XII EQUISETINE^ 479 



lie. It is at first a uniform membrane, closely applied to the 

 middle coat, but when placed in water it swells up and separates 

 completely from the exospore, or remains attached to it at one 

 point only, which marks the point of attachment of the elaters in 

 the ripe spores. The elaters arise from the epispore by its 

 splitting spirally into four bands (Fig. 280, C), due apparently 

 to thickening along these bands, leaving thin places between, 

 which are finally absorbed. The outside of the elaters becomes 

 cuticularised. The ripe spores contain numerous chloroplasts, 

 which only are evident in the latest stages of development. In 

 E. arvense the formation of the sporangia begins nearly a year 

 before the spores are shed, and they are completely developed 

 during the preceding autumn. The growth of the fertile 

 branch and the scattering of the spores take place very soon 

 after growth begins in the spring. Whether in cold climates 

 E. tehnateia behaves the same way I cannot state ; but in Cali- 

 fornia, where growth continues all the winter, the development 

 of the sporangia is gradual, and the fertile stems grow up and 

 scatter the spores as soon as they are ripe. The ripe sporangia 

 are oblong sacs, whose wall is composed for the most part of a 

 single layer of elongated cells, marked with spiral thickened 

 bands upon the dorsal surface and rings upon the ventral cells, 

 where the longitudinal slit by which the sporangium opens is 

 placed (Fig. 280, D, E). The internodes in the strobilus are 

 very little developed, but as the spores ripen there is a slight 

 elongation, by which the sporophylls are separated. 



Classification 



Milde ( i) divides the genus into two, Equisetum^ (Equiseta 

 phanopora), in which the accessory cells of the stoma are on a 

 level with the surface of the epidermis ; and Hippochwte (E. 

 cryptopora) , in which the stomata are sunk in depressions of the 

 epidermis. In the former group are two divisions, those which, 

 like E. arvense and E. tehnateia, have the fertile and sterile 

 branches different, and those where they are alike, e. g., E. linio- 

 sum (Fig. 280, A). Some species, e. g., E. pratense, have the 

 fertile stems at first colourless, but afterwards forming chloro- 

 phyll and developing branches. In Hippochccte, which includes 

 among American species E. hiemale, E. rohustum, E. variega- 

 ^ Euequisetum, Sadebeck. 



