422 THE MICROSCOPE. 



others from the same plant the crystals may be principally prismatic, 

 and are arranged as if they were beginning to assume a stellate form. 

 Some plants, as many of the cactus tribe, are made up almost entirely 

 of raphides. In some instances every cell of the cuticle contains a stel- 

 late mass of crystals ; in others the whole interior is full of them, ren- 

 dering the plant so exceedingly brittle, that the least touch will occa- 

 sion a fracture ; so much so, that some specimens of Cactus senilis, said 

 to be one thousand years old, which were sent a few years since to 

 Kew from South America, were obliged to be packed in cotton, with 

 all the care of the most delicate jewellery, to preserve them during 

 transport. 



Raphides, of peculiar figure, are common in the bark of many trees. 

 In the hicoory {Caryaalha) maybe observed masses of flattened prisms 

 having both extremities pointed. In vertical sections of the. stem of 

 Elmagnus angustifolia, numerous raphides of large size may be seen in 

 the pith. Raphides are also found in the bark of the apple-tree, and in 

 the testa of the seeds of the elm ; each cell contains two or more very 

 minute crystals. 



In fig. 203 we have other representations of the crystalline struc- 

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

 Devizia scabia. This insoluble material is called silica, and is abun- 

 dantly distributed throughout certain orders of plants, forming a skele- 

 ton after the soft vegetable matters have been destroyed : masses of it, 

 having the appearance of irregularly-formed blackened glass, may 

 always be found after the burning of hay or straw ; it 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 dissolve the tissue by 

 boiling it in nitric acid for several hours ■ the organic portion is thus 

 destroyed, and the siliceous matter remains a perfect cast of the ori- 

 ginal structure. 



In the GraminacecB, especially the canes ; in the Equisetum hye- 

 male, or Dutch rush ; in the husk of the rice, wheat, and other grains, — 

 silica is abundantly found. In the Pha/rus cristatus, an exotic grass, 

 fig. 203, No. 2, we have beautifully-arranged masses of silica with 

 raphides. The leaves of the Deutzia, No. 3, are remarkable for their 

 stellate hairs, developed from the cuticle of both their upper and under 

 surfaces; forming most interesting and attractive objects when ex- 

 amined under the microscope, either by polarised or condensed light. 



The most generally-distributed and conspicuous of the cell-contents 

 is Starch; at the same time it is one of great value and interest, per- 



