CLASSIFICATION OF THE PTERIDOPHYTES. 121 



fibro- vascular bundles (/&.) are arranged in a circle, alternating 

 with the air channels, and each one has running through it a 

 small air passage. 



The cone at the top of the branch is made up of closely set, 

 shield-shaped leaves, which are mostly six-sided, on account of 

 the pressure. These leaves (F, G) have short stalks, and are 

 arranged in circles about the stem. Each one has a number 

 of spore cases hanging down from the edge, and opening by a 

 cleft on the inner side (G, sp.}. They are filled with a mass 

 of greenish spores that shake out at the slightest jar when 

 ripe. 



The sterile branches (B) are more slender than the spore- 

 bearing ones, and the sheaths shorter. Surrounding the joints, 

 apparently just below the sheaths, but really breaking through 

 their bases, are circles of slender branches resembling the main 

 branch, but more slender. The sterile branches grow to a 

 height of forty to fifty centimetres, and from their bushy form 

 the popular name of the plant, "horse-tail," is taken. The 

 surface of the plant is hard and rough, due to the presence of 

 great quantities of flint in the epidermis, a peculiarity com- 

 mon to all the species. 



The stem is mainly composed of large, thin-walled cells, becoming 

 smaller as they approach the epidermis. The outer cells of the ground 

 tissue in the green branches contain chlorophyll, and the walls of some of 

 them are thickened. The fibro- vascular bundles differ entirely from those 

 of the ferns. Each bundle is nearly triangular in section ("), with the 

 point inward, and the inner end occupied by a large air space. The 

 tracheary tissue is only slightly developed, being represented by a few 

 vessels 1 (tr) at the outer angles of the bundle, and one or two smaller ones 

 close to the air channel. The rest of the bundle is made up of nearly 

 uniform, rather thin- walled, colorless cells, some of which, however, are 

 larger, and have perforated cross-walls, representing the sieve tubes of 



1 A vessel differs from a tracheid in being composed of several cells 

 placed end to end, the partitions being wholly or partially absorbed, so as 

 to throw the cells into close communication. 



