?8 BOTANY I'AKT i 



Crystals of Calcium Oxalate. Few plants are devoid of such 

 crystals. They are formed in the cytoplasm, within A r acuoles Avhk-h 

 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 are of consiiU-i 

 able size and can be seen to belong either to the tetragonal or 

 monosymmetrical crystal system ; or the crystals are so small and 

 numerous that their form cannot be clearly made out and they appear 

 as a crystalline sand filling the cell. Frequently they form CRYSTAL 

 AGGREGATES, Clusters of crystals radiating in all directions from a 

 common centre. In the Liliaceae, Orchidaceae, and other Mono- 

 cotyledons, compact bundles of needle-shaped crystals of calcium 

 oxalate, the so-called RAPHIDES, are especially frequent (Fig. 85). 

 Such crystal bundles are always enclosed in a large vacuole filled 

 with a mucilaginous substance. The degree of concentration of 

 the mother liquor from which the crystals have separated, deter- 

 mines, according to KNY ( 71 ), their form, whether tetragonal or 

 monoclinic. 



SILICEOUS BODIES, which are only soluble in hydrofluoric acid, are found in the 

 cytoplasm of many cells, especially of Palms and Orchids, and often completely 

 (ill the whole cell. 



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 considerable 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, an: 

 usually accepted as tests for the recognition of tannin, although equally applicable 

 for a whole group of similar substances. 



FATS and OILS in plants are mixtures of fatty-acid esters. Frequently, a> in 

 most Monocotyledons, a fatty oil appears in the 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 this form. But fatty substances may also 

 appear in the cytoplasm as irregularly shaped, more or less soft grains, as for 

 example in the vegetable butters and in the wax of various seeds ; they may even 

 be crystalline, as in the needle-like crystals of Para-nuts (Bertholletid exwlsa) and of 

 Nutmeg (Myristica fragrans). Special cytoplasmic structures in which the drops 

 of oil are embedded, are of frequent occurrence in the epidermal cells of Oreliidac < -.n- 

 and Liliaceae and go by the name of elaioplasts ( 72 ). 



CHOI.KSTK.UIN. Since this substance is a regular constituent especially of givi-n 

 cells, it probably plays an important part in metabolism ( 7:i ). 



(ii.vccici.N. This substance ( 74 ), related to starch, and of frequent occurrence 

 in animal tissues, fulfils, according to EKKKKA ( 75 ), the same functions in the 

 Fungi as sugar and starch in the higher plants. Cytoplasm containing glycogen 

 is coloured a reddish brown with a solution of iodine. This colour almost wholly 

 disappears if the preparation be warmed, but reappears on cooling. 



