72 



BOTANY 



albuminous substances, they may be stained, and also in that they are capable of 

 swelling by imbibition. Subjected to the action of water or a dilute solution of 

 caustic potash, they at first increase in size without losing their crystalline 

 outline. 



Crystals of Calcium Oxalate. — Few plants are devoid of such 

 crystals. They are formed in the cytoplasm, within vacuoles which 

 afterwards enlarge and sometimes almost fill the whole cell. In such 

 cases the other components of the cell become greatly reduced; "the cell 

 walls at the same time are often converted into cork, and the whole 

 cell becomes merely a repository for the crystal. The crystals may be 

 developed singly in a cell, in which case they belong either to the 

 tetragonal or monosymmetrical crystal system ; or, as is more frequently 

 the case, they form CRYSTAL aggregates, clusters of crystals radial 

 ing in all directions from a common centre. In the Liliaceae, Orchidaceae, 

 and other Monocotyledons, compact bundles of needle-shaped crystals of 

 calcium oxalate, the so-called raphides, are especially frequent (Fig. 80). 

 Such crystal bundles are always enclosed in a 

 large vacuole filled with a mucilaginous sub- 

 stance. The degree of concentration of the 

 mother liquor from which the crystals have 

 separated, determines, according to Kny, their 

 crystal form, whether tetragonal or monoclinic. 



Siliceous bodies, which are only soluble in hydro- 

 fluoric acid, are often found in the cytoplasm of many 

 cells, especially of Palms and Orchids, and often com- 

 pletely fill the whole cellular space. 



Tannin. — Highly refractive vacuoles filled 

 with a concentrated solution of tannin are 

 of frequent occurrence in the cytoplasm of 

 cortical cells, and may often grow to a con- 

 siderable size. 



The dark-blue or green colour reaction obtained on 

 treatment with a solution of ferric chloride or ferric 

 sulphate, and the reddish-brown precipitate formed with 

 an aqueous solution of potassium bichromate, are usually 

 accepted as tests for the recognition of tannin, although 

 equally applicable for a whole group of similar sub- 

 stances. 



Fig. 80.— Cell from the cortex 



of Dracaena rubra, filled fatty acid esters 



Fats and Oils in plants are mixtures of 



y acid esters. Frequently, as in species of 



^TSSSTS ^f" d ^e, a _ fatty oil appears in the 



of raphides, r. (x 160.) old chlorophyll grains. The occurrence of 



castor oil in the form of highly refractive drops 



in the cytoplasm of the aleurone-containing cells in the endosperm of 



the castor-oil seeds, has already been referred to. Oil usually occurs in 



