SECT. I 



CRYPTOGAMS 



457 



of the Conifers. The lowest whorl is sterile, and forms a collar-like protuberance. 

 The sporophylls (Fig. 420 B, C) are stalked and have a peltate expansion, on 

 the under side of which are borne the sac-like sporangia (5-10). In the young 

 sporangium the sporogenous tissue is surrounded by a wall consisting of several 

 cell layers, but eventually the tapetal cells of the inner layer become disorganised, 

 and their protoplasm penetrates between the developing spores. At maturity 

 the wall of the sporangium consists only of the outermost of the original layers, 

 the cells of which are provided with annular and spiral thickenings ; the sporangia 

 thus resemble tl> homologous pollen-sacs of Phanerogams. The dehiscence is deter- 

 mined by the cohesive force of the diminishing amount of water in the cells of the 



II 



FJI:. 4'J1. Xijuisetttui pratensf. I, Female prothallium from the under surface, showing the arche- 

 k'onia (.4). II, Male prothallium with antheridia (A); d, cover cells of antheridia. (/ x 17, 

 // x 12. After GOEBEL.) 



outer layer and the contraction of the thin parts of the cell walls on drying ( m ). 

 The sporangia split longitudinally, and set free a large number of green spores, 

 which are nearly spherical in shape, and have peculiarly constructed walls. In 

 addition to the intine and exine, the spores are overlaid with a perinium formed 

 from the protoplasm of the tapetal cells, and consisting of two spiral bands 

 which are attached to the spores only at their point of intersection (Fig. 420 D). 

 On drying, the spiral bands loosen and become uncoiled ; when moistened they 

 close again around the spore. By means of their hygroscopic movements they 

 serve to hook together the spores, and in this way assure the close proximity 

 of the unisexual prothallia which the latter produce (Fig. 420 E). 



In certain species some of the aerial haulms always remain sterile, branching 

 profusely, while others which produce the terminal cones either do not branch at 

 all, or only at a later stage, and then sparingly. This distinction between the 

 sterile and fertile haulms is most marked in Equisetum arvense and Equisetum 

 Telmateja, in both of which the fertile shoots are entirely unbranched, and 



