202 A TEXT-BOOK OF BOTANY. 



Scrophulariaceae, Polemoniaceae, Verbenaceae, Euphorbiaceae, and 

 Urticaceae. 



SILICA is seldom found as a cell-content, and when present 

 never occurs as a crystalline deposit, being usually in the form of 

 amorphous masses, termed 'silica-bodies' (Fig. 109). The 

 latter arise either in the cell-sap or the silica is deposited on the 

 cell-wall, ultimately filling the lumen of the cell. In the palms the 

 silica-bodies resemble stalkless cystoliths (Fig. 109, C). They may 

 also occur in the form of long rods, being more or less fusi- 

 form or rectangular, or in the form of discs showing a more or less 

 sphero-crystalline structure, or in other special forms (Fig. 109). 

 Silica-bodies have been found in the Palrnae, Orchidaceae, Podoste- 

 maceae, and Rutaceae. Silica is insoluble in any of the ordinary 

 solvents, being dissolved only by hydrofluoric acid. On incinerating 

 the tissues it is not destroyed. The presence of silica may be deter- 

 mined readily upon heating sections with sulphuric acid or any 

 reagent that destroys organic matter. 



Silica usually occurs as an incrustation in the cell-wall, being 

 found in epidermal cells, spinose hairs, and even the palisade and 

 mesophyll cells of quite a number of plants. Siliceous walls are 

 rather characteristic of the genera of the following families: 

 Acanthaceae, Aristolochiaceae, Bignoniaceae, Borraginaceae, 

 Burseraceae, Calycanthaceae, Campanulacese, Chloranthaceae, 

 Combretaceae, Compositae, Cucurbitaceae, Dilleniaceae, Euphor- 

 biaceae, Gesneraceae, Goodeniaceae, Hydrophyllaceae, Leguminosae, 

 Loranthaceae, Magnoliaceae, Melastomaceae, Menispermaceae, 

 Oleaceae, Piperaceae, Proteaceae, Rosaceae, Rubiaceae, Santalaceae, 

 Saxifragaceae, Urticaceae, and Yerbenaceae. 



TANNINS AND TANNIDES.- -There is a group of water-soluble 

 principles that occur in the cell-sap, especially of parenchyma 

 cells, of a large number of plants. They are derivatives of phenol 

 and phenol acids, and give either dark blue or green precipitates 

 with solutions of ferric chloride. They were formerly designated 

 as tannins and distinguished according to the plants from which 

 they were obtained ; thus we had chestnut tannin, oak tannin, etc. 

 Recent studies on the constitution of these substances show that 

 there are two principal groups of tannins, (i) being in the nature 

 of glucosides and (2) the other not yielding any dextrose on 



