CONTENTS OF THE TISSUES. 



63 



tened and distended by endosmosis, they forcibly discharge their 

 contents, in a curious manner, from an orifice at each end, as is 

 shown in Fig. 55. These acicular crystals are generally thought 



to consist of oxalate of lime ; according to Quekett, they are phos- 

 phate of lime. Of other crystals composed of inorganic acids and 

 an earthy base, the more usual are rhombic crystals of carbonate 

 of lime, found in Cacti ; and tabular, often twin crystals of sul- 

 phate of lime, which are " found in the Musacese and many Scita- 

 minese." Such are wholly formed of inorganic materials, derived 

 from the soil. 



92. Silex, likewise derived from the soil, very generally occurs 

 as a part of the deposit or incrustation on the walls of cells ; * but 

 it is not found in the form of crystals in their interior. In the Dia- 

 tomaceaB nearly the whole cell-wall is composed of this indestruc- 

 tible material ; consequently, the remains of these minute organ- 

 isms accumulate at the bottom of the water in which they live, so 

 as to form immense strata in many places. 



* This may be shown by carefully burning off the organized matter of the 

 tissue, and examining the undisturbed ashes by the microscope. 



FIG. 53. Raphides, or acicular crystals, from the stalk of the Rhubarb : three of the cells 

 contain starch or chlorophyll, and two of them raphides. 



FIG. 54. Raphides of an Arum, contained in a large cell; and 55, the same, detached from 

 the surrounding tissue, and discharging its contents upon the application of water. 



FIG. 56. Crystals from the Onion ; one of them a hemitrope. 



FIG. 57. Crystals of the inner bark of the Locust. 



FIG. 58. A glomerate mass of crystals from the Beet-root. 



FIG. 59,60. Crystals from the bark of Hickory. Figures 55-60, and also 51, are from 

 sketches kindly supplied by Professor Bailey of West Point. 



