PTERIDOPHYTES— EQUISETALES 



301 



cent alcohol for 2 or 3 days. The acid must then be washed out with 

 70 per cent alcohol. 



The strobilus. — E. arvense affords the most favorable material for 

 a study of the development of sporangia, since the strobilus contains 

 almost no silica and, even in its latest stages, is easily cut in parafhn. 

 In this species, the young strobih, in the Chicago region, can be dis- 

 tinguished from vegetative buds in July; sporogenous tissue is well 

 advanced by the middle of August; and the reduction divisions occur 



Fig. 94. — Equiseium arrense: A, section of a sporangiophore showing beginning of sporogenous 

 tissue, early Augu-st condition; B, topography of the strobilus at this stage. A X580; B X8. 



late in August or early in September (Fig. 94). The spores are not 

 shed until the following April. If you know a patch of this species 

 which "fruits" every year, dig up the horizontal underground stem in 

 July. The tip of the main axis is almost sure to be a strobilus. Dissect 

 away the scale leaves and fix the strobilus in chromo-acetic acid with 

 a httle osmic acid. August and September stages are easy to recognize. 

 If strobili are brought into the lal^oratory in December or January, 

 they shed their spores within a week. 



Equisetum telmateia, of our Pacific states, has the strobilus on a 

 comparatively soft, early shoot, as in E. arvense ; the vegetative shoot 



