Chapter 25

crossing of rhododendrons. Gallesio makes the same statement with respect to oranges. I have myself known extensive crossing to occur with the common rhubarb. For Leptosiphon, Verlot ‘Des Varieties’ 1865 page 20. I have not included in my list the Carnation, Nemophila, or Antirrhinum, the varieties of which are known to cross freely, because these plants are not always self-fertile. I know nothing about the self-fertility of Trollius Lecoq ‘De la Fecondation’ 1862 page 93, Mahonia, and Crinum, in which genera the species intercross largely. With respect to Mahonia it is now scarcely possible to procure in this country pure specimens of M. aquifolium or repens; and the various species of Crinum sent by Herbert ‘Amaryllidaceae’ page 32, to Calcutta, crossed there so freely that pure seed could not be saved.) Much other indirect evidence could be given with respect to the extent to which varieties of the same species spontaneously intercross.

Gardeners who raise seed for sale are compelled by dearly bought experience to take extraordinary precautions against intercrossing. Thus Messrs. Sharp “have land engaged in the growth of seed in no less than eight parishes.” The mere fact of a vast number of plants belonging to the same variety growing together is a considerable protection, as the chances are strong in favour of plants of the same variety intercrossing; and it is in chief part owing to this circumstance, that certain villages have become famous for pure seed of particular varieties. (10/39. With respect to Messrs. Sharp see ‘Gardeners’ Chronicle’ 1856 page 823. Lindley’s ‘Theory of Horticulture’ page 319.) Only two trials were made by me to ascertain after how long an interval of time, pollen from a distinct variety would obliterate more or less completely the action of a plant’s own pollen. The stigmas in two lately expanded flowers on a variety of cabbage, called Ragged Jack, were well covered with pollen from the same plant. After an interval of twenty-three hours, pollen from the Early Barnes Cabbage growing at a distance was placed on both stigmas; and as the plant was left uncovered, pollen from other flowers on the Ragged Jack would certainly have been left by the bees during the next two or three days on the same two stigmas. Under these circumstances it seemed very unlikely that the pollen of the Barnes cabbage would produce any effect; but three out of the fifteen plants raised from the two capsules thus produced were plainly mongrelised: and I have no doubt that the twelve other plants were affected, for they grew much more vigorously than the self-fertilised seedlings from the Ragged Jack planted at the same time and under the same conditions. Secondly, I placed on several stigmas of a long-styled cowslip (Primula veris) plenty of pollen from the same plant, and after twenty-four hours added some from a short-styled dark-red Polyanthus, which is a variety of the cowslip. From the flowers thus treated thirty seedlings were raised, and all these without exception bore reddish flowers; so that the effect of the plant’s own pollen, though placed on the stigmas twenty-four hours previously, was quite destroyed by that of the red variety. It should, however, be observed that these plants are dimorphic, and that the second union was a legitimate one, whilst the first was illegitimate; but flowers illegitimately fertilised with their own pollen yield a moderately fair supply of seeds.

We have hitherto considered only the prepotent fertilising power of pollen from a distinct variety over a plants’ own pollen,--both kinds of pollen being placed on the same stigma. It is a much more remarkable fact that pollen from another individual of the same variety is prepotent over a plant’s own pollen, as shown by the superiority of the seedlings raised from a cross of this kind over seedlings from self-fertilised flowers. Thus in Tables 7/A, B, and C, there are at least fifteen species which are self-fertile when insects are excluded; and this implies that their stigmas must receive their own pollen; nevertheless, most of the seedlings which were raised by fertilising the non-castrated flowers of these fifteen species with pollen from another plant were greatly superior, in height, weight, and fertility, to the self-fertilised offspring. (10/40. These fifteen species consist of Brassica oleracea, Reseda odorata and lutea, Limnanthes douglasii, Papaver vagum, Viscaria oculata, Beta vulgaris, Lupinus luteus, Ipomoea purpurea, Mimulus luteus, Calceolaria, Verbascum thapsus, Vandellia nummularifolia, Lactuca sativa, and Zea mays.) For instance, with Ipomoea purpurea every single intercrossed plant exceeded in height its self-fertilised opponent until the sixth generation; and so it was with Mimulus luteus until the fourth generation. Out of six pairs of crossed and self-fertilised cabbages, every one of the former was much heavier than the latter. With Papaver vagum, out of fifteen pairs, all but two of the crossed plants were taller than their self-fertilised opponents. Of eight pairs of Lupinus luteus, all but two of the crossed were taller; of eight pairs of Beta vulgaris all but one; and of fifteen pairs of Zea mays all but two were taller. Of fifteen pairs of Limnanthes douglasii, and of seven pairs of Lactuca sativa, every single crossed plant was taller than its self-fertilised opponent. It should also be observed that in these experiments no particular care was taken to cross-fertilise the flowers immediately after their expansion; it is therefore almost certain that in many of these cases some pollen from the same flower will have already fallen on and acted on the stigma.

There can hardly be a doubt that several other species of which the crossed seedlings are more vigorous than the self-fertilised, as shown in Tables 7/A, 7/B and 7/C, besides the above fifteen, must have received their own pollen and that from another plant at nearly the same time; and if so, the same remarks as those just given are applicable to them. Scarcely any result from my experiments has surprised me so much as this of the prepotency of pollen from a distinct individual over each plant’s own pollen, as proved by the greater constitutional vigour of the crossed seedlings. The evidence of prepotency is here deduced from the comparative growth of the two lots of seedlings; but we have similar evidence in many cases from the much greater fertility of the non-castrated flowers on the mother-plant, when these received at the same time their own pollen and that from a distinct plant, in comparison with the flowers which received only their own pollen.

From the various facts now given on the spontaneous intercrossing of varieties growing near together, and on the effects of cross-fertilising flowers which are self-fertile and have not been castrated, we may conclude that pollen brought by insects or by the wind from a distinct plant will generally prevent the action of pollen from the same flower, even though it may have been applied some time before; and thus the intercrossing of plants in a state of nature will be greatly favoured or ensured.

The case of a great tree covered with innumerable hermaphrodite flowers seems at first sight strongly opposed to the belief in the frequency of intercrosses between distinct individuals. The flowers which grow on the opposite sides of such a tree will have been exposed to somewhat different conditions, and a cross between them may perhaps be in some degree beneficial; but it is not probable that it would be nearly so beneficial as a cross between flowers on distinct trees, as we may infer from the inefficiency of pollen taken from plants which have been propagated from the same stock, though growing on separate roots. The number of bees which frequent certain kinds of trees when in full flower is very great, and they may be seen flying from tree to tree more frequently than might have been expected. Nevertheless, if we consider how numerous are the flowers, for instance, on a horse-chestnut or lime-tree, an incomparably larger number of flowers must be fertilised by pollen brought from other flowers on the same tree, than from flowers on a distinct tree. But we should bear in mind that with the horse-chestnut, for instance, only one or two of the several flowers on the same peduncle produce a seed; and that this seed is the product of only one out of several ovules within the same ovarium. Now we know from the experiments of Herbert and others that if one flower is fertilised with pollen which is more efficient than that applied to the other flowers on the same peduncle, the latter often drop off (10/41. ‘Variation under Domestication’ chapter 17 2nd edition volume 2 page 120.); and it is probable that this would occur with many of the self-fertilised flowers on a large tree, if other and adjoining flowers were cross-fertilised. Of the flowers annually produced by a great tree, it is almost certain that a large number would be self-fertilised; and if we assume that the tree produced only 500 flowers, and that this number of seeds were requisite to keep up the stock, so that at least one seedling should hereafter struggle to maturity, then a large proportion of the seedlings would necessarily be derived from self-fertilised seeds. But if the tree annually produced 50,000 flowers, of which the self-fertilised dropped off without yielding seeds, then the cross-fertilised flowers might yield seeds in sufficient number to keep up the stock, and most of the seedlings would be vigorous from being the product of a cross between distinct individuals. In this manner the production of a vast number of flowers, besides serving to entice numerous insects and to compensate for the accidental destruction of many flowers by spring-frosts or otherwise, would be a very great advantage to the species; and when we behold our orchard-trees covered with a white sheet of bloom in the spring, we should not falsely accuse nature of wasteful expenditure, though comparatively little fruit is produced in the autumn.

ANEMOPHILOUS PLANTS.

The nature and relations of plants which are fertilised by the wind have been admirably discussed by Delpino and Hermann Muller; and I have already made some remarks on the structure of their flowers in contrast with those of entomophilous species. (10/42. Delpino ‘Ult. Osservazioni sulla Dicogamia’ part 2 fasc. 1 1870 and ‘Studi sopra un Lignaggio anemofilo’ etc. 1871. Hermann Muller ‘Die Befruchtung’ etc. pages 412, 442. Both these authors remark that plants must have been anemophilous before they were entomophilous. Hermann Muller further discusses in a very interesting manner the steps by which entomophilous flowers became nectariferous and gradually acquired their present structure through successive beneficial changes.) There is good reason to believe that the first plants which appeared on this earth were cryptogamic; and judging from what now occurs, the male fertilising element must either have possessed the power of spontaneous movement through the water or over damp surfaces, or have been carried by currents of water to the female organs. That some of the most ancient plants, such as ferns, possessed true sexual organs there can hardly be a doubt; and this shows, as Hildebrand remarks, at how early a period the sexes were separated. (10/43. ‘Die Geschlechter-Vertheilung’ 1867 pages 84-90.) As soon as plants became phanerogamic and grew on the dry ground, if they were ever to intercross, it would be indispensable that the male fertilising element should be transported by some means through the air; and the wind is the simplest means of transport. There must also have been a period when winged insects did not exist, and plants would not then have been rendered entomophilous. Even at a somewhat later period the more specialised orders of the Hymenoptera, Lepidoptera, and Diptera, which are now chiefly concerned with the transport of pollen, did not exist. Therefore the earliest terrestrial plants known to us, namely, the Coniferae and Cycadiae, no doubt were anemophilous, like the existing species of these same groups. A vestige of this early state of things is likewise shown by some other groups of plants which are anemophilous, as these on the whole stand lower in the scale than entomophilous species.

There is no great difficulty in understanding how an anemophilous plant might have been rendered entomophilous. Pollen is a nutritious substance, and would soon have been discovered and devoured by insects; and if any adhered to their bodies it would have been carried from the anthers to the stigma of the same flower, or from one flower to another. One of the chief characteristics of the pollen of anemophilous plants is its incoherence; but pollen in this state can adhere to the hairy bodies of insects, as we see with some Leguminosae, Ericaceae, and Melastomaceae. We have, however, better evidence of the possibility of a transition of the above kind in certain plants being now fertilised partly by the wind and partly by insects. The common rhubarb (Rheum rhaponticum) is so far in an intermediate condition, that I have seen many Diptera sucking the flowers, with much pollen adhering to their bodies; and yet the pollen is so incoherent, that clouds of it are emitted if the plant be gently shaken on a sunny day, some of which could hardly fail to fall on the large stigmas of the neighbouring flowers. According to Delpino and Hermann Muller, some species of Plantago are in a similar intermediate condition. (10/44. ‘Die Befruchtung’ etc. page 342.)

Although it is probable that pollen was aboriginally the sole attraction to insects, and although many plants now exist whose flowers are frequented exclusively by pollen-devouring insects, yet the great majority secrete nectar as the chief attraction. Many years ago I suggested that primarily the saccharine matter in nectar was excreted as a waste product of chemical changes in the sap; and that when the excretion happened to occur within the envelopes of a flower, it was utilised for the important object of cross-fertilisation, being subsequently much increased in quantity and stored in various ways. (10/45. Nectar was regarded by De Candolle and Dunal as an excretion, as stated by Martinet in ‘Annal des Sc. Nat.’ 1872 tome 14 page 211.) This view is rendered probable by the leaves of some trees excreting, under certain climatic conditions, without the aid of special glands, a saccharine fluid, often called honey-dew. This is the case with the leaves of the lime; for although some authors have disputed the fact, a most capable judge, Dr. Maxwell Masters, informs me that, after having heard the discussions on this subject before the Horticultural Society, he feels no doubt on this head. The leaves, as well as the cut stems, of the manna ash (Fraxinus ornus) secrete in a like manner saccharine matter. (10/46. ‘Gardeners’ Chronicle’ 1876 page 242.) According to Treviranus, so do the upper surfaces of the leaves of Carduus arctioides during hot weather. Many analogous facts could be given. (10/47. Kurr ‘Untersuchungen uber die Bedeutung der Nektarien’ 1833 page 115.) There are, however, a considerable number of plants which bear small glands on their leaves, petioles, phyllodia, stipules, bracteae, or flower peduncles, or on the outside of their calyx, and these glands secrete minute drops of a sweet fluid, which is eagerly sought by sugar-loving insects, such as ants, hive-bees, and wasps. (10/48. A large number of cases are given by Delpino in the ‘Bulletino Entomologico’ Anno 6 1874. To these may be added those given in my text, as well as the excretion of saccharine matter from the calyx of two species of Iris, and from the bracteae of certain Orchideae: see Kurr ‘Bedeutung der Nektarien’ 1833 pages 25, 28. Belt ‘Nicaragua’ page 224, also refers to a similar excretion by many epiphytal orchids and passion-flowers. Mr. Rodgers has seen much nectar secreted from the bases of the flower-peduncles of Vanilla. Link says that the only example of a hypopetalous nectary known to him is externally at the base of the flowers of Chironia decussata: see ‘Reports on Botany, Ray Society’ 1846 page 355. An important memoir bearing on this subject has lately appeared by Reinke ‘Gottingen Nachrichten’ 1873 page 825, who shows that in many plants the tips of the serrations on the leaves in the bud bear glands which secrete only at a very early age, and which have the same morphological structure as true nectar-secreting glands. He further shows that the nectar-secreting glands on the petioles of Prunus avium are not developed at a very early age, yet wither away on the old leaves. They are homologous with those on the serrations of the blades of the same leaves, as shown by their structure and by transition-forms; for the lowest serrations on the blades of most of the leaves secrete nectar instead of resin (harz).) In the case of the glands on the stipules of Vicia sativa, the excretion manifestly depends on changes in the sap, consequent on the sun shining brightly; for I repeatedly observed that as soon as the sun was hidden behind clouds the secretion ceased, and the hive-bees left the field; but as soon as the sun broke out again, they returned to their feast. (10/49. I published a brief notice of this case in the ‘Gardeners’ Chronicle’ 1855 July 21 page 487, and afterwards made further observations. Besides the hive-bee, another species of bee, a moth, ants, and two kinds of flies sucked the drops of fluid on the stipules. The larger drops tasted sweet. The hive-bees never even looked at the flowers which were open at the same time; whilst two species of humble-bees neglected the stipules and visited only the flowers.) I have observed an analogous fact with the secretion of true nectar in the flowers of Lobelia erinus.

Delpino, however, maintains that the power of secreting a sweet fluid by any extra-floral organ has been in every case specially gained, for the sake of attracting ants and wasps as defenders of the plant against their enemies; but I have never seen any reason to believe that this is so with the three species observed by me, namely, Prunus laurocerasus, Vicia sativa, and V. faba. No plant is so little attacked by enemies of any kind as the common bracken-fern (Pteris aquilina); and yet, as my son Francis has discovered, the large glands at the bases of the fronds, but only whilst young, excrete much sweetish fluid, which is eagerly sought by innumerable ants, chiefly belonging to Myrmica; and these ants certainly do not serve as a protection against any enemy. Delpino argues that such glands ought not to be considered as excretory, because if they were so, they would be present in every species; but I cannot see much force in this argument, as the leaves of some plants excrete sugar only during certain states of the weather. That in some cases the secretion serves to attract insects as defenders of the plant, and may have been developed to a high degree for this special purpose, I have not the least doubt, from the observations of Delpino, and more especially from those of Mr. Belt on Acacia sphaerocephala, and on passion-flowers. This acacia likewise produces, as an additional attraction to ants, small bodies containing much oil and protoplasm, and analogous bodies are developed by a Cecropia for the same purpose, as described by Fritz Muller. (10/50. Mr. Belt ‘The Naturalist in Nicaragua’ 1874 page 218, has given a most interesting account of the paramount importance of ants as defenders of the above Acacia. With respect to the Cecropia see ‘Nature’ 1876 page 304. My son Francis has described the microscopical structure and development of these wonderful food-bodies in a paper read before the Linnean Society.)

The excretion of a sweet fluid by glands seated outside of a flower is rarely utilised as a means for cross-fertilisation by the aid of insects; but this occurs with the bracteae of the Marcgraviaceae, as the late Dr. Cruger informed me from actual observation in the West Indies, and as Delpino infers with much acuteness from the relative position of the several parts of their flowers. (10/51. ‘Ult. Osservaz. Dicogamia’ 1868-69 page 188.) Mr. Farrer has also shown that the flowers of Coronilla are curiously modified, so that bees may fertilise them whilst sucking the fluid secreted from the outside of the calyx. (10/52. ‘Nature’ 1874 page 169.) It further appears probable from the observations of the Reverend W.A. Leighton, that the fluid so abundantly secreted by glands on the phyllodia of the Australian Acacia magnifica, which stand near the flowers, is connected with their fertilisation. (10/53. ‘Annals and Magazine of Natural History’ volume 16 1865 page 14. In my work on the ‘Fertilisation of Orchids’ and in a paper subsequently published in the ‘Annals and Magazine of Natural History’ it has been shown that although certain kinds of orchids possess a nectary, no nectar is actually secreted by it; but that insects penetrate the inner walls and suck the fluid contained in the intercellular spaces. I further suggested, in the case of some other orchids which do not secrete nectar, that insects gnawed the labellum; and this suggestion has since been proved true. Hermann Muller and Delpino have now shown that some other plants have thickened petals which are sucked or gnawed by insects, their fertilisation being thus aided. All the known facts on this head have been collected by Delpino in his ‘Ult. Osserv.’ part 2 fasc. 2 1875 pages 59-63.)

The amount of pollen produced by anemophilous plants, and the distance to which it is often transported by the wind, are both surprisingly great. Mr. Hassall found that the weight of pollen produced by a single plant of the Bulrush (Typha) was 144 grains. Bucketfuls of pollen, chiefly of Coniferae and Gramineae, have been swept off the decks of vessels near the North American shore; and Mr. Riley has seen the ground near St. Louis, in Missouri, covered with pollen, as if sprinkled with sulphur; and there was good reason to believe that this had been transported from the pine-forests at least 400 miles to the south. Kerner has seen the snow-fields on the higher Alps similarly dusted; and Mr. Blackley found numerous pollen-grains, in one instance 1200, adhering to sticky slides, which were sent up to a height of from 500 to 1000 feet by means of a kite, and then uncovered by a special mechanism. It is remarkable that in these experiments there were on an average nineteen times as many pollen-grains in the atmosphere at the higher than at the lower levels. (10/54. For Mr. Hassall’s observations see ‘Annals and Magazine of Natural History’ volume 8 1842 page 108. In the ‘North American Journal of Science’ January 1842, there is an account of the pollen swept off the decks of a vessel. Riley ‘Fifth Report on the Noxious Insects of Missouri’ 1873 page 86. Kerner ‘Die Schutzmittel des Pollens’ 1873 page 6. This author has also seen a lake in the Tyrol so covered with pollen, that the water no longer appeared blue. Mr. Blackley ‘Experimental Researches on Hay-fever’ 1873 pages 132, 141-152.) Considering these facts, it is not so surprising as it at first appears that all, or nearly all, the stigmas of anemophilous plants should receive pollen brought to them by mere chance by the wind. During the early part of summer every object is thus dusted with pollen; for instance, I examined for another purpose the labella of a large number of flowers of the Fly Ophrys (which is rarely visited by insects), and found on all very many pollen-grains of other plants, which had been caught by their velvety surfaces.

The extraordinary quantity and lightness of the pollen of anemophilous plants are no doubt both necessary, as their pollen has generally to be carried to the stigmas of other and often distant flowers; for, as we shall soon see, most anemophilous plants have their sexes separated. The fertilisation of these plants is generally aided by the stigmas being of large size or plumose; and in the case of the Coniferae, by the naked ovules secreting a drop of fluid, as shown by Delpino. Although the number of anemophilous species is small, as the author just quoted remarks, the number of individuals is large in comparison with that of entomophilous species. This holds good especially in cold and temperate regions, where insects are not so numerous as under a warmer climate, and where consequently entomophilous plants are less favourably situated. We see this in our forests of Coniferae and other trees, such as oaks, beeches, birches, ashes, etc.; and in the Gramineae, Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all these trees and plants being fertilised by the wind. As a large quantity of pollen is wasted by anemophilous plants, it is surprising that so many vigorous species of this kind abounding with individuals should still exist in any part of the world; for if they had been rendered entomophilous, their pollen would have been transported by the aid of the senses and appetites of insects with incomparably greater safety than by the wind. That such a conversion is possible can hardly be doubted, from the remarks lately made on the existence of intermediate forms; and apparently it has been effected in the group of willows, as we may infer from the nature of their nearest allies. (10/55. Hermann Muller ‘Die Befruchtung’ etc. page 149.)

It seems at first sight a still more surprising fact that plants, after having been once rendered entomophilous, should ever again have become anemophilous; but this has occasionally though rarely occurred, for instance, with the common Poterium sanguisorba, as may be inferred from its belonging to the Rosaceae. Such cases are, however, intelligible, as almost all plants require to be occasionally intercrossed; and if any entomiphilous species ceased to be visited by insects, it would probably perish unless it were rendered anemophilous. A plant would be neglected by insects if nectar failed to be secreted, unless indeed a large supply of attractive pollen was present; and from what we have seen of the excretion of saccharine fluid from leaves and glands being largely governed in several cases by climatic influences, and from some few flowers which do not now secrete nectar still retaining coloured guiding-marks, the failure of the secretion cannot be considered as a very improbable event. The same result would follow to a certainty, if winged insects ceased to exist in any district, or became very rare. Now there is only a single plant in the great order of the Cruciferae, namely, Pringlea, which is anemophilous, and this plant is an inhabitant of Kerguelen Land, where there are hardly any winged insects, owing probably, as was suggested by me in the case of Madeira, to the risk which they run of being blown out to sea and destroyed. (10/56. The Reverend A.E. Eaton in ‘Proceedings of the Royal Society’ volume 23 1875 page 351.)

A remarkable fact with respect to anemophilous plants is that they are often diclinous, that is, they are either monoecious with their sexes separated on the same plant, or dioecious with their sexes on distinct plants. In the class Monoecia of Linnaeus, Delpino shows that the species of twenty-eight genera are anemophilous, and of seventeen genera entomophilous. (10/57. ‘Studi sopra un Lignaggio anemofilo delle Compositae’ 1871.) The larger proportion of entomophilous genera in this latter class is probably the indirect result of insects having the power of carrying pollen to another and sometimes distant plant much more securely than the wind. In the above two classes taken together there are thirty-eight anemophilous and thirty-six entomophilous genera; whereas in the great mass of hermaphrodite plants the proportion of anemophilous to entomophilous genera is extremely small. The cause of this remarkable difference may be attributed to anemophilous plants having retained in a greater degree than the entomophilous a primordial condition, in which the sexes were separated and their mutual fertilisation effected by means of the wind. That the earliest and lowest members of the vegetable kingdom had their sexes separated, as is still the case to a large extent, is the opinion of a high authority, Nageli. (10/58. ‘Entstehung und Begriff der Naturhist. Art’ 1865 page 22.) It is indeed difficult to avoid this conclusion, if we admit the view, which seems highly probable, that the conjugation of the Algae and of some of the simplest animals is the first step towards sexual reproduction; and if we further bear in mind that a greater and greater degree of differentiation between the cells which conjugate can be traced, thus leading apparently to the development of the two sexual forms. (10/59. See the interesting discussion on this whole subject by O. Butschli in his ‘Studien uber die ersten Entwickelungsvorgange der Eizelle; etc. 1876 pages 207-219. Also Engelmann “Ueber Entwickelung von Infusorien” ‘Morphol. Jahrbuch’ B. 1 page 573. Also Dr. A. Dodel “Die Kraushaar-Algae” ‘Pringsheims Jahrbuch f. Wiss. Bot.’ B. 10.) We have also seen that as plants became more highly developed and affixed to the ground, they would be compelled to be anemophilous in order to intercross. Therefore all plants which have not since been greatly modified, would tend still to be both diclinous and anemophilous; and we can thus understand the connection between these two states, although they appear at first sight quite disconnected. If this view is correct, plants must have been rendered hermaphrodites at a later though still very early period, and entomophilous at a yet later period, namely, after the development of winged insects. So that the relationship between hermaphroditism and fertilisation by means of insects is likewise to a certain extent intelligible.

Why the descendants of plants which were originally dioecious, and which therefore profited by always intercrossing with another individual, should have been converted into hermaphrodites, may perhaps be explained by the risk which they ran, especially as long as they were anemophilous, of not being always fertilised, and consequently of not leaving offspring. This latter evil, the greatest of all to any organism, would have been much lessened by their becoming hermaphrodites, though with the contingent disadvantage of frequent self-fertilisation. By what graduated steps an hermaphrodite condition was acquired we do not know. But we can see that if a lowly organised form, in which the two sexes were represented by somewhat different individuals, were to increase by budding either before or after conjugation, the two incipient sexes would be capable of appearing by buds on the same stock, as occasionally occurs with various characters at the present day. The organism would then be in a monoecious condition, and this is probably the first step towards hermaphroditism; for if very simple male and female flowers on the same stock, each consisting of a single stamen or pistil, were brought close together and surrounded by a common envelope, in nearly the same manner as with the florets of the Compositae, we should have an hermaphrodite flower.

There seems to be no limit to the changes which organisms undergo under changing conditions of life; and some hermaphrodite plants, descended as we must believe from aboriginally diclinous plants, have had their sexes again separated. That this has occurred, we may infer from the presence of rudimentary stamens in the flowers of some individuals, and of rudimentary pistils in the flowers of other individuals, for example in Lychnis dioica. But a conversion of this kind will not have occurred unless cross-fertilisation was already assured, generally by the agency of insects; but why the production of male and female flowers on distinct plants should have been advantageous to the species, cross-fertilisation having been previously assured, is far from obvious. A plant might indeed produce twice as many seeds as were necessary to keep up its numbers under new or changed conditions of life; and if it did not vary by bearing fewer flowers, and did vary in the state of its reproductive organs (as often occurs under cultivation), a wasteful expenditure of seeds and pollen would be saved by the flowers becoming diclinous.

A related point is worth notice. I remarked in my Origin of Species that in Britain a much larger proportion of trees and bushes than of herbaceous plants have their sexes separated; and so it is, according to Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in the ‘London Catalogue of British Plants’ that there are thirty-two indigenous trees and bushes in Great Britain, classed under nine families; but to err on the safe side, I have counted only six species of willows. Of the thirty-two trees and bushes, nineteen, or more than half, have their sexes separated; and this is an enormous proportion compared with other British plants. New Zealand abounds with diclinous plants and trees; and Dr. Hooker calculates that out of about 756 phanerogamic plants inhabiting the islands, no less than 108 are trees, belonging to thirty-five families. Of these 108 trees, fifty-two, or very nearly half, have their sexes more or less separated. Of bushes there are 149, of which sixty-one have their sexes in the same state; whilst of the remaining 500 herbaceous plants only 121, or less than a fourth, have their sexes separated. Lastly, Professor Asa Gray informs me that in the United States there are 132 native trees (belonging to twenty-five families) of which ninety-five (belonging to seventeen families) “have their sexes more or less separated, for the greater part decidedly separated.”) It is, however, doubtful how far this rule holds good generally, and it certainly does not do so in Australia. But I have been assured that the flowers of the prevailing Australian trees, namely, the Myrtaceae, swarm with insects, and if they are dichogamous they would be practically diclinous. (10/61. With respect to the Proteaceae of Australia, Mr. Bentham ‘Journal of the Linnean Society Botany’ volume 13 1871 pages 58, 64, remarks on the various contrivances by which the stigma in the several genera is screened from the action of the pollen from the same flower. For instance, in Synaphea “the stigma is held by the eunuch (i.e., one of the stamens which is barren) safe from all pollution from her brother anthers, and is preserved intact for any pollen that may be inserted by insects and other agencies.”) As far as anemophilous plants are concerned, we know that they are apt to have their sexes separated, and we can see that it would be an unfavourable circumstance for them to bear their flowers very close to the ground, as their pollen is liable to be blown high up in the air (10/62. Kerner ‘Schutzmittel des Pollens’ 1873 page 4.); but as the culms of grasses give sufficient elevation, we cannot thus account for so many trees and bushes being diclinous. We may infer from our previous discussion that a tree bearing numerous hermaphrodite flowers would rarely intercross with another tree, except by means of the pollen of a distinct individual being prepotent over the plants’ own pollen. Now the separation of the sexes, whether the plant were anemophilous are entomophilous, would most effectually bar self-fertilisation, and this may be the cause of so many trees and bushes being diclinous. Or to put the case in another way, a plant would be better fitted for development into a tree, if the sexes were separated, than if it were hermaphrodite; for in the former case its numerous flowers would be less liable to continued self-fertilisation. But it should also be observed that the long life of a tree or bush permits of the separation of the sexes, with much less risk of evil from impregnation occasionally failing and seeds not being produced, than in the case of short-lived plants. Hence it probably is, as Lecoq has remarked, that annual plants are rarely dioecious.

Finally, we have seen reason to believe that the higher plants are descended from extremely low forms which conjugated, and that the conjugating individuals differed somewhat from one another,--the one representing the male and the other the female--so that plants were aboriginally dioecious. At a very early period such lowly organised dioecious plants probably gave rise by budding to monoecious plants with the two sexes borne by the same individual; and by a still closer union of the sexes to hermaphrodite plants, which are now much the commonest form. (10/63. There is a considerable amount of evidence that all the higher animals are the descendants of hermaphrodites; and it is a curious problem whether such hermaphroditism may not have been the result of the conjugation of two slightly different individuals, which represented the two incipient sexes. On this view, the higher animals may now owe their bilateral structure, with all their organs double at an early embryonic period, to the fusion or conjugation of two primordial individuals.) As soon as plants became affixed to the ground, their pollen must have been carried by some means from flower to flower, at first almost certainly by the wind, then by pollen-devouring, and afterwards by nectar-seeking insects. During subsequent ages some few entomophilous plants have been again rendered anemophilous, and some hermaphrodite plants have had their sexes again separated; and we can vaguely see the advantages of such recurrent changes under certain conditions.

Dioecious plants, however fertilised, have a great advantage over other plants in their cross-fertilisation being assured. But this advantage is gained in the case of anemophilous species at the expense of the production of an enormous superfluity of pollen, with some risk to them and to entomophilous species of their fertilisation occasionally failing. Half the individuals, moreover, namely, the males, produce no seed, and this might possibly be a disadvantage. Delpino remarks that dioecious plants cannot spread so easily as monoecious and hermaphrodite species, for a single individual which happened to reach some new site could not propagate its kind; but it may be doubted whether this is a serious evil. Monoecious plants can hardly fail to be to a large extent dioecious in function, owing to the lightness of their pollen and to the wind blowing laterally, with the great additional advantage of occasionally or often producing some self-fertilised seeds. When they are also dichogamous, they are necessarily dioecious in function. Lastly, hermaphrodite plants can generally produce at least some self-fertilised seeds, and they are at the same time capable, through the various means specified in this chapter, of cross-fertilisation. When their structure absolutely prevents self-fertilisation, they are in the same relative position to one another as monoecious and dioecious plants, with what may be an advantage, namely, that every flower is capable of yielding seeds.