182 WAYSIDE WEEDS. 



grass tribeSj and more especially in tliose of the 

 horse-tailsj or " jointed ferns," we do not, however, 

 yet number the latter among our acquaintance. 



In these plants the cells of the cuticle contain the 

 silex grains so abundantly, and withal so symmetri- 

 cally arranged, that it is possible for all the real 

 vegetable substances to be removed, and still the 

 stem will retain its form. Moreover, the presence of 

 this crystalline sand, for it is nothing else, fine and 

 minute as it must necessarily be, renders these com- 

 paratively unvalued plants of some commercial 

 importance as pohshing agents to the cabinet-maker, 

 the whitesmith, and others. 



It is not, however, colour only which the 

 plant cuticle allows to pass, but, what is much 

 more important to the vegetable economy, it per- 

 mits the free ingress and egress of moisture, and of 

 air or gases, not indeed through its entire surface, 

 but by means of the httle breathing pores or 

 stomates (Fig. 109 a), which are so thickly 

 scattered over the plant exterior ; in most, on the 

 under surfaces of the leaves chiefly, the principal 

 exception to this being in the case of floating leaves 

 of aquatic plants, where, obviously, the stomates 

 would be useless on the under surface, and, conse- 

 quently, we find them on the upper. Each little 

 stomate, as represented, is composed of a couple of 

 oblong cells, with an opening between them commu- 

 nicating with the cell tissues which compose the sub- 



