CHAPTER XXIV 



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 means "horse bristle," and the name 

 "Horsetail Family" is given in the manuals. The name, "scouring 

 rush," is more appropriate, because the rough stems have been used 

 for scouring kettles. The roughness is due to silica. Species, like E. 

 hiemale, which contain 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 fluviatile 

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

 secured in a few minutes. Fix in the Chicago chromo-acetic-osmic 

 solution. 



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. Dissect away very 

 carefully the scale leaves covering the growing point. To show the 

 segmentation, building up the root or the stem from a single apical 

 cell, the sections should not be too thin; 10 or 12 ju is not too thick. 



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

 it can be cut in paraflSn without any difficulty. The adult vegetative 

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

 be fixed in formalin acetic alcohol and — after washing in 70 per cent 

 alcohol — should be treated with 20 per cent hydrofluoric acid in 70 per 



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