Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/448

Rh 430 REPRODUCTION [VEGETABLE. upon which are developed, as lateral buds, moss-plants consisting of stem and leaves and bearing the sexual reproductive organs. Similarly, when the oospore has been formed by fertilization in the archegonium, it does not give rise to the sexual moss-plant, but to the asexual sporogonium. There is thus in the life-history of a Moss a regular alternation of a sexual with an asexual genera- tion ; the former may be conveniently termed the oophore, the latter the sporophore ; the asexually produced spore always gives rise to the oophore (moss-plant), the sexually produced spore (oospore) of the moss-plant always gives rise to the sporophore (sporogonium). This kind of life-history is not peculiar to the Muscinese, but it can be traced, more or less clearly, in all the vascular plants (Pteridophyta, Phanerogams). In the Isosporous Vascular plants, those, namely, which produced spores of one kind only (Filices, Equisetaceae, Lycopo- diacese), the asexually produced spore gives rise, on germination, to a small inconspicuous organism consisting entirely of cellular tissue, termed the prothallium, on which the sexual reproductive organs, the antheridia and archegonia, are borne ; the oospore produced by fertiliza- tion in the archegonium gives rise to the well-developed plant, consisting of stem, root, and leaves, which produces the sporangia and spores. The prothallium, derived from the asexually produced spore, is clearly the sexual genera- tion or oophore ; the fully-developed plant derived from the sexually produced spore is the asexual generation or sporophore. In the Heterosporous Vascular plants, those, namely, which produce spores of two kinds (Rhizocarpae, Ligulatae, Phanerogams), the spores likewise give rise to prothallia, though they may be rudimentary. The micro- spore gives rise to a prothallium which is reduced to a single antheridium, and which, with the exception of Salvinia among the Rhizocarps, and of the Phanerogams, does not project from the spore ; in Salvinia and in the Phanerogams it projects from the spore in the form of a closed tube, which is known in the Phanerogams as the pollen-tube. Similarly the macrospore of these plants gives rise to a small cellular prothallium bearing one or more archegonia, which in the Rhizocarps extends beyond the limits of the spore, but does not become free from it ; in the Ligulatae (Selaginella and Isoetes) the prothallium is only partially exposed by the rupture of the coats of the spore, and in the Phanerogams, where it is termed the endosperm, it remains permanently and completely enclosed within the spore (embryo-sac). In the Heterosporous Vascular plants, then, as in the Isosporous, the asexual generation or sporophore is that which is termed the plant, which is highly differentiated morphologically and histolo- gically, and which produces the sporangia and the spores ; the sexual generation or oophore is here represented by two prothallia, the one exclusively male, the other exclu- sively female, derived respectively from the microspores and the macrospores. Such a life-history can be stated generally in the follow- ing manner : twice in its course the individual consists of a single cell, the spore, which in the one case has been produced asexually, in the other sexually; the sexual generation (oophore) springs from the asexually produced spore, and gives rise to the sexually produced spore (oospore) from which the asexual generation (sporophore) is developed. It is of interest to note that in most cases the organism developed from the sexually produced spore is much more highly organized than that developed from the asexually produced spore. orraa- A peculiarity of the macrospores of the Phanerogams is worthy on of of mention here, as it leads to the production of that structure, the sed. seed, which is characteristic of Phanerogams; the production of a seed constitutes, in fact, the only real and constant distinction between Phanerogams and Cryptogams. As a rule the asexually produced spores of plants become free from the sporangium in which they have been formed. In Phanerogams this is true only of the microspores (pollen-grains), the macrospores (embryo-sacs) remaining permanently enclosed in the sporangia (mucellus) producing them. This being the case, their germination, i.e., the formation of endosperm, must take place within them whilst enclosed in the sporangium ; and, further, tin- formation of the female reproductive organs and the development of the embryo must take place under these circumstances also. The result is the production of a seed. In a typical seed three generations of the plant are represented ; they are as follows : sporangial tissue belonging to the parent sporophore = perisperm ; tissue belonging to the oophore = endosperm ; the new sporophore = embryo. In the seed the development of the new asexual plant proceeds to a certain limit When this is reached the development ceases, and it is only when the seed is placed under favourable conditions that the further development of the embryo, i.e., the germination of the seed, can take place. It has now been shown that in the life-history of the Alterm Muscineae and in plants above them in the vegetable tio n of kingdom there is a regular alternation ,of generations, and ge the question now arises as to how far this is true of the TJ^I" life-history of plants lower than the Mosses, that is, in the phytes Thallophytes. It is clear that no such alternation can take place in the life-history of those which are known to reproduce only either sexually or asexually, nor in that of those individuals which produce spores both sexually and asexually, either simultaneously or at different times. The following are instances of the above-mentioned possible cases : TJudlophytes reproduced only by asexually produced sjwres : Algx: Cyanophycete or Phycochromacere ; Protococcaceae ; Sphacelarioe and Laminarieae so far as known at present Fungi : Schizomycetes ; Saccharomycetes ; Myxomycetes ; some Chytridieae ; probably many Mucoriui ; a few Peronosporeae (probably Phytophthora infestans and Pythium intermedium) ; some Ascomycetes and Ure- dinese ; Basidiomycetes. Thallophytes reproduced only by sexually produced spores : Algse, : Conjugate ; Fucaceae ; Sphceropleae. The case of the Characeae will be subsequently discussed. Fungi : a few Peronosporeae (Pythium vexans, Artotrogus) ; Ancylistes Closterii ; Aplanes Braunii among the Sapro- legnieae ; some Ascomycetes, Eremascus, Sorclaria (Hypocopra), Ascobolusfurfuraceus, Pyroncma conflucns, Gymnoascus, Collemaceae and other Lichen- Fungi ; some Uredineae in which only aecidiospores are known. Thallophytes in which the same form produces spores both asexually and sexually : Algse : Vaucheria, Hydrodictyon, Ulothrix, (Edogonium, some Floridese (e.g., Polysiphonia variegata). Fungi : Mucorini ; most Peronosporeae and Saprolegniese ; Monoblepharis ; among the Ascomycetes, the Erysi- pheae, Eurotium, Penicillium, Nectria ; some Uredinese (Uromyces appendiculatus, U. Bchenis, U. Scrophulariie, U. Oestri, Pucdnia Berberidis). In some of the Algae, as in the Volvocineae, in the life- in Al history of which distinct sexual and asexual forms occur, no alternation of generations can be traced, since there is no certainty as to the nature of the form arising from any given spore ; the form developed from the asexually pro- duced spore is not, as in the typical life-history of the Mess, necessarily sexual, nor is the individual produced from the sexually produced spore necessarily asexual. But in the life-history of some others an alternation of genera- tions is traceable. Thus in the Siphonaceous Acetabularia the plant produces spores; these, as mentioned above, behave on germination as gametangia ; the gametes con- jugate to form a zygospore, and from the zygospore the asexual Acetabularia springs. Here there is a distinct and regular alternation of generations ; the Acetabularia- plant is the asexual generation (sporophore), the gamet- angia alone representing the sexual generation (oophore). The life-history of Botrydium, another Siphonaceous Alga, is essentially the same as that of Acetabularia, but it is frequently less regular ; thus the resting-spores, instead of producing gametes, may directly germinate to form a