506 



PHYSIOLOGY. 



the substance of the parenchyma is rendered quite gritty to the touch by 

 crystals of calcium oxalate and phosphate (fig. 554 ) ; the Musacege con- 

 tain crystals of calcium sulphate, &c. The large spicular cells of Arauca- 

 ria and of Welwitschia, already referred to, are covered with small 

 crystals. 



Fte. 554. 



Fig. 555. 



Clustered crystals from Acic-ular crystals (raphides) in a cell 



Cactus. of Polyanthes tuberosa, magnified 



400 diam. : a, a single crystal, more 

 magnified. 



Crystals usually occur free in the cavity of the cell ; but in some 

 plants, especially in the Urticaceae, we find them accumulated on a 

 clavate process, formed of cellulose, developed from the side-wall 

 of the cell : these are called cystolitties. 



These curious structures are well seen in the subepidermal cells of the 

 leaf of Ficus elastica and other species also in Parietaria, the Mulberry, &c. 



Other important substances, such as the vegetable alkaloids and the 

 great number of organic acids usually associated with them, exist either 

 dissolved in the cell-sap, intermixed with the protoplasm, or diffused in 

 the solid cell-structures as impregnating or incrusting substances. With 

 regard to these, microscopic investigation has not hitherto afforded any 

 information. 



Sect. 3. COMBINATIONS OF CELLS. 



Tissues consist of collections of cells of uniform character com- 

 bined together by apposition or by more or less complete union of 

 their outer surfaces. They are produced by the aggregation and 

 juxtaposition of cells of equal age or degree, originally separate and 

 distinct, or more frequently by cells which are formed by repeated 

 subdivisions of preexisting cells. In the one case the tissue is mul- 

 tiple from the first as to its elementary constituents ; in the other it is, 



