Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/169

CRYPTOGAMS.] BOTANY 159 or sporidia arc subsequently developed. In Lichens no sexual organs have as yet been clearly demonstrated. Amongst Fungi and Lichens there are certain bodies to be noted, which may bo connected with sexual repro duction, but the function of which is at present unknown. Embedded in the margin of the thallus in Lichens and on various parts of the plant in Fungi, certain hollow urn- shaped bodies arc found, which have been termed sperma- gones (conceptacles) (fig. 334). These are usually closed all but a small opening at the apex. From their base rise peg-shaped projections or sterigmata (fig. 335), bearing at FIG. ""A. Two spennagones on thalli of Lichens. FIG. 335. Sterigmata a and spermatia 6 of Cladoniafui.briata. their summit elongated, sometimes curved, bodies, termed spermatia, which, when mature, are discharged through the apex of the spermagone. These are by some con sidered the analogues of the antherozoids in other Cryptogams. There are also found other conceptacles, to which the name of pycnidia or pycnides is given. These contain large spores, which have been termed stylosporea (fig. 33G), but their homology is at present unknown. Another process of reproduction is seen amongst Crypto gams. This is termed conjugation (fig. 333). It differs from the process of fertilization just described in that it consists of a union of the contents of tw r o similar cells, whilst fertilization essentially consists in the mutual influence of dissimilar cells. It is well seen in many Algae, such as Diatoms, Volvocinerc, Conjugate, also in some Fungi, such as species of llhizopus and Zyzygites. It may consist in the coining together of two cells, either moving spores, as in Pandorinia,or motionless, as in Diatoms, which completely fuse, arid a compound spore, or zygospore, results ; or pro longations of the wall of two adjacent cells meet, the partition between the two gives way and the contents from the one pass into the other, and a zygospore is formed, as in Zygnema. Zygospores germinate only after a long period of rest. Amongst Thallogens especially, though it also occurs in other Cryptogams, the asexual mode of reproduction is very common. But the manner of formation and the nature of the spores is diversified. Amongst Alga3 two modes of asexual reproduction are seen, by motile spores and by motionless or resting spores. In the former case the contents of a cell form a new cell, which escapes from the mother-cell, and moves about by means of cilia, which are formed either all round it, as in Vaucheria (fig. 337), or as a tuft at one end, as in (Edogonium. These are termed swarm spores, or zoospores, and frequently are of different sizes, being termed respectively microgonidia and macrogonidia. After swim ming about for a time they fix themselves, and develop small rhizi>ids from one extremity, the other growing up into the plant. The motionless spores are seen in the Floridea , where they are formed in cells, four in each, attached together in a row, or as a tetrahedron, constituting the tetraspore (fig. 338). Amongst Fungi asexual reproduction is very common. Swarm spores are rarely formed ; but the mode of formation of motionless spores is very various. They may be produced in the interior of distinct sacs called Fig. 337. FIG. 330. I tap id in a, stylosporea 6, free stylospores e. from pycnides of I.ccldia. Kio. 337. X.oosporc of an Alga (raiicheria), surrounded by moving cilia. FIG. 338. Tetrasporc ( of one of the rose-coloured Seaweeds (Callithamnior. crttciatum). thecce, asd, or cystidia (fig. 339), when they are denom inated cndospores or ascosporcs, and the plants are said to be thccasporous or ascosporous. When they are de veloped on the exterior of sacs called basidia (fig. 340), they arc denominated exosjwrcs, and the plants are basi- diosporous. When produced in the midst of a gelatinous mass, without any evident differentiation, they are called myxospores, the plants being myxosporoiis. Both the endo- spores and the exospores may by division become septate, and form compound spores, each of the secondary spores so formed being termed a merispore. Four or eight basidiospores are usually produced from each basidium. In the asci numerous spores are formed. Other forms of spores seen in Fungi arc the conidia. These are stalked spores which arise from a mycelium. In Lichens we find asci which produce spores or sporidia just as in Fungi ; and in addition there also exist spermo- gones with their sterigroata and spermatia as well as pycnides and stylospores. Besides the spores there are organs called soredia in Lichens. These are groups of small round cells or gonidia (fig. 97), surrounded by hyphae, which are capable of independent growth after removal from the thallus. These gonidia are now regarded by many as an Alga, upon which the hypha, which is supposed to be an ascomycetous Fungus, is parasitic. In (Jhara a curious mode of asexual reproduction is seen which presents several well-marked modifications, the pro cess always proceeding at the nodes, and consisting in the growth of a new axis from a nodal cell either isolated or iinitcd with other cells. In all vascular Cryptogams, Muscincre, and many Thai- Alternation logons, that process of development in which successive dif- of genera- ferent generations are produced finally returning to the tions> original form, usually termed alternation of generations,

is well ?eeu. The number of such generations varies.