Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/447

Rh VEGETABLE.] REPRODUCTION 429 F. serratus, Himanthalia lorea) some individuals bear only antheridia, and others only oogonia; among the Muscineae the plants are frequently either male or female; in the Isosporous Vascular Cryptogams the prothallia are usually hermaphrodite ; but exclusively male and female prothallia occur not unfrequently in the Filices and as a rule in the Equisetaceoe. In the Heterosporous Vascular Cryptogams dioecism is brought about in a somewhat different manner : these plants have, as mentioned above, two kinds of spores, macrospores and microspores ; the former on germination always give rise to a female (archegoniate) prothallium, the latter to a male (antheridial) prothallium; hence the male and female organs are necessarily borne on distinct organ- isms. In some Phanerogams even the two kinds of spores are produced by separate individuals, the flowers of the one producing pollen-grains (microspores) but no embryo-sacs, those of the other producing embryo-sacs (macrospores) but no pollen-grains. More special arrangements are to be found in the flowers of Phanerogams for ensuring cross-fertiliza- tion, and preventing self-fertilization, e.g., the development of highly-coloured perianth-leaves and the secretion of nectar to attract insects, dichogamy, heterostylism, &c. ; but it is impossible to do more than mention these here. But besides these obvious structural arrangements for ensuring cross-fertilization and preventing self-fertilization there are in some cases imperceptible physiological con- ditions which even more certainly lead to the same results. It appears, namely, that in such cases no sexual process can take place between reproductive cells or organs of closely-allied origin. Thus, among the Algae, in Ecto- carpus and in Acetabularia, conjugation only takes place between planogametes derived from distinct gametangia, and in Dasycladus it only takes place between piano- gametes derived from gametangia borne by different indi- viduals. Again, in many Phanerogams, as Darwin has shown, the pollen of one flower is quite incapable of fertilizing the oospheres of its own ovules, and the pollen- grains from another flower borne by the same plant is but slightly, if at all, more potent. The pollen from the flower of another individual of the same species is potent, and this the more so the wider the difference between the individuals ; the pollen from an individual of a different variety is more potent than that from an indi- vidual of the same variety. The effect of the sexual process is not necessarily con- fined to the cells or organs which directly take part in it ; not infrequently it makes itself felt in adjoining organs, stimulating them to active growth, giving rise to the formation of a fruit or fructification. Thus in the Mu- corini an outgrowth of filaments, forming an incomplete or complete (Mortierella) investment to the zygospore, takes place from the sexual organs after conjugation ; in Coleochsete the oogonium becomes surrounded, after the fertilization of the oosphere, by an investment formed by outgrowths from the adjacent vegetative cells ; a cellular investment is formed in the same way round the fertilized procarpium in most Florideae, and round the fertilized ascogonium in the Ascomycetes. The most familiar case of fruit-formation is that occurring in the Phanerogams ; here in many instances the carpels, in some the floral leaves of the perianth, and in some the floral receptacle (torus) grow actively after the fertilization of the oospheres has taken place, giving rise to a mass of succulent parenchymatous tissue. In the Orchideae the develop- ment of the ovules does not take place at all until the flower has been pollinated. Germination of the Spores and Alternation of Generations. The spores of plants may either germinate immediately on their production, or they may undergo a, longer or shorter period of quiescence ; those which are destined for immediate germination have, as described above, a thin cell-wall, whereas those which are capable of undergoing a period of quiescence have a thick cell-wall In some cases the spores are incapable of immediate germination, notably sexually produced spores; for instance, among the Algae, immediate germination is only known to take place in the case of the zygospores of Botrydium and Ectocarpus, and of the oospore of Fucus ; among the Fungi the zygospores of the Mucorini and the oospores of the Peronosporeae and Saprolegnieae pass through a period of quiescence. The mode of germination is not always the same. In Germina- most cases the spore gives rise directly to a new organism, tio n of either by protruding one or more filamentous outgrowths, s P res - or by the division of its protoplasm to form the tissue of the embryo. In some cases the spore behaves like a reproductive organ ; from its protoplasm are formed a larger or smaller number of cells, either motile or non- motile, which are set free. In Acetabularia, and under certain circumstances in Botrydium, the asexually pro- duced spore behaves like a sexual reproductive organ (gametangium), giving rise to a number of planogametes ; similarly the spore of Protomyces produces within itself a number of conjugating sporidia. In some Peronosporeae (always in Cystopus ; occasionally, according to circum- stances, in Pythium, Phytophthora, and Peronospora) the asexually produced spore behaves like a sporangium, and gives rise to a number of zoospores from each of which a new individual is developed. This happens occasionally also in sexually produced spores. Among the Fungi, the formation of zoospores in the oospore occurs in various species of Peronosporeae and Saprolegniese. Among the Algae, zoospores are formed in the zygospores of Pandorina and Ulothrix, and in the oospores of CEdogonium and Sphaeroplea. Cases of a similar kind are known also in the Phanerogams ; thus in some Coniferae, and notably in the Gnetaceous Epliedra altissima, a process of cell-forma- tion goes on in the oospore, resulting in the formation of a Polyem- a larger or smaller number of cells from each of which an bryony. embryo plant is developed. These cases, in which, namely, the spore, whether sexually or asexually produced, gives rise to a number of cells, each of which is capable, by itself, of developing into a new individual, are instances of what is known as polyembryony. In some cases the cells formed in the sexually produced spore do not each give rise to a new individual ; this obtains in the Hydro- dictyeae. In Hydrodictyon utriculatum the protoplasm of the zygospore gives rise to two or four large zoospores which eventu- ally come to rest and remain quiescent for several months ; these resting spores are termed, on account of their form, polyhedra. On germination, the protoplasm of the polyhedron gives rise to a number of small zoospores, the endospore protruding as a delicate vesicle, within which the zoospores are in active movement ; the zoospores eventually come to rest, without escaping from the en- dospore, and arrange themselves so as to form the meshes of a small sac-like net, which is a young Hydrodictyon ; the endospore is then disorganized, and the young net is set free a^ an independ- ent ccenobium. Occasionally it happens that a portion only of the spore gives rise to the embryo. This is the case in the sexually produced spores (oospores) of the Characese, and in those of the Coniferse ; in Selaginella and in the Angiospermous Phanerogams one-half of the oospore gives rise to a filamentous structure, the suspensor, the other half to the main body of the embryo. It is very commonly the case that the spore, on germina- tion, gives rise to an organism unlike that by which the spore was produced. In a Moss, for example, the asexu- Alterna- ally produced spores are developed by an organism, the tion of sporogonium, consisting typically of a longer or shorter. e stalk, the seta, bearing a capsule (theca) which produces the spores. When one of these spores germinates, it does not give rise to another sporogonium, but to an incon- spicuous, usually filamentous, structure, the protonema,