LESSON 13.] 



SILICA. 



31) 



form in size ; the stomata are apparent as nucleated spots in a tortu- 

 ous line, which occupies the central portion of each bar. 



232. The various species of Horse-tails (Equisetum) are remarl a- 

 ble for the large amount of flint found in their cuticular covering. 



FIG. 67. 



Campilodiscus clupens, from 

 the JSerg-inehl. 



Silica of the Out. 



FIG. 68. 





Silica of the Wheat. 



One species (E. hyemale), or Dutch rush, or scouring rush, presents a 

 flinty layer, which forms a fine object under the Microscope (Fig. 70). 



233. The flint in this plant appears to be an accurate cast of the 

 tissue (cuticle) with which it is in contact ; every cell, and the nucleus 

 of every cell, is faithfully represented. 



234. In addition to this structure of the cells, the stomata are 



FIG. 69. 



FIG. 70. 



Silica of Eice. Equisetuiu Hyemale. 



well represented, and, as will be seen by reference to the Fig. (a, a), 

 they are double the one internal, the other external. The internal 

 breathing-mouths are the large circles, with irregular openings; 

 through these openings, and through the transparent sides of the cir- 

 cles, the external stomata may be seen. These consist of a pair of 

 lips, which open in the direction of the long axis of the plant, and, 



