11. In that most remarkable plant the Sa prolegnia ferox, which is structurally so closely related to Vaucheria, though separated from it by the absence of green colouring matter, we find a corresponding analogy in the history of the development. Its vegetative life is, in fact, divisible into two well-marked periods, each characterised by a special mode of germ formation. During the first, the only one with which we have at present to do, swarms of zoospores which rapidly succeed each other, are formed at the closed terminations of the c)lindrical filaments. The mode of their origin, agrees with that of the aggregate zoo spore of Vaucheria. The protoplasma accuinu lates in the swollen extremity of the filament, and a septum is formed in exactly the same manner as in that plant ; while the mass of protoplasma is now observed to be limited by a distinct surface. At this point the resem blance ceases ; the protoplasmic membrane divides, just as in the spore-cases of the zo osporous Algm, into particles, which, as the period of maturity is approached, become more and more easily distinguishable from each other. These particles are the future zo ospores. Soon they detach themselves from their connection with the membrane which encloses them, and with each other, and pre sent the globular or ovoidal form characteristic of their perfect condition. In the meantime the external tube membrane buds out at its apex, so as to form a conical projection; as the zoospores become ripe, a gentle oscillatory motion is seen in the upper part of the spore case. This is accompanied with a compres sion of its contents, in consequence of which its membrane gives way at its weakest poin t,— viz. the apex of the terminal conical projec tion. In its most perfect condition, the zo ospore of Saprolegnia consists of a pyriform, protoplasmic, membraneless corpuscle, which is furnished with a pair of cilia, emanating from its apex. It is remarkable for the short dura tion of its motion, the cessation of which is immediately followed by germination.* 12. In the process of the formation of zo ospores in Saprolegnia, we have an intermediate step between that of the zoosporous Algm on the one hand, and that of a class of plants which is usually placed among the Fungi on the other.
I allude to the Fungi included in the class Cystosporem of Leveille ; on the intimate structure of this, as well as of many other allied groups, there are as yet but few re searches. VVe have, however, enough in the beautiful monograph of Cohn, on Pilobolus, to enable us to discover that it is structurally more closely allied to the Algm than to the Fungi. We shall take Pilobolus as an illus trative example.
13. Pilobolus has an ephemeral existence. The spore germinates about mid-day; the plant grows till evening, ripens during the night. In the morning the spore-case bursts, and the whole disappears, leaving scarcely a trace of its former existence.
In correspondence with the future mode of life of the plant, the spore-cell displays in its germination, a tendency to development in two opposite directions, by the formation of two sacculations, the first, cylindrical—the root; the second, ellipsoidal—the stem. Shortly afterwards the young plant is seen to consist of two cells, of which the inferior is elongated and branched at its lower extremity—root-cell; while the superior is ellipsoid, and acuminated above. The former contains a quantity of pro
toplasma, which lines, as a distinct layer, the internal surface of its wall. The first change which is observed consists in the accumulation of this protoplasma towards the apex of the cell, at which point the membrane buds out, so as to form a bead-like head. Within the cavity of this organ —the future spore-case, further accumulation takes place, until it is entirely filled with a coloured granular material; while the rest of the cell, from which it is as yet undi vided, contains only a clear fluid. The pro cess is completed by the formation of a septum just as in Vaucheria, which takes place early in the morning. This is immediately followed by the "cleaving" of the protoplasma, and its division into numerous small cells, which are the future spores. As the plant reaches the termination of its existence, the cell on which the spore-case is supported, enlarges at its upper part from the increase of its fluid con tents ; the septum is pushed upwards, and presses on the contents of the spore-case. At last in the course of the forenoon, the tension of the wall of the spore-case becomes so great that it gives way at its junction with the sup porting cell with such force, that it is thrown like a miniature bomb for several inches.* 14. The Fungi which agree in their develop ment with the species above described, are limited in number, and belong for the most part to the genera Pilobolus and Ascophora (Mucor). The formation of the spore differs entirely from the process of strangulation, which Schleiden considers as characteristic of the Fungi. On the other hand, the analogies between Pilobolus and Vaucheria are of the closest kind ; even the ephemeral periods observed in the development and ripening of the reproductive apparatus, being the same. The root-cell of Pilobolus the inferior of the three of which the whole plant is composed, is as permanent as the tubular frond of a Conferva. From it emanate tubular, unjointed root-like processes, from the upper surface of which spring out at intervals young spore cases, in every respect similar to the first formed plant. These creeping rootlets con stitute the vegetative system of the plant, which, like that of the Fungi, is perennial.
15. Reproduction by conjugatzon.—From the number of the observations which, during the last few years, have been made on the sub ject of the phenomena of conjugation, no less than from the ,vatiety of the conditions under which they have presented themselves, we are bound to assign them an important place in a systematic description of the repro ductive process. Decaisne included in his group Synsporere all the Algre in which the phenomena in question were then known to present themselves — namely, the genus Zygnema and its allies, along with Closterium, which last, for the same reason, he separated from the Desmidem. The beautiful researches of Mr. Ralfs have taught us that all the genera of the Desmidere conjugate in the same manner as Closterium. More recently analo gous phenomena have been observed in the Vaucheriacem, and in that remarkable plant Saprolegnia ferox, which so closely resem bles Vaucheria in every respect, except its green colour. We shall describe in succes sion each of the examples which have been mentioned.