The Pteridophytes 



489 



having four long appendages, that coil around 

 moist and uncoil when dry. The spores contain 

 do not withstand drying, and they 

 die unless germinated within a 

 month. 



The gametophytes. In most 

 species of equisetum the gameto- 

 phytes are irregularly lobed, thal- 

 loid structures which grow on 

 moist banks of streams. They 

 are bisexual, producing both an- 

 theridia and archegonia. In the 

 most-specialized species {E. ar- 

 vense) the gametophytes are usu- 

 ally unisexual ; that is, about one 

 half produce antheridia only and 

 the other half archegonia. Fer- 

 tilization takes place when a swim- 

 ming sperm fuses with an Qgg cell. 

 An oospore results, which germi- 

 nates at once as in the ferns, 

 and from it the sporophyte de- 

 velops (Fig. 304, C, D). 



the spore when 

 chlorophyll and 



Fig. 302. The common field equisetum 

 {Equisetum arvense). Rootstock with 

 sterile branches {B), spore-bearing 

 branches {A), and tubers (C). D shows 

 the spores with their appendages. 



THE LYCOPODS (LYCOPODIALES) 



Another, and perhaps the most 

 ancient, group of Pteridophytes 

 includes the lycopods or club 

 mosses. Only two genera remain, 

 Lycopodium (100 species) and Selaginella (500 species). Both 

 consist of scale-leafed creeping plants, often several feet in 

 length, with upright or inclined branches. The stems branch 

 by repeated forking, and this is true even of the fossil tree forms. 

 In all cases the leaves are scale-like. In the more primitive spe- 



