SECT. I 



CRYPTOGAMS 



461 



and for that purpose tlieir cortical tissue under the epidermis is provided with 

 chlorophyll. 



The sPoiiANGiA are borne on specially-shaped leaves or sporophylls. The 

 sporophylls are developed in whorls, but are closely aggregated at the tips of the 

 erect feitile shoots into a cone (Fig. 408), which is sometimes spoken of as a flower, 

 from the correspondence in its structure to the male flower of the Conifers. The 

 lowest Avhorl is sterile, and forms a collar-like protuberance. The sporophylls (Fig. 

 408, B, G) are stalked and have a peltate expansion, on the luider side of which are 

 borne the (5-10) sac-like sporangia. In the young sporangium the sporogeuous 

 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 



/ // 



Fig. 409. — Equisetumpratensc. I, Female protliallium from the under surface, sliowing the arche- 

 gonia (A). II, Male prothallium with antheridia (A) ; (/, cover cells of antheridia. (/ x 17, 

 // X 12. After GoEBEL.) 



only of the outermost of the original layers, the cells of w'hich are provided with 

 annular and spiral thickenings ; the sporangia thus resemble the homologous pollen- 

 sacs of Phanerogams. The dehiscence is determined by the cohesive force of the 

 diminishing amount of water in the cells of the outer layer and the contraction of the 

 thin parts of the cell walls on drying (^*^). 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 deposited by the protoplasm of the tapetal cells, and 

 consisting of two sjjiral bands which are attached to the spores only at tlieir point 

 of intersection (Fig. 408, D). On drying, the spiral bands loosen and become un- 

 coiled ; 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 anisexual prothallia which the latter produce 

 (Fig. 408, U). 



