CHAPTER XXI 



PTERIDOPHYTES 

 EQUISETALES 



This order was large and prominent in the Carboniferous age, 

 but now only a single family, the Equisetaceae, survives. Its only 

 genus, Equisetum, contains 24 pieces, 10 of which occur within the 

 Gray's Manual range. Equisetum is often called the " scouring 

 rush," because the rough stems have been used for scouring kettles. 

 The roughness is due to silica. Species, like E. hiemale, which con- 

 tain much silica, must be treated with hydrofluoric acid before the 

 older parts can be cut in paraffin. 



Vegetative Structure. The roots are very small, but have large 

 cells and easily yield good preparations. If a handful of Equisetum 

 limosum or E. hiemale growing in water be pulled up, scores of root- 

 tips may be secured in a few minutes. Fix in chromo-acetic acid 

 with a little osmic acid. In case of such small objects it is a good 

 plan to add a few drops of eosin to the alcohol during the process of 

 dehydrating, in order that the material may be seen more easily. 

 The slight staining does no damage, even if more critical stains are 

 to be used after the sections are cut. Longitudinal sections of the 

 roots may also be obtained by cutting transverse sections of the 

 nodes. 



The growing points of stems may be cut with ease in paraffin. 

 E. arvense is particularly favorable on account of the numerous apical 

 cells which may be found in a single preparation. 



The " fertile' 1 stem of Equisetum arvense is so free from silica 

 that it can be cut in paraffin without any difficulty. The adult 

 vegetative stem of E. arvense, and all stems which contain so much 

 silica, must be treated with hydrofluoric acid before imbedding in 

 paraffin. However, nearly all of these stems can be cut freehand, 

 before fixing, without removing the silica. Fix freehand sections 

 in 95 per cent alcohol. Material for paraffin sections should be 



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