236 



THE MICROSCOPE. 



of plants, in sections taken from grass, and the leaf of 

 Deutzia scabra. This insoluble material is called silica, 

 and is abundantly distributed throughout certain orders 

 of plants, leaving a skeleton after the soft vegetable 

 matters have been destroyed : masses of it, having the 

 appearance of irregularly-formed blackened glass, will 

 always be found after the burning of hay or straw ; which 

 is caused by the fusion of the silica contained in the 

 cuticle combining with the potash in the vegetable tissue, 

 thus forming a silicate of potash (glass). To display this 

 siliceous structure, it is necessary to cut very thin slices 

 from the cuticle, and mount them in fluid or Canada 

 balsam. 



In the Graminacece, especially the canes ; in the Equi- 

 setum hyemale, or Dutch rush ; in the husk of the rice, 



wheat, and other grains, 

 silica is abundantly found. 

 In the Pharus cristatus, 

 an exotic grass, fig. 142, 

 we have beautifully-ar- 

 ranged masses of silica with 

 raphides. The leaves of 

 Deutzia j fig. 141, are re- 

 markable for their stel- 

 late hairs developed from 

 the cuticle, of both their 

 upper and under surfaces ; 

 forming most interesting 

 arid attractive objects when 

 examined under the micro- 

 scope, either with polarised 

 or condensed light. 



Silica is found in all Ru- 

 biacece ; both in the stem 

 and leaves, and if present in sufficient thickness, depolarises 

 light. This is especially the case in the prickles, which 

 all these plants have on the margin of the leaves and the 

 angles of the stem. One of the order Composites, a plant 

 popularly known as the "sneeze wort," (Archillce ptarmica') 

 has a large amount of silica in the hairs found on the 

 double serratures of its leaves ; commonly said to be the 



Fig. 143. Portion of the husk of Wheat, 

 showing siliceous crystals. 



