2. In the above-described plants we have examples of the occurrence of zoosporous reproduction under the most simple condi tions. In the history of their development we have an epitome of that of all the more simple Algm. In the family of Protococci the type may be said to be included to which all the green and olive-coloured Algm, with the exception, on the one hand, of the Des mideoe and their allies, on the other of the Fucacere, may be referred. Among the Des midem, indeed, is placed one genus, that of Pediastrum, in which the zoosporous is the only form of reproduction which has been observed. Pediastrum consists of a disc of cells, which are usually eight in number, and contain a protoplasma, which possesses a generally diffused green colour. The first step in the reproductive process consists in the separation of the protoplasma into a green and a colourless portion. The former, after collecting into a central mass, becomes divided into numerous secondary masses, the number of which is always a power of two. From the latter is formed a transparent gelatine-like in vestment which lines the parent cell. After the completion of these changes the original cell wall is ruptured, and the whole contents escape in a mass. No sooner has this taken place than the corpuscles into which the green protoplasma has divided, commence an active motion in the interior of the gelatinous cell in which they are included, and in fact display in every respect the form and peculiarities of zoospores. They are not, however, as in every' other example with which we are ac quainted, destined to display their activity beyond the narrow limits within which they originate. In a short time their motions be come languid, and finally cease, while they arrange themselves in a beautifully regular geometrical order which corresponds exactly to that of the cells that constitute the adult Pediastrum. The next change observed is the disappearance of the gelatinous membrane, and the investment of each of the zoospores with a distinct covering of its own. From this there results a disc-like body, which, in a short time, assumes all the characteristics of the original parent.* 3. Taking these simplest of unicellular plants as our point of departure, we pass to the consideration of the confervoid Algw, many of which, though they are but little elevated above the Protococci as regards their struc tural elements, present a general appearance which at first sight recalls that of plants very much higher in the scale of organisation. Thus in Bryopsis and its allies, in which the tubular frond branches in the most compli tated manner, the whole consists essentially but of a single cell, the cavity of which is continuous throughout. When the formation of zoospores is about to take place, all that is observed in a Bryopsis is the accumulation of the green granular protoplasma towards the extremities of the tubular branches. In these situations the cavity of the tube becomes cornpletely filled, while at some point in the neighbourhood of each accumulation, the tube membrane becomes sacculated so as to present a nipple-shaped projection. In the meantime the accumulated protoplasrna is observed to have given rise, by. its division, to numerous green bodies, the forms of which cannot y et be distinguished, owing to the closeness with which they are packed together. No sooner, however, is this process complete, than a re markable phenomenon, corresponding to that already described in Protococcus, manifests itself. The crowded zoospores, now com pletely developed, at once commence their characteristic motions. From this results an appearance of confused agitation, to which the term " swarming " has been applied by the Germans. A minute aperture, or pore, is then found at the extremity of the nipple-like projection, apparently in consequence of the absorption of the cell-membrane at its apex. The zoospores now begin to escape, at first one by one, afterwards more rapidly, until at last a few only are left occupying the cavity of the tube.* 4. In the simplest forms of jointed confer void s, the frond consists of a series of cells super posed one upon the other, each of which is capable of producing zoospores independently of the rest. In the vegetative state, each contains only a green protoplasma. The re productive process is the same in every respect as in the Bryopsidem, the opening by which the zoospores make their exit, being situated at the upper part of the cell, imme diately below the septum, which divides it from its successor. In other cases (as in Mi crospora), the zoospores escape by a kind of dislocation of the tube, each cell dividing into two in a plane parallel to the septa.* 5. The Ulvacem, among which the frond has no longer the form of a filament, but assumes that of a membranous expansion of juxtaposed cells, still present the same phenomena to our notice. In the cells set apart for the forma tion of zoospores, the green protopIasma is increased in quantity, at the same time that it becomes accumulated towards one point of the cell-wall. As the zoospores are formed,
they are observed to converge with their apices towards this point. The phenomena attending their escape from the parent cell are similar to those which we have already noticed.
6. In some genera, which seem to be closely related in form and structure to the Bryop sidere, we observe this important difference, that the zoospores are developed in an organ specially destined to this purpose, which presents peculiarities of form distinguishing it from every other part of the branching tubular frond. Thus in the genus Derbesia distinct spore cases are to be observed, the cavity of which does not communicate with that of the frond. These organs, which are of an oval form, take their origin in the same manner as the ordinary vegetative branches of which they are modifications. A young branch which is destined to become a spore case, instead of elongating indefinitely, begins, after having arrived at a certain length, to swell out into an ovoid vesicle, in the cavity of which a rapid accumulation of protoplasma takes place. The next change which oc curs is the separation of this protoplasma from that of the rest of the plant with which it was before continuous, so as to give rise to an oval and opaque mass, which soon becomes surrounded by a distinct membrane. As the result of the division of this mass, a number of pyriform zoospores, each of which is fur nished with a crown of cilia, are set free. Many other genera have been described by Derbes and Soliert, in which the relations of the spore cases to the frond are similar to those which exist in Derbesia,"although the forms presented by the organs in question are infinitely various.
7. The researches of the authors above alluded to, along with those of M. Thuret, have shown us that in many families of the olive coloured Algm, the occurrence of zoosporous reproduction is no less general. The zo ospores, however, although they resemble in their general form those of the plants which we have been considering, differ from them not only in respect of their olive colour, but in the arrangement of their cilia. These organs, which are always two in number, are usually of unequal length, and emanate not from the beak, but from the reddish-coloured point in its neighbourhood. The longest is directed forwards, being closely applied to the colourless beak ; while the other, which seems during the motions of the spore to serve as a rudder, assumes an opposite direction. In many genera a peculiarity exists, the significa tion of which is not yet completely under stood—that, namely, of a double fructification. The ovoidal sporangia (oosporangia, Thur.), which have been frequently described as single spores, in reality contain numerous zoospores. The other form (tricno-sporangium, Thur.)con sists of a series of small cells joined together so as to form a narrow and generally short fila ment. Each of the cells contains a zoospore, which, according to the observations of Thuret, is no less capable of germinating than the one produced by the oosporangium, In the genus Cutleria there is observed, for the first time, another feature of great interest and importance ; namely, the appecrance of two kinds of organs which seem to he opposed to each other as regards their reproductive func tions. The sporangia (trichosporangia) of Cutleria, not only differ from those of other genera, in respect of their greater size, but present well-marked distinctive peculiarities of structure. The frond consists of coloured, irregularly divided on each the resulting cavities, zoospores are to be found, which, though they altogether resemble in structure those of the other olive-coloured Algoe, are about three times as large. The supposed antheridia of Cutleria communicate to the tufts of which they fortn a part, their characteristic orange colour. The organs themselves are elongated, sausage-shaped vesicles: they contain a greyish, granular matter, in which, as the organ becomes ripe, indications may be observed of a division into several concentric layers ; the more internal of these layers being distinguished from those next the surface by the greater intensity of the orange colour which they present. After side of which, tufts (soli), consisting of the reproductive organs, intermixed with hair like bodies, are scattered at irregular intervals. The sporangia, and so-called antheridia, are borne by different individuals, but their po sitions and arrangements on the frond are identical. The former consist of oblong or club-shaped bodies, which are supported by hyaline pedicles, set into their inferior extre mities. The cavity of each sporangium is divided by three transverse partitions into four cavities, each of which is again bisected by a longitudinal median septum. In each of the discharge of the contents of the antheri dium, it may be observed to consist of a transparent vesicle, which, like the analogous female organ, is divided by transverse and longitudinal septa into eight communicating cavities.