Part 7

Order 8. =Valoniaceæ.= The thallus is generally multicellular, without differentation into stem- or leaf-structures, but the cells are sometimes united together and form a leaf-like reticulate expansion (_e.g. Anadyomene_). Zoospores are known in some, and they are then formed directly in the vegetative cells. In others (_e.g. Valonia_), a mass of protoplasm, which maybe separated through the damaging of a cell, can surround itself with a cell-wall, and grow into a new plant. No other modes of reproduction are known. The most important genera are: _Valonia_, _Siphonocladus_, _Chamædoris_, _Struvea_, _Microdictyon_, _Anadyomene_. They are all salt water forms.

As already pointed out, the _Valoniaceæ_ occupy a somewhat central position among the Siphoneæ, and present points of similarity and contrast with the _Botrydiaceæ_ and the _Bryopsidaceæ_ through _Valonia_, with the _Dasycladaceæ_ through _Chamædoris_, and also with the _Cladophoraceæ_ through _Siphonocladus_, and _Struvea_.

Order 9. =Dasycladaceæ.= The thallus consists of an axile longitudinal cell, destitute of transverse walls, attached at the base by root-like organs of attachment, and producing acropetally whorls of united, single or branched, leaf-like structures with limited growth. Asexual reproduction is wanting. Sexual reproduction by conjugation of gametes which arise in separate, fertile leaves, either directly or from aplanospores, which develope into gametangia. The principal genera are: _Acetabularia_, _Dasycladus_, _Neomeris_, _Cymopolia_. All marine.

The curiously shaped _Acetabularia mediterranea_ grows gregariously on limestone rocks, and shells of mussels in the Mediterranean; it resembles a minute umbrella with a small stem, sometimes as much as nine centimetres in height, and a shade which may be more than one centimetre in diameter. The cell-membrane is thick, and incrusted with carbonate and oxalate of lime. Only the lower, root-like part of the thallus, which penetrates the calcareous substratum survives the winter, and may grow up into a new plant. The sterile leaves, which drop off early, are dichotomously branched and formed of cylindrical cells separated from each other by cross-walls, but they are not grown together. The shade is formed by a circle of 70–100 club-shaped rays (fertile leaves) grown together, in each ray 40–80 aplanospores are formed, which become liberated at the breaking of the shade, and later on are changed to gametangia (compare _Botrydium_) which open by a lid and allow a large number of egg-shaped gametes with two cilia to escape. Gametes from various gametangia conjugate with one another; the product of the conjugation swarms about for some time, rounds off, and then surrounds itself with a cell-wall. The zygote germinates after a period of rest and then produces a sexual plant. The aplanospores (gametangia) thus represent the sexual generation.

Class 7. =Characeæ.=

The thallus has a stem with nodes and internodes; and whorls of leaves, on which may be developed the antheridia and oogonia, are borne at the nodes. Vegetative reproduction by bulbils and accessory shoots. Zoospores are wanting. The antheridia are spherical, and contain a number of filaments in which the spirally coiled spermatozoids, each with two cilia, are formed. The oogonium is situated terminally, and is at first naked, but becomes later on surrounded by an investment, and forms after fertilisation the so-called “fruit.” The oospore, after a period of rest, germinates by producing a “proembryo,” from which the young sexual plant arises as a lateral branch. The Characeæ are distinguished by the structure of their vegetative system as well as by the spirally-coiled spermatozoids, and stand as an isolated group among the Thallophytes, of which, however, the Siphoneæ appear to be their nearest relations. They were formerly, but wrongly, placed near the Mosses. The class contains only one order, the Characeæ.

Order 1. =Characeæ.= Algæ with a peculiar odour, often incrusted with lime, and of a brittle nature. They generally grow gregariously in large masses at the bottom of fresh and brackish water, and are from a few inches to more than a foot in height. The stem has long internodes which in _Nitella_ are formed of one cylindrical cell; in _Chara_ of a similar cell, but closely surrounded by a cortical layer of smaller ones. The protoplasm in contact with the cell-wall exhibits in a well-marked degree the movement of rotation (cyclosis), carrying the chlorophyll corpuscles along with it. The internodes are separated from each other by a layer of small cells (nodal cells) from which the leaves are produced. The leaves are borne in whorls of from 5–12 which regularly alternate with one another as in the higher verticillate plants; a branch is borne in the axil of the first formed leaf of each whorl (Fig. 61 _A_, _n_).

The leaves are constructed in the same manner as the stem; they are divided into a series of joints, but have only a limited power of growth; their terminal cell, too, is not enclosed by a cortex. Leaflets are borne at their nodes. The growth of the stem is unlimited, and proceeds by means of an apical cell (Fig. 62 _s_). The apical cell divides into a segment-cell and a new apical cell. The segment-cell then divides by a transverse wall into two cells, one lying above the other; the lower one, without any further division, becomes one of the long, cylindrical, internodal cells (Fig. 62 _in_), and the upper one (Fig. 62 _n_) divides by vertical walls to form the nodal cells. The cortical cells (Fig. 62 _r_) which surround the long internodal cells of _Chara_, are derived from the divisions of the nodal cells; the cells covering the upper portion of an internodal cell being derived from the node immediately above it, and those in the lower part of the internode from the node below it.

The organs of reproduction are very conspicuous by their colour and form. They are always situated on the leaves, the plants being very frequently monœcious. The antheridia (Fig. 61 _B_, _a_) are modified leaflets or the terminal cell of a leaf; they are spherical and become red when mature. Their wall consists of 8 “shields,” _i.e._ of plate-like cells, 4 of which cover the upper half, and are triangular; the 4 round the lower half, to which the stalk of the antheridia is attached, being quadrilateral, with sides of unequal length. The shields (Fig. 61 _C_) have dentated edges, with the teeth fitting into one another, and their faces ornamented with ridges. From the centre of the internal face of each shield (_C_) a cylindrical cell, the _manubrium_, projects nearly as far as the centre of the antheridium; at the inner end of each of the manubria a spherical cell, the _capitulum_, is situated. Each capitulum bears six secondary capitula, from each of which four long coiled filaments (_C_, _D_) project into the cavity of the antheridium. These filaments are divided by transverse walls into from 100–200 discoid cells, in each of which a biciliated, coiled spermatozoid is developed (_D_, _E_) from the nucleus. The spermatozoids escape from their mother-cell and are set free by the shields separating from one other.

The female organ of reproduction (Fig. 61 _B_, 63) is a small modified shoot, whose apical cell functions as an oogonium, its protoplasm forming the oosphere, which has a colourless receptive-spot at the summit (Fig. 63 _u_). The oogonium is situated on a nodal cell, from which 5 cells grow out in a circle and coil round the oogonium, covering it with a close investment. These cells divide once or twice at the top, so that 5 or 10 small cells are cut off, which project above the oogonium and form the so-called “crown” (Fig. 63 _k_). The crown either drops off at fertilisation, or its cells separate to form a central canal for the passage of the spermatozoids. The wall of the oosphere[9] above the receptive spot becomes mucilaginous, and allows the spermatozoid to fuse with the oosphere. The oospore, on germination (Fig. 64 _sp_), becomes a small filamentous plant of limited growth (Fig. 64 _i_, _d_, _q_, _pl_)--the proembryo--and from this, as a lateral outgrowth, the sexual generation is produced.

The order is divided into two sub-orders:--

A. NITELLEÆ. The crown consists of 10 cells; cortex absent: _Nitella_, _Tolypella_.

B. CHAREÆ. The crown consists of 5 cells; cortex present: _Tolypellopsis_, _Lamprothamnus_, _Lychnothamnus_, _Chara_.

_Chara crinita_ is parthenogenetic; in large districts of Europe only female plants are found, yet oospheres are formed capable of germination.

About 40 species of fossilized _Chara_, determined by their carpogonia, are known in the geological formations from the Trias up to the present day.

Class 8. =Phæophyceæ (Olive-Brown Seaweeds).=

The Phæophyceæ are Algæ, with chromatophores in which the chlorophyll is masked by a brown colour (phycophæin). The product of assimilation is a carbohydrate (fucosan), _never true starch_. In the highest forms (_Fucaceæ_), the thallus presents differentiation into stem, leaf, and root-like structures. The asexual reproduction takes place by means of zoospores. The sexual reproduction is effected by the coalescence of motile gametes, or by oogamous fertilisation. The swarm-cells are _monosymmetric_, each moved by two cilia which are true protoplasmic structures, and generally _attached laterally_ (Fig. 65). The Phæophyceæ are almost entirely saltwater forms; a few species of _Lithoderma_ live in fresh water.

The class is divided into two families:--

1. PHÆOSPOREÆ: 1 Sub-Family, Zoogonicæ; 2 Sub-Family, Acinetæ.

2. CYCLOSPOREÆ: Fucaceæ.

Family 1. =Phæosporeæ.=

The family consists of multicellular plants, whose cells are firmly united together to form a thallus; this, in the simplest cases, may be a branched filament of cells (_Ectocarpus_), or, in the highest, may resemble a stem with leaves (_Laminariaceæ_), while all transitional forms may be found between these two. The thallus grows by intercalary divisions (_e.g. Ectocarpus_), or by an apical cell (_e.g._ _Sphacelaria_); pseudo-parenchymatous tissue may sometimes be formed by cells, which were originally distinct, becoming united together. The size of the thallus varies; in some species it is quite small--almost microscopical,--while in the largest it is many metres in length.

The vegetative cells in the lower forms are nearly uniform, but in those which are more highly developed (_Laminariaceæ_ and _Fucaceæ_), they are sometimes so highly differentiated that mechanical, assimilating, storing and conducting systems may be found; the last named systems are formed of long cells with perforated, transverse walls, which bear a strong resemblance to the sieve-tubes in the higher plants.

The colouring matter in the living cells (“phæophyl”) contains chlorophyll; but this is concealed by a brown (“phycophæin”), and a yellow (“phycoxanthin”) colouring material, and hence all these Algæ are a lighter or darker _yellow-brown_. Starch is not formed. Asexual reproduction takes place, (1) by zoospores which arise in unilocular zoosporangia, and are monosymmetric, with two cilia attached laterally at the base of the colourless anterior end (Fig. 65), the longer one being directed forwards and the shorter backwards; or (2) by aplanospores (?).

Sexual reproduction has only been discovered in a few cases, and takes place by means of gametes (oogamous fertilisation perhaps occurs in the Tilopteridæ). The gametes have the same structure as the zoospores, and arise in multilocular gametangia; these, like the zoosporangia, are outgrowths from the external surface, or arise as modifications from it. The conjugating gametes may be similar (_e.g. Ectocarpus pusillus_), or there may be a more or less pronounced difference of sex, an indication of which is found in _Ectocarpus siliculosus_ (Fig. 66). When the gametes in this species have swarmed for a time, some, which are generally larger, are seen to attach themselves by one of the cilia, which by degrees is shortened to form a kind of stalk (compare the upper gamete in Fig. 66 _II_); these are the female gametes, which now become surrounded by a number of males endeavouring to conjugate with them, but only one succeeds in effecting fertilisation. The protoplasm of the two gametes coalesces (Fig. 66 _III_), and a zygote (_e_) is formed. The male gametes which do not conjugate may germinate, but the plants derived from them are much weaker than those produced by the zygotes. Strongly pronounced sexual differences are found in the Cutleriaceæ, in which order the male and female gametes arise in separate gametangia (Fig. 67 _A_). The male gametes (Fig. 67 _D_) are much smaller than the female gamete (Fig. 67 _C_); the latter, after swarming for a short time, withdraws the cilia, and is then ready to become fertilised (Fig. 67 _B_, _E_), thus we have here a distinct transition to the oogamous fertilisation which is found in the Fucaceæ. Alternation of generations is rarely found.

1. Sub-Family. =Zoogonicæ.=

Reproduction by means of gametes and zoospores.

Order 1. =Ectocarpaceæ.= The thallus consists of single or branched filaments with intercalary growth, extending vertically from a horizontal, branched filament or a disc, but sometimes it is reduced to this basal portion only. Zoosporangia and gametangia (for fertilisation see Fig. 66) are either outgrowths or arise by the transformation of one or several of the ordinary cells. The most common genera are: _Ectocarpus_ and _Pylaiella_.

Order 2. =Choristocarpaceæ.= _Choristocarpus_, _Discosporangium_.

Order 3. =Sphacelariaceæ.= The thallus consists of small, parenchymatous, more or less ramified shoots, presenting a feather-like appearance. In the shoots, which grow by means of an apical cell (Fig. 68 _S_), a cortical layer, surrounding a row of central cells, is present. Sporangia and gametangia are outgrowths from the main stem or its branches. _Sphacelaria_, _Chætopteris_ are common forms.

Order 4. =Encoeliaceæ.= _Punctaria_, _Asperococcus_, _Phyllitis fascia_.

Order 5. =Striariaceæ.= _Striaria_, _Phlœospora_.

Order 6. =Dictyosiphonaceæ.= _Dictyosiphon._

Order 7. =Desmarestiaceæ.= _Desmarestia aculeata_ is common.

Order 8. =Myriotrichiaceæ.= _Myriotrichia._

Order 9. =Elachistaceæ.= _Elachista fucicola_ is a common epiphyte on species of _Fucus_.

Order 10. =Chordariaceæ.= The shoot-systems are often surrounded by mucilage. _Chordaria_; _Leathesia difformis_ occurs as rounded, brown-green masses of the size of a nut, generally attached to other Seaweeds.

Order 11. =Stilophoraceæ.= _Stilophora rhizodes_ is common.

Order 12. =Spermatochnaceæ.= _Spermatochnus paradoxus_ is common.

Order 13. =Sporochnaceæ.= _Sporochnus._

Order 14. =Ralfsiaceæ.= _Ralfsia verrucosa_ is common as a red-brown incrustation on stones and rocks at the water’s edge.

Order 15. =Lithodermataceæ.= Some species of the genus _Lithoderma_ occur in fresh water.

Order 16. =Laminariaceæ.= The thallus is more or less leathery, and has generally a root-like lower part (Fig. 69) which serves to attach it, and a stalk or stem-like part, terminated by a large leaf-like expansion. Meristematic cells are situated at the base of the leaf, and from these the new leaves are derived. The older leaf thus pushed away by the intercalary formation of the younger ones, soon withers (Fig. 69). Gametes are wanting. Zoosporangia are developed from the lower part of a simple, few-celled sporangiophore, which is an outgrowth from a surface-cell and has a large club-formed apical cell. The sporangia are aggregated into closely packed sori, which cover the lower part of the terminal leaf, or occur on special, smaller, lateral, fertile fronds (_Alaria_). Most of the species belonging to this order live in seas of moderate or cold temperature and occur in the most northern regions that have yet been explored, forming their organs of reproduction during the cold and darkness of the arctic night. _Laminaria_ is destitute of a midrib and has only one terminal leaf. _L. digitata_ has a broad leaf, which, by the violence of the waves, is torn into a number of palmate strips (Fig. 69). _L. saccharina_ has a small, undivided leaf. _Alaria_ has a midrib and special fertile fronds. _A. esculenta_ occurs plentifully on the west coast of Norway and on the shores of Great Britain. _Chorda filum_, a common seaweed, is thick, unbranched, and attains a length of several metres, without any strong demarcation between stalk and leaf. Some attain quite a gigantic size, _e.g. Macrocystis pyrifera_, whose thallus is said sometimes to be more than 300 metres in length. The _Lessonia_-species, like the above, form submarine forests of seaweed on the south and south-west coasts of South America, the Cape, and other localities in the Southern Hemisphere.

USES. The large Laminarias, where they occur in great numbers, are, like the Fuci, used for various purposes, for example, in the production of iodine and soda, and as an article of food (_Laminaria saccharina_, _Alaria esculenta_, etc.). _Laminaria saccharina_ contains a large quantity of sugar (mannit) and is in some districts used in the preparation of a kind of syrup; in surgical operations it is employed for the distension of apertures and passages, as for instance the ear-passage. It is by reason of the anatomical peculiarities and structure of the cell-walls, that they are employed for this purpose. The cell-walls are divided into two layers, an inner one which has very little power of swelling, and an outer one, well developed and almost gelatinous--the so-called “intercellular substance”--which shrivels up when dried, but can absorb water and swell to about five times its size. The stalks of _Laminaria clustoni_ are officinal.

Order 17. =Cutleriaceæ.= The thallus is formed by the union of the originally free, band-shaped shoots. The growth is intercalary. Sexual reproduction by the conjugation of male and female gametes. An asexual generation of different appearance, which produces zoospores, arises from the germination of the zygote. _Cutleria_, _Zanardinia_.

Sub-Family 2. =Acinetæ.=

Branched, simple cell-rows with intercalary growth. The organs of reproduction are partly uni-and partly multicellular; in the unicellular ones a cell without cilia is formed, which may be destitute of a cell-wall, but has one nucleus (oosphere?), or which has a cell-wall and contains several (generally four) nuclei (aplanospores?); in the multicellular, monosymmetric swarm-cells with two cilia (spermatozoids?) are formed. The fertilisation has not been observed.

Order 1. =Tilopteridaceæ.= _Haplospora_, _Tilopteris_.

Family 2. =Cyclosporeæ.=

The individuals are multicellular, with growth by an apical cell. The thallus--often bilateral--is differentiated into a root-like structure (attachment-disc), and stem, sometimes also into leaves (_Sargassum_). Sometimes a differentiation occurs into various tissue-systems, viz. an external assimilating tissue, a storing tissue, a mechanical tissue of thickened, longitudinal, parenchymatous, strengthening cells, and a conducting tissue of sieve-cells, or of short sieve-tubes with perforated walls. Colouring material, as in Phæosporeæ. Vegetative reproduction can only take place by means of detached portions of the thallus (_Sargassum_), which are kept floating by means of bladders (Fig. 70 _A_, _a_, Fig. 72). Zoospores are wanting.

The sexual reproduction takes place by oogamous fertilisation. The oogonia and antheridia are formed inside special organs (conceptacles), and are surrounded by paraphyses. The conceptacles (Fig. 70 _B_, Fig. 71 _b_) are small, pear-shaped or spherical depressions, produced by a special ingrowth of the surface cells of the thallus, and their mouths (_ostioles_) project like small warts; they are either situated near the end of the ordinary branches of the thallus (_Fucus serratus_, Fig. 71 _a_) which may be swollen on this account (_Fucus vesiculosus_, Fig. 70 _A_, _b_), or on special short branches (_Ascophyllum_, _Sargassum_). The vertical section of a conceptacle is seen in Fig. 70 _B_ (see also Fig. 71 _b_) where, in addition to the paraphyses, oogonia only are seen (_F. vesiculosus_ is diœcious--male plant, yellow-brown; female plant, olive-brown); but in some species antheridia, together with oogonia, are produced in the same conceptacle. The oogonia are large, almost spherical cells, situated on a short stalk, in each of which are formed from 1–8 (in _Fucus_, 8; in _Ascophyllum_, 4; in _Halidrys_, 1; in _Pelvetia_, 2) rounded, immotile oospheres. The wall of the oogonium ruptures, and the oospheres, still enclosed in the inner membrane, are ejected through the mouth of the conceptacle, and float about in the water, being finally set free by the bursting of the inner membrane. The antheridia are oblong cells (Fig. 70 _C_, _a_), many of which are produced on the same branched antheridiophore (Fig. 70 _C_); the numerous spermatozoids are provided with 2 cilia and are very small (Fig. 70 _D_, two antheridia surrounded by spermatozoids, one being open). The spermatozoids, still enclosed by the inner membrane of the antheridium, are similarly set free, and fertilisation takes place in the water, numerous spermatozoids collecting round the oosphere (Fig. 70 _E_), which is many times larger, and by their own motion causing it to rotate. After fertilisation, the oospore surrounds itself with a cell-wall and germinates immediately, attaching itself (Fig. 70 _F_) to some object, and by cell-division grows into a new plant.