Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/711

] others in which chlorophyll is normally developed, are entirely destitute of it. These considerations have re cently led Cohn and Sachs to treat Fungi and Algce as an assemblage of organisms the classification of which is to be attempted on purely morphological grounds. But the assemblage of plants formed by Algce and Fungi which thus appear to require classification anew, has been further increased by the addition of two other groups, both re garded quite recently as entirely distinct. In 1868 Schwen- dener proposed his now well-known theory as to the true nature of Lichens ; and although his views have been vigorously attacked, chiefly by writers who seemed to feel that they had a vested interest in their autonomy, the weight of testimony, in the case of those who have examined the matter in a wider spirit, has been to strongly confirm Schwendener s hypothesis. Lichens must now be regarded as composite structures, partly consisting of an alga, partly of a fungus. Quite lately Sachs has pointed out (in the 4th edition of his Lehrbuch der Botanik) that Characece may be compared with the structure and mode of reproduction of some of the Floridece. Sachs has proposed a classification of the Thallophyta, which appears to be the best that our present knowledge admits of. He divides them into four classes Protophyta, Zygosporece, Oosporece, and Carposporece.

1. The Protophyta include the simplest plants, and those in which at present gamogenesis is not known to occur. In the Cyanophyccoe the protoplasm of the cells is destitute of a nucleus, and, besides containing chlorophyll, is tinged with a peculiar bluish colouring matter, known by the name of phycocyan. In Palmellacece this peculiar pigment is absent. Eugleneoe is a group of well-known but little understood organisms, which must also be placed here provisionally. To these must be added Schizomycetes (Bacteria), which are the agents of putrefactive changes in nitrogenous organic matters, and Saccharomyces (yeast), which bring about the phenomena known as &quot; fermentation.&quot; The Schizomycetes appear to bo allied in some respects to Chroococcacece and Oscillatoriaceos amongst the Cyanophycecz. The true posi tion of Saccharomyces must for the present be held as pro blematical ; we are still without evidence to conclusively decide in favour either of its autonomy or of its being a peculiar condition of a member of some group of Fungi of more complex development.

2. The Zygosporece are an assemblage of organisms, none of the members of which attain any high degree of morphological complexity, and in all the subordinate divisions of which the simplest form of gamogenesis, known as conjugation, has been observed to take place. Conjugation only differs from the normal process of fer tilization in the two protoplasmic bodies which take part in it being precisely similar in bulk and form. Till Thwaites pointed out the contrary in 1848, it was not supposed to D3 entitled to recognition as a sexual process. But fer tilization, as ordinarily understood, only differs in the two conjugating bodies being unlike that is, in their having undergone differentiation into antherozoid and Oosphere, the male and female bodies respectively. The Zygosporece may be divided, perhaps artificially, into two groups, according as the conjugating cells are motile or non-motile. In many of the simpler green Algce it has long been known that two kinds of zoospores, differing in size, are produced. To these the terms macrozoospore and micro- zoospore may be applied. The function of the macrozoo- spores is purely agamogenetic. The microzoospores, on the other hand, meet in pairs, and fuse into a single pro toplasmic body, which Areschoug has termed a zygozoo- spore. The process was first observed by Pringsheim in Pan- dorina. It has also been observed in Chlamydomonas. On these grounds Sachs has placed the whole group of the Volvocinacece amongst the Zygosporece. This has, however, been reasonably objected to by Cohn, inasmuch as in Eu- dorina as described by Carter, and in Volvox as observed by himself, there is a true process of fertilization by means of antherozoids, and not a simple conjugation. It may be allowed, therefore, provisionally to break up the Volvo cinacece, or rather to restrict the name to the two last-named genera, which may then be removed to the next class, using that of Pandorinece for Yolvocinaceous forms, which only exhibit conjugation. Hydrodictyece should probably be placed here, and also Confervacece and Ulvacece in which the conjugation of zoospores has been observed by Ares choug. Sachs has proposed to associate with the Zygosporece the Myxomycetes, in which the formation of the plasmodium is a kind of complex conjugation. Closely allied in some respects to the Myxomycetes are Chytridinece and, more doubtfully, Protomyces. In the second division, in which the conjugating cells are non-motile, must be placed all the more familiar instances of the process the Desmidiece, the Diatomacece, the Zygne- macece, and the Zygomycetes as a fungoid type.

3. The Oosporeæ must include the Volvocinacece, in the limited sense already explained, and also Sph&roplea a form divorced on much the same grounds from Confervacece. Fucus affords the best known type of reproduction belong ing to this group. The antherozoids are ciliate bodies dis charged from antheridia ; the oospheres are naked proto plasmic masses originally contained in oogonia. The antherozoids gather round them in such numbers as to impart to them a movement of rotation. They eventually completely blend with the oosphere, which becomes sur rounded with a coat, and sinks to the bottom of the fluid in which it has hitherto been suspended. It is now an oospore, and speedily undergoes a process of cell division, which gives rise to an individual of the new generation. Near the Fucacece a place must probably be assigned to the Phceosporece. The remaining groups which fall into Oosporece are Cceloblastce and (Edogcmiece. Cctloblastce have their protoplasm unsegmented throughout the vegeta tive portions of the organism. They include forms which are partly algoid, such as Vaucheria and the Siphophycece generally, partly fungoid, as Peronosporece and Saprolegniece. In all other respects, except the presence or absence of chlorophyll, they closely agree with one another, and the consideration of this fact has led, perhaps more than any other fact, to the breaking down of the barrier between Algse and Fungi.

4. The Carposporeæ agree with the Oosporece in so far that the two sexual organs contribute in very different proportions to the formation of the sexual product. While the male only stimulates its development, the female supplies the material for the whole subsequent growth. The female organ, or carpogonium, may consist of one or more cells. The male organ varies very considerably in the different subordinate groups. Fertilization may, as in the Oosporece, be effected by antherozoids (which may be actively motile or passively locomotive), or by a kind of conjugation, or even by a mere apposition of the male organ and subse quent diffusion of the fertilizing medium. The product of the act of fertilization is sometimes a single cell developing directly into a new individual (Chara). In other cases the fertilized female organ produces zoospores, and still more commonly a multicellular mass in which spores are finally developed. This involves an alternation of the generations of the type of that met with in the sporocarp of Muscinece. 