that Prof. Penzig found no evidence of mangrove-settlements on the shores of Krakatoa fourteen years after the eruption.

Yet suited as Rhizophora seedlings are for crossing tracts of sea, I regard them as quite unfitted for being transported by the currents unharmed across an ocean. The plumular bud is insufficiently protected for such a long voyage of many months, and perhaps of years. Though the horizontal position of the seedling would secure the plumule against being scorched in the sun, it increases considerably the risk of injury from direct impact.

As bearing on their capacity for dispersal in other fashions, it may be remarked that Rhizophora seedlings can withstand long drying. Five which had been kept dry for nine weeks, after having been found stranded on a beach, were planted in the mud of a mangrove-swamp. In a fortnight two of them were developing the first leaves and throwing out roots. As long as they are protected by a covering of vegetable _débris_ and sand, the stranded seedlings might retain their vitality for months.

BRUGUIERA RHEEDII (Blume)

This species is reduced in Hooker’s _Flora of British India_ to Bruguiera gymnorhiza (Lam.), and thus viewed it has a very wide range in the Old World, corresponding very much to that of Rhizophora mucronata, namely, tropical East Africa, tropical East Asia to the Liukiu Islands, the Indian Archipelago, New Guinea, tropical Australia, and Western Polynesia, as in New Caledonia, Fiji, Tonga, and Samoa. There are four or five species of the genus, but all are confined to the Eastern Hemisphere, none occurring in America.

As with the species of Rhizophora, this plant is indebted for its present dispersal to the floating seedling, which, however, often falls from the tree whilst still attached to the fruit, but is generally freed in a day or two. The seedlings float for a long time in sea-water. I kept one of them afloat for 117 days, when it was quite sound and healthy. They appear to be better fitted than the species of Rhizophora for the “rough-and-tumble” of ocean transport, since the plumule is much less prominent, projecting only one line (2·5 mm.) or less, whilst with the two Fijian species of Rhizophora the plumule measures from seven to twelve lines (18 to 30 mm.). In the latter part of the year they are to be found in abundance in the floating drift of rivers, and there they readily develop the first leaves and roots. They are also frequent in the sea off the coasts, and they are stranded in large numbers on the beaches, where they readily strike into the sand when partially buried amongst the vegetable drift.

The empty flowers and the germinated fruits containing the cotyledons are very common in floating drift. They look much alike, but the flowers are much smaller and possess the long style, whilst the fruits contain the cotyledons at the bottom of the seed-cavity.

As with Rhizophora, there is a rather curious adjustment of the buoyancy of the seedling to the density of sea-water. About 75 per cent. of those afloat in the fresh-water of rivers assume the vertical position, the plumular end protruding between two and five lines (5 to 12 mm.) above the surface, while the remainder float horizontally or nearly so. In sea-water about 50 per cent. float either vertically or steeply inclined, and the other half float horizontally.

With regard to the times of flowering and fruiting, it may be remarked that the trees are mostly in flower during the hot months from November to February, and that the fruiting is in active operation in the latter half of March. The floating seedlings occur in abundance in the river-drift at the end of the year, a circumstance which corresponds with the fact that a period of six months passes between the fertilisation of the ovule and the fall of the seedling into the water.

Fertilisation, or, more correctly speaking, the discharge of the pollen, takes place after the opening of the flower, and not before, as in the case of the species of Rhizophora. The flower-bud is at first erect, but subsequently it begins to bend downwards, and ultimately it hangs more or less vertically. The provision to secure fertilisation under these circumstances is rather curious. Without some such contrivance as is below described, the pollen would merely fall out of the flower. Each petal has its sides rolled or folded inwards so as to completely inclose two stamens. In the bud the folded petals are white and flexible, but as the flower expands they redden and become dry and elastic, and are only prevented from flying open with a spring by the interlocking of the hairy tips of their lobes. Whilst the folded petals are becoming stiff and elastic during the opening of the flower, the inclosed stamens are at the same time preparing themselves for their function. The anthers are dehiscing and the filaments are acquiring elasticity. All is now ready, and a slight shake or a touch puts the mechanism into action. The petals unfold themselves with a spring, and the stamens thus suddenly exposed and released fly forward, and a little shower of pollen is thrown towards the centre of the flower. This process is accomplished in ordinary fine weather during the first twenty-four or thirty-six hours after the expansion of the flower. When the opening occurs in the early morning, half of the stamens will be found released in the evening and the rest on the following day. During the next day or two the petals and the stamens fall out of the flower. In wet weather, the petals never acquire elasticity, and in consequence do not unfold. In this case pollenisation is never effected, and the folded petals soon fall to the ground, carrying the stamens within them. Cross-fertilisation would be much more likely to occur with species of Bruguiera (if, as is probable, the same process of pollenisation is usually followed) than with species of Rhizophora, since the stamens are securely inclosed in the petals for some hours after the expansion of the flower.

Nearly eight weeks pass between the date of fertilisation and the commencement of germination. This is somewhat similar to the period given for Rhizophora mangle, namely, nine weeks, and it obviously leaves little or no time for any stage of quiescence or dormant vitality in the case of the seed. The changes which the fruit undergoes in this interval are a considerable increase in girth and a thickening of the calycine walls, together with a contraction of the mouth of the tube. However, I found no method sufficiently accurate for recording the rate of increase of the fruit.

It is known that germination is in progress when the end of the hypocotyl begins to lift up the lining membrane at the bottom of the calycine tube (see Figs. 21 to 26). The floor of the tube begins to bulge up, but since this cannot be well seen at first, a better index is afforded in the elevation of the style which accompanies it. The top of the style preserves previous to this time a constant level with regard to the tips of the calycine teeth. But this does not indicate the actual beginning of germination. As shown in Fig. 21, the seed lies about two and a half lines (6 mm.) below the floor of the calycine tube, and the tip of the hypocotyl has to penetrate the intervening tissues before it can push up the lining membrane and raise the style. Judging from the subsequent rate of growth, seven or eight days at least, and perhaps as much as two weeks, are requisite for this purpose. It is not necessary to give further details here, and it may be at once stated that the average of numerous observations on the length of the interval between fertilisation and the elevation of the style was sixty-four days, the range being fifty-nine to sixty-nine. After deducting ten days for the time occupied for the radicle in reaching the floor of the calycine tube (see Figs. 22 and 23), we obtain, as already remarked, nearly eight weeks as the time elapsing between fertilisation and germination.

The radicle or hypocotyl, therefore, in the first stage of germination pierces the tissues above it and reaches the floor of the calycine tube. It does not, however, pierce the lining membrane of the tube but pushes it upward until it ruptures about 4 millimetres below the base of the style which is carried up with it. Thus a kind of cap is formed, as shown in Fig. 24, which does not fall off from the end of the hypocotyl until it has protruded rather more than an inch. The hypocotyl attains a length varying between 5 and 11 inches, the average being about 8 inches.

The whole period may be thus divided up:—

(1) Period between fertilisation and germination 7-1/2 weeks.

(2) Period between the beginning of germination and the 1-1/2 protrusion of the point of the hypocotyl at the floor of the weeks. calycine tube

(3) Period occupied in the growth of the hypocotyl 8 inches 18 weeks. outside the fruit and terminating in the fall of the seedling

——

Total 27 weeks.

The total period of twenty-seven weeks between fertilisation and the fall of the seedling is thus six weeks shorter than that estimated for Rhizophora mangle. On comparing the two tables it will be seen that the difference mainly lies in the length of the second period, namely, that between the commencement of germination and the protrusion of the hypocotyl from the fruit. With Rhizophora mangle the fruit grows considerably in length during this period of the germinating process. On the other hand with Bruguiera rheedii there is, during this period, practically no increase in the length of the fruit, and the radicle has only to penetrate the tissues, 2-1/2 lines in thickness, between the seed and the floor of the calycine tube.

In the mode of separation of the seedling there are very marked differences between this species of Bruguiera and the species of Rhizophora. With Bruguiera rheedii the four small cotyledons, which are united at the base, are, however, left behind at the bottom of the seed-cavity, when the seedling is detached. But there is no expulsion of the seedling, the connection being ultimately severed at the contracted base of the cotyledons inside the fruit. When the seedling is full-sized the nutritive supply begins to fail, and in consequence the pressure of the sides of the fruit on the inclosed plumular end of the seedling becomes slacker, the union with the cotyledons becomes weaker, and the connection of the fruit with its peduncle at the basal joint becomes slighter. Usually the fruit falls before the seedling is ready to drop out, and the connection is severed after a few days’ flotation in the water; but sometimes the union between the seedling and fruit is weaker than that between the fruit and its peduncle, and in that case the seedling falls and leaves the fruit containing the cotyledons on the tree. The whole process of separation is much simpler than with species of Rhizophora. Here it is mainly a matter of the failure of the nutritive supply, whilst with Rhizophora it is almost a process of parturition.

Haberlandt, in the memoir before quoted, describes quite a different mode of detachment in the case of Bruguiera eriopetala. Here the seedling falls normally whilst still attached to the fruit, and the separation is subsequently effected by the expansion of the mouth of the calyx-tube due to the swelling of the “endosperm-neck” from the entrance of water.

_Summary_

(1) There are four typical mangroves of the Rhizophoraceæ in Fiji, Bruguiera rheedii, Rhizophora mucronata (the Asiatic species), Rhizophora mangle (the American species), and the Selala, a seedless form intermediate between the two species of Rhizophora just named, but nearest to the Asiatic species.

(2) It is shown that the sterility of the Selala is connected with the impotent character of the pollen; and since the ovules appear capable of fertilisation this is held to indicate that cross fertilisation has not been in operation in producing the barren form.

(3) Good reasons are given for the belief that the Asiatic species of Rhizophora is the parent of the Selala, not as the result of a cross between the Asiatic and American species, but as connected with dimorphism, the Asiatic species producing two kinds of offspring, one of them with impotent pollen.

(4) In support of this view it is pointed out that there are two forms of Rhizophora mangle in Ecuador, one of which comes near the Fijian Selala, though producing seed. There could thus be no question of crossing, since but one species occurs there.

(5) The Selala reproduces itself in a vegetative fashion when growing, as it often does, in an inclined position. The parent trunk dies and the primary branches supported by the aërial roots, remain alive and in their turn give rise to secondary branches similarly supported.

(6) Although, as a rule, only one of the four ovules of Rhizophora becomes a seed, occasionally a fruit contains more than one seed. With R. mangle in Fiji about one per cent. of the germinating fruits displayed more than one hypocotyl.

(7) As a result of a protracted series of observations in Fiji, it was established that in the case of a seedling of average length of Rhizophora mangle a period of thirty-three weeks elapsed between the date of fertilisation of the ovule and the detachment of the seedling from the tree. In the instance of R. mucronata it was placed at forty-two weeks. A period of thirty-eight weeks, or nine to ten months, is regarded as typical for the genus.

(8) It is established that normally there is no rest-period for the seed in the case of Rhizophora, the seed at once beginning to germinate on reaching maturity. In those exceptional instances, however, where there is more than one seed, it is shown that in some cases the seeds do not begin to germinate together, and that a rest-period of at least some weeks can be at times postulated for one of the seeds.

(9) An analogy exists between the process of expulsion ending in the detachment of the seedling of Rhizophora from the fruit and the process of parturition.

(10) Experiments show that Rhizophora seedlings can float unharmed in sea-water for a period of at least three or four months. Though nine-tenths or more float in sea-water, as much as a fourth or a half sink in fresh-water. As a rule they float vertically in fresh-water and horizontally in sea-water, the horizontal position safe-guarding the plumule against the risk of being withered up by the sun in a calm sea.

(11) It is shown that in the case of Bruguiera rheedii the seedlings when detached from the tree can float unharmed in sea-water for months. In their specific weight they display a similar fine adjustment to the density of sea-water, as is above described in the case of Rhizophora.

(12) With this species of Bruguiera, fertilisation takes place not in the unopened flower, as in Rhizophora, but after the flower’s expansion; and a very singular mechanism is here described which secures the completion of the process.

(13) A period of twenty-seven weeks elapses between the fecundation of the ovule and the detachment of the seedling from the tree in the case of Bruguiera rheedii; and it is shown that there is normally little or no room for any rest-period, and that, as with Rhizophora, the seed on reaching maturity begins to germinate.

(14) Though the seedlings of Rhizophora and Bruguiera could be transported in safety a few hundred miles across the sea, it is held that they could never cross the Pacific and reproduce the plant. That the American species of Rhizophora has reached the Western Pacific from the New World is not accepted. Rather is its present distribution regarded as representing its original wide range over much of the tropical zone.