Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/444

Rh 426 REPRODUCTION [VEGETABLE. The lowest Fungi in which a sexual process has been observed are the Chytridiaceae (Polyphagua Englcnte), the Mucorini, and the Entomophthorene. In these, when it occurs, it takes the form of conjugation, with the production of zygospores. Conjugation is here effected in essentially the same manner as that described above for the conjugation among the Algae, by the fusion of two similar sexual organs. Conjugation of planogametes has been observed by Sorokin in Tetrachytrium and in Haplocystis, pro- bably belonging to the Chytridiacere, a fact of some interest as it is the only case of this form of the sexual process known in the Fungi In Protomyces and the Ustilaginece a process takes place which appears to be of a sexual nature, resembling the conjugation of the Mucorini. Certain reproductive cells of an elongated form, termed sporidia, are produced, which become connected by a transverse canal so that they then resemble the letter H. No zygospore is formed, but the conjugated sporidia are its equivalent. The question of the sexual nature of this process is still under discus- sion, but it is made highly probable by the fact that, in all fully investigated cases, the sporidia are incapable of independent germination. In the allied groups of the Peronosporese and Saprolegnieae two kinds of sexual organs are present, male and female, which corre- spond to the antheridia and oogonia of the Algae. The female organ is here also termed an oogonium, and, like that of the Alga, it may produce one oosphere (Peronosporece) or many (most Saprolegniese). The male organ is also usually termed an anthe- ridium, but it is sometimes spoken of as a pollinodium on account of the mode in which it effects the fertilization of the oosphere. In one of the plants allied to these groups namely, in Monoble- pharis sphserica, according to Cornu the protoplasm of the antheridium becomes differentiated into motile antherozoids ; this is the only case of the kind known among the Fungi. In the other members of these groups in which fertilization takes place the antheridium lies in contact with the oogonium, and produces at its apex a delicate tubular outgrowth, which bores its way through the wall of the oogonium and comes into contact with-an oosphere ; the tube then opens, and protoplasm from the antheridium passes through into the oosphere and fuses with it to form an oospore. Sexual In the Ascomycetes sexual organs are very commonly present, organs of but it is only in a few cases that a sexual process has been observed Ascomy- actually to take place. The sexual organs differ considerably in cetes. their form in the different genera. In some (e.g., Gymnoascus, Eremascus, Eurotium, Penicillium) the sexual organs are similar ; they are unicellular or multicellular hyphae, but in some the female organ, termed in the Ascomycetes the ascogonium or carpo- gonium, may be distinguished from the male organ, which is a pollinodial antheridium like that of the Peronosporese, in that it is wound into a close spiral. In others (e.g., Erysipheae, Ascobolus, Pyronema [Peziza] conftuens) the sexual organs are readily dis- tinguishable. In the Erysipheae and in Pyronema the ascogonium is a single relatively large ovoid cell ; that of Pyronema produces a delicate tubular outgrowth, the trichogyne ; the antheridium also is unicellular, but it is more slender. In Ascobolus the ascogonium consists of a row of five or six relatively large cells ; the antheridium is a slender multicellular hypha. In all these cases the sexual organs are developed in such close proximity to each other that they come into contact. In other Ascomycetes l>elonging to the Discomycetous Lichens (Colleina, Synechoblastus, Leptogium, Physma), and to the Pyrenomycetes (Polystigma), the antheridial filaments, termed sterigmata, are developed at a dis- tance from the female organ in separate receptacles, spermogonia. In this case the formation of male cells is a necessity. Accordingly cells, termed spermatia, are produced from the sterigmata by abstriction, which resemble the male cells of the Floridese in that they are non-motile and have a cell-wall. With regard to the process of fertilization, it must be premised that in no Ascomycete is the protoplasm of the ascogonium ever differentiated into an oosphere. When the sexual organs are ascogonia and pollinodial antheridia, fertilization takes place by the fusion of the undifferentiated protoplasmic contents of the two organs, a mode of fertilization which recalls the conjugation in the Mucorini, and in the Conjugate among the Algae. This has only been observed to take place in Eremascus and in Pyronema; in the latter the antheridium applies its apex to the trichogyne of the ascogonium, the intervening walls are absorbed, and the pro- toplasm of the two organs coalesces. Although this process of fertilization resembles the conjugation occurring in the Mucorini, the product is very different. The product of conjugation in the Mucorini is a single cell, the zygospore ; the product of fertiliza- Asco- tion in the Ascomycetes is a number of cells, termed ascospores or spores carpospores. Thus in Eremascus the product of fertilization is a the pro- unicellular capsule, the ascus, in which eight ascospores are duct of formed. In Pyronema the fertilized ascogonium enlarges and fertiliza- gives rise to a number of outgrowths which produce asci ; at the tion. same time a number of hyphaj grow up from below around the developing asci, some of which produce delicate filaments, termed paraphyscs, which lie amongst the asci, whereas others form an investing wall. The result is the formation of a fructification termed an apothcciiim. Within each ascus eight spores are formed. In those Ascomy retes which have spermatia fertilization is effected, as in the Floridere among the Algae, by the fusion of a spcrmatium with the trichogyue. The result is the saino as in Pyronema : the fertilized ascogonium gives rise to hyj>lui> which bear asci, and these, together with sterile hyphre, form a fructi- fication. It is of interest to note the similarity between the products of fertilization in these Ascomycetes and in the Floridese. In both cases the female organ produces no differentiated oosphere, and in both cases the product of its fertilization is a many-spored fructi- fication. It was pointed out, in speaking of the Floridese, that each carpospore is the equivalent of a fertilized oosphere (oospore) ; this holds good also with regard to the carpospores (ascospores) of the Ascomycetes. It may be that a similar sexual process takes place in the other forms mentioned above, viz., the Erysipheae, Penicillium, Sordaria, &c., but it has not been observed ; in any case, the ascogonium in all these plants gives rise to asci and ascospores, and a more or less complex fructification is produced. But th^ere is also some ground Seru; for believing that in some at least of these cases the sexual organs, clegei - though morphologically differentiated, are functionless. For there tion are clear indications of sexual degeneration in the Ascomycetes. Asco In some cases, for instance (e.g., Chsetomium, Melanospora), no cetes antheridium can be distinguished, but the ascogonium eventually produces asci nevertheless. In others (e.g., Xylaria, as far as known at present) no male organ is produced, but there is an ascogonium which does not, however, give rise to asci ; the asci, as well as the rest of the fructification, arise from the vegetative hyphae. In others, again (e.g., Claviceps, Cordiceps, Pleospora), all trace of the sexual organs has disappeared, but a fructification containing ascospores is produced, as in Xylaria, from the vegeta- tive hyphos. In others, finally, no ascospores are known, the only reproductive cells being the characteristic asexually produced stylospores. In the remaining groups of Fungi, the Uredinese and the Basidio- Fun a! mycetes, no sexual reproduction is known. In the Basidiomycetes whi. , no kind of sexual organ has been discovered. In the Uredinea? sexi ' spermatia are commonly produced, as in the Ascomycetes mentioned repi c- above, but no female organ is known ; however, fructifications tion u termed eecidia are in some cases developed. These resemble kno j somewhat those of the Ascomycetes, but differ in that here the spores (ascidiospores) are formed by abstriction, and not in asci as in the Ascomycetes. It may be suggested that the ascospores of the asexual Ascomy- cetes and the aecidiospores of the Uredineae should not be included in an account of the sexual reproduction of the Fungi. It is true that these spores are asexually produced, but their evident homo- logy with the sexually produced spores makes it inconvenient to treat of them apart. They differ from the sexually produced spores in that they are developed apogamously. Under the name Archegoniata we may conveniently group An together the Muscineae and the Vascular Cryptogams ( Plerido- goi . 1 phyta). The sexual organs, as also the process of fertilization, are essentially the same throughout. The female organ produces a single oosphere, and is termed an ctrchcgonium ; it is essentially similar to the oogonium of the Thallophytes, the only difference being that, whereas the archcgonium is multicellular, the oogonium is unicellular. The male organ, here also termed the antheridium, is likewise multicellular, and gives rise to a larger or smaller number of motile antherozoids. Fertilization is effected by the fusion of an antherozoid with the oosphere, which then clothes itself with a cell-wall and becomes an oospore. In the Phanerogams the sexual organs are essentially of the Ph > nature of archegonia and of antheridia, but they are somewhat gai modified and are called by other names. The female organ of the Gynmosperms, termed a corpusculum, closely resembles the archc- gonium of the Archegoniata, and produces a single oosphere. In the Angiosperms, the female organ is much reduced, consisting only of three cells, one of which is the oosphere, the other two being the syncrgidas which assist in the process of fertilization ; the organ is termed the egg-apparatus. The male organ in the Phanerogams is a unicellular filament termed the pollen-tube ; its protoplasm does not undergo differentiation into antherozoids. The sexual organs of the Phanerogams recall those of the Perono- sporese and the SaprolegnietE ; in both cases the female organ pro- duces an oosphere, and in neither does the protoplasm of the antheridium produce antherozoids. The process of fertilization will be described subsequently. Physiology of Reproduction. From the fact that in even the most highly organized plants an isolated portion of one member is capable of producing, not merely a member like itself, but other members also, so that a new