814 ECOLOGY 



all with relatively thin cell walls, while toward the close of the season there are 

 developed teleutospores (figs. 1126, 1127), which are thick-walled and are capable 

 of enduring the winter. 



Asexual spores in the bryophytes. In most liverworts and mosses 

 there is a well-defined alternation of generations (p. 822), spores being 

 characteristic of one generation, the sporophyte, and sex organs being 

 equally characteristic of another generation, the gametophyte. The 

 spores are scattered chiefly by the wind, their minute size and the 

 generally stalked and therefore elevated capsules facilitating such dis- 

 persal (figs. 977, 231, 254). The spores contain chlorophyll, so that 

 independence is possible from the outset, if the sporelings (i.e. the ger- 

 minating spores) are exposed to light. Sometimes (as in Riccia and 

 Phascum) the spores are exposed to dispersing agents only upon the 

 decay of the capsule wall, but more commonly there is definite dehis- 

 cence. In the Jungermanniales and in Anthoceros the capsule wall 

 splits into valves (figs. 235, 239), and in the Marchantiales and in most 

 mosses there is a lid or operculum (fig. 250). Most moss capsules are 

 fringed toward the tip with a peristome (figs. 263, 264), whose hygro- 

 scopic teeth open when the weather is dry and close when it is moist; 

 these movements effect the detachment of the operculum, and probably 

 are of some value in facilitating the removal of spores from the capsule. 

 In most liverworts long, fiber-like, spirally thickened bodies, known as 

 elaters (fig. 230), occur among the spores, and, like the peristome teeth 

 of mosses, they exhibit hygroscopic movements which are thought to 

 facilitate spore removal. As a rule, the spores of liverworts soon lose 

 their capacity for germination, but the spores of mosses may retain their 

 vitality for a long time; cases are on record, where moss spores have 

 germinated, after having lain dry in a herbarium for fifty years. 



Asexual spores in the pteridophytes. In the Filicales the spores com- 

 monly are borne in sporangia on the backs of ordinary foliage leaves 

 (figs. 1128, 1129), but in some cases (as in Onodea and Osmunda) 

 special leaf regions or entire leaves are spore-bearing, while other leaf 

 regions or entire leaves are foliage organs; comparable to the latter are 

 the Ophioglossales (figs. 352-354). In Equisetum the sporangia are 

 borne on a special structure, the strobilus (fig. 332), and, as in liverworts, 

 there are elaters (figs. 337, 338) which assist to some extent in dispersal. 

 In Lycopodium the sporangia may be arranged in the axils of foliage 

 leaves (fig. 265) or in a strobilus (fig. 266). In the above pteridophytes 

 all the spores are alike, that is, homosporous, but in the water ferns 



