CEYSTALS IN PLANTS. 



339 



yabra. 



crystals will be principally prismatic, and are arranged aa 

 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, rendering the 

 plant so exceedingly brittle, 

 that the least touch will 

 occasion a fracture; so much 

 so, that some specimens of 

 Cactus senilis, said to be a 

 thousand years old, which 

 were sent a few years since 

 to Kew from South America, 

 were obliged to be packed 



-^.i n .1 Fig. 184 Siliceous cuticle from 



in COtton, With all the Care surface of leaf of Deutzia scab 



of the most delicate jewel- 

 lery, to preserve them during transport. 



Raphides, of peculiar figure, are common in the bark of 

 many trees. In the Hiccory 

 (Carya alba) may be ob- 

 served masses of flattened 

 prisms having both extre- 

 mities pointed. In vertical 

 sections from the stem of 

 Elceagnus angustifolia, nu- 

 merous raphides of large size 

 are embedded in the pith. 

 Raphides are also found in 

 the bark of the apple-tree, 

 and in the testa of the seeds 

 of the elm ; every cell con- 

 tains two or more very 

 minute crystals. 



In figs. 184 and 185 we 

 have other representations 

 of the crystalline structure 



z 2 



. 185. Siliceous cuticl", of Qrtut 

 (Pharus cristatut), 



