then come to rest and sink to the bottom, where they remain heaped in green masses. What is their future history, has not yet been ascertained; but there is evidence from analogy that they are "antheridial" cells, which have for their office to fecundate the true germ-cells by contained antherozoids. No development of "resting-spores," however, has yet been observed. The rapidity of the growth of this curious plant, is not one of the least remarkable parts of its history. The individual cells of which the net is composed, at the time of their emersion as gonidia, measure no more than 1-2500th of an inch in length; but in the course of a few weeks, they grow to a length of from 1-12th to 1-3d of an inch.

198. Almost every pond and ditch contains some members of the family Confervacea; but they are especially abundant in moving water; and they constitute the greater part of those green threads, which are to be seen attached to stones, with their free ends floating in the direction of the current, in every running stream, and upon almost every part of the sea-shore, and which are commonly known under the name of "silk-weeds" or "crow-silk." Their form is usually very regular; for each thread is a long cylinder, made up by the union of a single file of short cylindrical cells united to each other by their flattened extremities; sometimes these threads give off lateral branches, which have the same structure. The endochrome, though usually green, is occasionally of a brown or purple hue; it is sometimes distributed uniformly throughout the cell (as in Fig. 107), whilst in other instances it is arranged in a pattern of some kind, as a network or a spiral; but this may be only a transitional stage in its development. The plants of this order are extremely favorable subjects for the study of the method of cellmultiplication by binary subdivision. This process usually takes place only in the terminal cell; and it may be almost always observed there, in some one of its stages. The first step is seen to be the subdivision of the endochrome, and the inflection of the primordial utricle around it (Fig. 107, A, a); and thus there is gradually formed a sort of hour-glass contraction across the cavity of the parent cell, by which it is divided into two equal halves (B). The two surfaces of the infolded utricle produce a double layer of cellulose membrane between them; this is not confined, however, to the contiguous surfaces of the young cell, but takes place over the whole exterior of the primordial utricle, so that the new septum is continuous with new layers that are formed throughout the interior of the cellulose wall of the original cell (c). Sometimes, however, as in Conferva glomerata (a common species), new cells may originate as branches from any part of the surface, by a process of budding; which, notwithstanding its difference of mode, agrees with that just described in its essential character, being the result of the subdivision of the original cell. A certain portion of the primordial utricle seems to undergo

increased nutrition, for it is seen to project, carrying the cellulose envelope before it, so as to form a little protuberance; and this sometimes attains a considerable length, before any separation of its cavity from that of the cell which gave origin to it, begins

b

FIG. 107.

B

to take place. This separation is gradually effected, however, by the infolding of the primordial utricle, just as in the preceding case; and thus the endochrome of the branch cell is completely severed from that of the stock. The branch then begins to elongate itself by the subdivision of its firstformed cell; and this process may be repeated for a time in all the cells of the filament, though it usually comes to be restricted at last to the terminal cell. The Confervaceae multiply themselves by "zoospores," which are produced within their cells, and are then set free, just as in the Ulvacea (§ 195); in most of the genera, the endochrome of each cell divides into numerous zoospores, which are of course very minute; but in Edogonium,-a fresh-water genus distinguished by the circular markings which form rings round the extremities of many of the cells, and by many interesting peculiarities of growth and reproduction,'-only a beneath the mucous investment, single large zoospore is set free from each cell; and its liberation is accomplished by the almost complete fission of the wall of the cell through one of these rings, a small part only remaining uncleft, which serves as a kind of hinge, whereby the two parts of the filament are prevented from being altogether separated. Sometimes the zoospore does not completely extricate itself from the parent cell; and it may begin to grow in this situation, the root-like processes which it puts forth being extended into the cavity. A true sexual generation has been observed in several Confervaceæ; and is probably universal throughout the group. Thus in Sphæroplea annulina, according to the recent observations of Dr. Cohn, the ring-like masses of endochrome, of which several are found in each cell, resolve themselves in certain of the cells into minute bodies resembling the antherozoids of Chara (Fig. 112, u), which, after moving freely within these cells, escape through apertures in their walls, and then penetrate

Process of cell-multiplication in Conferva glomerata :-A, portion of filament with incomplete separation at a, and complete partition at b; B, the separation completed, and new cellulose partition being formed at a; c, formation of additional layers of cellulose wall, c,

d, and around the primordial utricle, a, which encloses the endochrome, b.

See the account of these processes in the "Micrographic Dictionary," p. 468. 2. Monatsberichte der Königl. Akad, der Wissenschaften," Mai, 1855.

apertures in the walls of certain other cells, within each of which the endochrome has coalesced into a globular mass; over this mass the antherozoids spread themselves, and seem to dissolve away upon its surface; and after this process has taken place, the mass of endochrome acquires a firm envelope, and becomes a "resting-spore," which, when set free by the rupture of the parent cell-wall, germinates into a new plant. A curious variation of this process is seen in Edogonium; for instead of the antherozoids escaping freely from the "sperm-cells" which produced them, they are discharged en masse, included within a capsule which is furnished with cilia, and which so resembles a "zoospore" as to be easily mistaken for it; and it is only when this has attached itself, and has set free its contents by the falling off of a sort of lid, that the antherozoids are enabled to perform their fertilizing office. The same thing happens in some other Confervaceæ; in which, however, the antheridial capsules, being smaller than the zoospores, are distinguished as microgonidia, whilst the latter are known as macrogonidia. The offices of these different classes of reproductive bodies are only now beginning to be understood; and the inquiry is one so fraught with Physiological interest, and, from the facility of growing these plants in artificial Aquaria, may be so easily pursued, that it may be hoped that Microscopists will apply themselves to it so zealously, as not long to leave any part of it in obscurity.

199. The family Conjugated agrees with that of the Confervaceo in its mode of growth, but differs from it in the plan in which its generative process is performed; this being accomplished by an act of "conjugation," resembling that which has been described in the simplest Protophytes. These plants are not found so much in running streams, as in waters that are perfectly still, such as those in ponds, reservoirs, ditches, or marshy grounds; and they are for the most part unattached, floating freely at or near the surface, especially when buoyed up by the bubbles of gas which are liberated from the midst of them under the influence of solar light and heat. In an early stage of their growth, whilst as yet the cells are undergoing multiplication by subdivision, the endochrome is commonly diffused pretty uniformly through their cavities (Fig. 109, A); but as they advance towards the stage of conjugation, the endochrome ordinarily arranges itself in regular spirals (B), but occasionally in some other forms. The act of "conjugation" usually occurs between the cells of two distinct filaments, that happen to lie in proximity to each other; and all the cells of each filament generally take part in it at once. The adjacent cells put forth little protuberances, which come into contact with each other, and then coalesce by the breaking down of the intervening partitions, so as to establish a free passage between the cavities of the conjugating cells. In some genera of this family (such as Mesocarpus), the conjugating

cells pour their endochromes into a dilatation of the passage that has been established between them; and it is there that they commingle, so as to form the spore or the embryo-cell. But in the Zygnema (Fig. 108), which is among the commonest and best known forms of Conjugateæ, the endochrome of one cell passes over entirely into the cavity of the other; and it is within the latter that the spore is formed (c), the two endochromes coa

FIG. 108.

C

b

Various stages of the history of Zygnema quininum :-A, three cells, a, b, c, of a young filament, of which bis undergoing subdivision; B, two filaments in the first stage of conjugation, showing the spiral disposition of their endochromes, and the protuberances from the conjugating cells; c, completion of the act of conjugation, the endochromes of the cells of the filament a having entirely passed over to those of filament b, in which the sporangia are formed.

lescing into a single mass, around which a firm envelope gradually makes its appearance. Further, it may be generally observed, that all the cells of one filament thus empty themselves, whilst all the cells of the other filaments become the recipients; here, therefore, we seem to have a foreshadowing of the sexual distinction of the generative cells into "sperm-cells" and "germcells," which we have just seen to exist in the Confervaceæ (§ 198). And this transition will be still more complete, if (as Itzigsohn has affirmed) the endochrome of certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly colorless spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to the antherozoids of the truly sexual Protophytes.'

1 This group of plants seems to serve as the connecting link between those simple Protophytes in which the sexes are not yet differentiated, and those higher forms in which the distinction between the "sperm-cells" and "germ-cells" is very apparent. For let it be supposed that in Spharoplea (§ 198) a conjugation of two adjacent cells were to take place, at that stage in their development in which the endochrome is uniformly arranged in rings, no differentiation of sexes yet showing itself,-the process would in all respects correspond with that of the ordinary Conjugatea. Again, whilst in Mesocarpus, the two conjugating cells appear to take (as in the Desmideæ, § 169) a precisely similar share in the formation of their product, the first stage of differentiation

FIG. 109.

200. The Chatophoraceae constitute another beautiful and interesting little group of Confervoid plants, of which some species inhabit the sea, whilst others are found in fresh and pure water, rather in that of gently moving streams, however, than in strongly flowing currents. Generally speaking, their filaments put forth lateral branches, and extend themselves into arborescent fronds; and one of the distinctive characters of the group is afforded by the fact, that the extremities of these branches are usually prolonged into bristle-shaped processes (Fig. 109). As in many preceding cases, these plants multiply themselves by the conversion of the endochrome of certain of their cells into "zoospores;" and these, when set free, are seen to be furnished with four large cilia. "Resting-spores have also been seen in many species; and it is probable that these, as in Confervaceæ, are true generative products of the fertilization of the contents of "germ-cells" by "antherozoids" developed within "sperm-cells" (§ 198). Nearly allied to the preceding are the Batrachospermea, whose name is indicative of the strong resemblance which their beaded filaments bear to frog-spawn; these exhibit a somewhat greater complexity of structure, and afford objects of extreme beauty to the Microscopist. The plants of this family are all inhabitants of fresh water, and they are chiefly found in that which is pure and gently flowing. "They are so extremely flexible," says Dr. Hassall, "that they obey the slightest motion of the fluid which surrounds them; and nothing can surpass the ease and grace of their movements. into sperm-cells and germ-cells is manifested in Zygnema, by the passage of the whole endochrome of those of one filament into the cavities of the other, and by the formation of the spores within the latter. In Spirogyra, moreover, the endochrome of one set of cells becomes converted into antherozoids before conjugation, whilst that of the other aggregates into a sporangial mass; thus exhibiting the second stage of differentiation. Further, there are certain species which agree with the ordinary Conjugate in their general habit, and which form "resting spores" like theirs, but in which no conjugation has been observed; and it seems not improbable that in these, as in Sphæroplea, the antherozoids make their way out of the sperm-cell by minute apertures in its wall, and swim freely about before finding their way into the germ-cell through the apertures in its wall;-still, however, performing by this means the very same act, as that which is accomplished by the more direct process of conjugation,-viz., the introduction of the contents of the sperm cell into the interior of the germ-cell.