174 BOTANY AND PHARMACOGNOSY. 



of the leaf, the walls of which form an inward protrusion into the 

 cell and become impregnated with and encrusted by calcium car- 

 bonate, giving rise to more or less stalked bodies known as cysto- 

 liths (Fig. 221). The calcium carbonate dissolves on the 

 application of acetic acid, leaving a core which responds to the 

 tests for cellulose. Cystoliths are not of common occurrence, 

 being found with but few exceptions in the two families Acan- 

 thaceae and Moracese, and in a few species of the Cucurbitacese. 

 In the leaves of the cultivated rubber plant the cystoliths have 

 long stalks, whereas in cannabis indica (Fig. 279), they are 

 sessile. 



Tannin and Tannoids. — Tannins are astringent principles 

 which belong to the class of phenol acids and give blue or green 

 precipitates with iron salts. The tannoids, in addition, precipitate 

 albuminous compounds, and when applied to animal hides con- 

 vert them into leather. These principles are widely distributed, 

 occurring dissolved in the cell-sap, in parenchyma cells or 

 in distinct reservoirs or vessels, and vary in amount from 

 I per cent, or less to as high as 70 per cent, in Chinese galls. 

 Tannin occurs in relatively large amount in the following fam- 

 ilies : Anacardiacese, Betulaceae, Cornacese, Ericaceae, Fagacese,. 

 Geraniaceas, Hamamelidacese, Lythracese, Leguminosse, Myrtacese, 

 Plumbaginacese, Polygonacese, Rosacese, Rubiaceas, Salicacese, 

 Saxifragacese and Filices. It not only occurs in the normal cells 

 of plants but also in the excrescences known as galls, which are 

 produced by the sting of various species of the insects Cynips 

 and Aphis. 



The following microchemical reagents are used in the study 

 of the tannins and tannoids: Carbonates of the alkalies, chromic 

 acid, cupric acetate, iron acetate, lime water, methylene blup,, 

 potassium dichromate, sodium wolframate, ammonium molybdate, 

 bromine water. 



Mucilages and Gums. — By the terms mucilages and gums 

 are meant those substances which are soluble in water, or swell 

 very perceptibly in it, and which, upon the addition of alcohol, 

 are precipitated in the form of a more or less amorphous or gran- 

 ular mass. Mucilage originates in the plant as acell-content, or 

 as a modification of the wall. In the former case it arises as a 



