CLASSIFICATION OF THE PTEEIDOPHYTES. 119 



tions of some of the commonest genera are shown in Figure 

 70, E, J. 



CLASS II. HORSE-TAILS (Equisetince) . 



The second class of the pteridophytes includes the horse- 

 tails (Equisetince) of which all living forms belong to a single 



p.- 



FIG. 72. A, spore-bearing stem of the field horse-tail (Equisetum), x 1. x, 

 the spore-bearing cone. JB, sterile stem of the same, x %.. C, underground 

 stem, with tubers (o),x%. D, cross-section of an aerial stem, x 5. /.&. 

 fibro-vascular bundle. E, a single fibre-vascular bundle, x 150. tr. vessels. 

 F, a single leaf from the cone, x 5. G, the same cut lengthwise, through a 

 spore sac (sp.), x 5. jj, a spore, x 50. /, the same, moistened so that the 

 elaters are coiled up, x 150. J, a male prothallium, x 50. an. an antheridium. 

 K, spermatozoids, x 300. 



genus (Equisetum) . Formerly they were much more numerous 

 than at present, remains of many different forms being espe- 

 cially abundant in the coal formations. 



