148 THE ELEMENTS OF VEGETABLE HISTOLOGY 



larger crystals, and occurs in many powders. In 

 materials containing sand or earth, care must be 

 taken not to confuse crystalline quartz particles with 

 calcium oxalate crystals. Various types of plant 

 crystals are illustrated in Plate 53. 



Tannins. — These materials occur as brownish or 

 blackish masses without characteristic form. 



Gums. — The gum deposits may fill the entire cell 

 and are usually observed by examining the material 

 in an aqueous mounting medium which causes swell- 

 ing . of the contained gum. The mucilage formed 

 upon the addition of water appears in the form of 

 highly refractive globules, which remain light colored 

 after the diaphragm is closed (Plate 50, Nos. 3 

 and 4). 



Resins. — Resin deposits in plant cells differ greatly 

 in color and lack characteristic form. 



Fats and Oils. — These substances appear as highly 

 refractive globules which may be mistaken for air 

 bubbles in well-lighted fields. When the diaphragm 

 is closed, the outer margin of an air bubble increases 

 in thickness, while that of an oil globule does not. 



Silica Deposits. — This material occasionally occurs 

 in epidermal and fiber cells. The deposits appear 

 as white, spherical bodies with numerous small spines 

 or projections upon their surface. 



Calcium Carbonate. — This substance appears in 

 grayish masses of indefinite form and may possibly 

 be mistaken for compound starch grains. The deposits 

 are termed cystoliths and are composed of numerous 

 small globular particles of carbonate (Plate 52, Nos. 

 6, 7, 8). Aside from the absence of a hilum in the 

 small globules forming the masses, they may be dis- 



