CHAPTER XII 



EQUISETINE^ 



All of the living representatives of the second class of the 

 Pteridophytes may without hesitation be referred to die single 

 genus Equisehim, with about twenty-five species, some of which, 

 e. g., E. arvcnse, are almost cosmopolitan. In the largest 

 species, E. giganteum, the stems reach a height of lo metres or 

 more, but are slender, not more than 2 to 3 cm. in diameter, and 

 supported by the surrounding trees and bushes. The smallest 

 species is E. scirpoides (Fig. 281, B), whose slender stems are 

 seldom more than 15 to 20 cm. in length, and often one milli- 

 metre or less in diameter. In spite of these differences in size, 

 the structure is remarkably uniform, both in gametophyte and 

 sporophyte. The following account is based mainly upon a 

 study of E. tclmateia,^ but applies" to the other species that have 

 been studied. 



The Gametophyte 



The ripe spore of Equisetum is globular and shows no 

 trace of the ventral ridges usually evident in tetrahedral spores. 

 Four distinct membranes surround it, the inner one (intine) 

 being exceedingly delicate, but with care showing the cellulose 

 reaction (Buchtien ( i ) ) . Outside of this are the exospore and 

 the elaters, between which lies ^another layer, ''Mittelhaut" of 

 Strasburger ((11), p. 199), belonging to the exospore. The 

 well-known elaters (Fig. 257, A) form two strips attached in 

 the middle and terminating in spoon-shaped appendages. The 

 elaters are usually more or less spirally twisted, and when dry 

 show faint oblique striations, except on the expanded ends. 

 They are extremely hygroscopic, and respond instantly to any 



^ E. maximum Lam. 



443 



