Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/449

Rh VEGETABLE.] KEPBODUCTION 431 Botrydium-plant, a process which is clearly a case of apogamy ; and, further, the Botrydium-plant does not necessarily produce resting-spores, but may produce zoospores by which it is directly reproduced. In Coleo- chsete, too, alternation of generations is indicated. The oospore produces by division a small individual which is always asexual, giving rise to zoospores which likewise produce asexual individuals ; this asexual reproduction may continue through a number of generations until eventually a sexual individual is developed. In Coleo- chsete it is only in the case of the sexually produced spore that the nature of the resulting organism is known ; it always gives rise to an asexual form, whereas the asexually produced spores give rise to an individual which may be either sexual or asexual. In the Characese the oospore always gives rise to an imperfectly developed form, the proembryo, which is to be regarded as the sporophore ; however, it never produces spores, but gives rise to the sexual Chara-plant (oophore) vegetatively by budding. The study of the life-history of the Fungi is attended with considerable difficulty, partly on account of the fact that in many cases the development of the sexual organs is dependent upon a combination of external circumstances which may but rarely present itself, and partly on account of there being frequently a great difference in habit between the sexual and asexual forms of the same plant, a difference which is sometimes accentuated, in parasitic Fungi, by the occurrence of the two forms on different plants as hosts (hetercecisni). But there are Fungi in the life-history of which alternation of generations has been ascertained. Before entering upon an account of these, it must be stated that the term "sexually produced spore" will be applied not only to those the formation of which is known to be preceded by a sexual process, but also to those which are formed probably or actually without a sexual process in a word, apogamously but which may be considered, as pointed out above, to be homologous with those which are actually sexually produced. In Mucor Mucedo and Phycomyces nitens among the Mucorini, for instance, the zygospore gives rise on germination to an imperfectly developed individual (pro- mycelium) which is entirely asexual and produces spores ; one of these spores, in turn, gives rise to an individual which produces spores asexually but may also bear sexual reproductive organs. Essentially the same life-history may be traced in certain Peronosporese (Phytophthora omnivora, Pythium proliferum). In these cases the form developed from the sexually produced spore is always asexual, where- as that derived from the asexually produced spore may be sexual, but it always produces spores asexually ; hence there is not a strict alternation of an asexual and a sexual generation. In others the alternation is complete. In the Ustilaginese, for instance, the asexually produced spore gives rise to an imperfectly developed mycelium (promycelium), which is the sexual generation; it produces the sporidia, which conjugate in pairs, and from the product of conjugation springs the mycelium, which bears asexually produced spores. Essentially the same life-history has been traced in some Ascomycetes and Uredineae. In Claviceps (Ascomy- cete) the sexually produced spore (ascospore) gives rise to an asexual form, long regarded as a distinct genus under the name of Sphacelia, from the spores of which the sexual form is reproduced. In Sclerotinia (Peziza) Fuckeliana (Ascomycete), a similar regular alternation of generations occasionally but not always occurs ; the ascospore may give rise to an asexual form, long known as Botrytis cinerea, and when it does so the alternation of generations is complete ; but it may give rise to another sexual genera- +' in which case no- alternation takes place. In tion. Polystigma (Ascomycete), the ascospore gives rise to a promycelium which bears sporidia, and these sporidia give rise to the sexual form. In Endophyllum (Uredinese) the life-history is precisely the same as that of Polystigma: the promycelium is the sporophore generation, the myce- lium developed from the sporidium the oophore. In other Uredinese the life-history is slightly modified in that asexually produced spores of at least two kinds make their appearance. The sexually produced spore (aecidiospore) gives rise to a mycelium which, in Gymno- sporangium and Hemipuccinia, bears asexually produced spores, teleutospores ; in Puccinia Graminis the formation of teleutospores is preceded by that of somewhat different spores, the uredospores; in any case the teleutospore gives rise, on germination, to a second asexual generation, the promycelium, which bears sporidia ; from these the sexual aecidium-bearing generation is developed. The rule that, in the alternation of generations, the alter- Alterna- nate generations are developed from spores produced either tion of sexually or asexually is not, however, without exceptions, & for in some instances the one generation may spring vege- terfered" tatively from the other without the intervention of a spore, with by This is brought about by the suppression either of theapogamy sexually produced spore or of the asexually produced spore; or a P s " the former is an instance of apogamy, the latter of apospory. Thus in the apogamous Ferns mentioned above, the asexual generation (sporophore) is developed as a bud upon the sexual generation (oophore) ; and in Botrydium the gametangium which is the representative of the sexual generation may, instead of producing gametes, produce zoospores, in which case the new asexual individual is not developed from a sexually produced zygospore, but from an asexually produced zoospore. Similarly, in the apo- sporous Ferns and Mosses and in the Characese the oophore is developed as a bud from the sporophore. The alternation of generations may be also interfered with by a combination of apogamy and apospory. This is the case, namely, when one generation gives rise to its like by vegetative budding, sporophore to sporophore, oophore to oophore. For instance, when, as in the Phanerogams mentioned above (Ccelebogyne, Funkia, Nothoscordum, Citrus), embryos are produced vegetatively from the tissue of the nucellus, that is, sporophore from sporophore, the typically intervening formation of spores, first, by the asexual method and, secondly, by the sexual method is suppressed. This is necessarily always the case among Phanerogams when one plant is produced vegetatively from another. A striking instance of the same thing has been observed by Goebel in some species of Isoetes, in which an Isoetes plant was produced on the leaf in place of a sporangium. Similarly when oophore springs vegetatively from oophore, the typically intervening formation, first, of the sexually produced spore and, secondly, of the asexu- ally produced spore, is suppressed. This occurs when, as mentioned above, a Moss-plant gives rise by budding or by means of gemmse to another Moss-plant, and when a Fern- prothallium gives rise to another by means of gemmae. BIBLIOGRAPHY. General: Sachs, Text-book, 2d English edition, 1882; Goebel, Gfrundzuge dor Systematik, 1882; Vines, Alternation of Generations in Thallophy tes, " Journal .of Botany, 1879; Pringsheim, " Ueber den Generationswechsel bei den Thallophyten," Jahrb. f. Wiss. Sot., xi., 1877. Algae, : Falkenberg, in Schenk's Handbuch der Botanik, ii., 1882; Vines, "The Pro-embryo of Chara, " Journal of Botany, 1878. Fungi: De Bary, Vergleichende Morphologic und Biologic der Pilze, 1884. Ferns: Cramer, " Ueber die gescblechtslose Vermehrung des Farn-prothaliums," in Denk- schriften der Schweitz. naturforsch. Gesellschaft, xxviii., 1880; Bower, "On Apospory in Ferns," Jour. Linn. Soc., Botany, vol. xxi., 1885. Mosses: Pringsheim, MonatsbericM d. Akad. d. Wiss. in Berlin, 1876 ; Stahl, Botanische Zeitung, 1876. Physiology : Strasburger, Ueber Befruchtung und Zelltheilung, 1878, and Neue Untersuchungen uber den Befruchtungsvorgang bei den Phanero- gamen, 1884. (S. H. V.)