576 ELEMENTS OF PLANT HISTOLOGY AND MICROTECHNIC 



however, have absorbed it and are colored by it. It is colored a purple-red by 

 potassium hydrate, and an orange color by acids. In dry roots it takes on the form 

 of red flakes, and in the injured cells of fresh material it assumes the same form. 

 It may be extracted by alcohol from its yellow solution in uninjured tissues, but 

 in the red flake form it is not dissolved by alcohol. 



Rutin, C/zHsoOzs. This glucoside is widely distributed in plants. It crystal- 

 lizes from an aqueous solution in the form of minute light yellow crystals. The 

 yellow color of straw is, in part, due to it. When treated with ammonia or lime- 

 water, rutin forms a deep yellow solution, which turns to brown on exposure to 

 the air. 



Salicin, CuHisO;. Salicin is a glucoside which occurs in particular abundance 

 in the cortex of many poplars and willows. It may be dissolved by water, but more 

 readily by boiling water, by aqueous solutions of alkalies, and by acetic acid. It 

 is insoluble in ether. It crystallizes in the form of needles, scales, or thin plates. 

 It is colored by concentrated sulphuric acid, and, on the addition of a little water, 

 a red powder is thrown down in the sulphuric acid solution. 



Santalin. Santalin is the coloring matter of the red sandal-wood, Pterocarpus 

 santalinus. Santalin is insoluble in water, but is soluble in ether with a yellow 

 color and with 80 per cent, alcohol it gives a blood-red solution. Stronger alcohols 

 give the same result. It is also soluble in acetic acid and in aqueous alkaline 

 solutions. 



Saponin, CigHjoOio- This glucoside occurs in solution in the cell-sap. When 

 treated with a mixture of equal parts of alcohol and sulphuric acid, a yellow color 

 is produced which soon changes to red, and later to violet. If it is then treated 

 with a concentrated solution of chloride of iron, a brown or bluish-brown precipi- 

 tate is formed, the intensity of the bluish color increasing with the amount of 

 saponin present. 



Seminose. Seminose is one of the products resulting from the hydrolysis of 

 hemicellulose by sulphuric acid. It is dextrorotary, reduces Fehling's solution, 

 and is fermentable. 



Silica, SiO 2 . Silica occurs in the skeletons of diatoms, and as incrustations over 

 the epidermis of the Equisetacea^ and Gramineae. It also sometimes occurs in 

 masses in the interior cells. It may be isolated from the organic matter with which 

 it is associated by burning over a flame bits of epidermis incrusted by it, or diatoms, 

 which are placed in a drop of concentrated sulphuric acid on a piece of platinum 

 foil. By this treatment the organic matter will be destroyed, and the silica will 

 remain behind as a pure white ash. The silica may also be obtained pure by plac- 

 ing bits of tissues incrusted by it in a drop of concentrated sulphuric acid, and then 

 after a time adding 20 per cent, chromic acid, and following this with additions of 

 still stronger chromic acid until a considerable strength has been reached, and, 

 finally, washing in water and alcohol. Silica is distinguished by being insoluble 

 in all the acids excepting hydrofluoric acid. Silicious skeletons may be removed 

 from diatoms by placing the latter in hydrofluoric acid which is contained in a 

 platinum vessel. The vessel should be kept on a water-bath, and the diatoms 

 should remain in the acid for 24 hours. At the end of this time the acid should be 

 thoroughly washed out from the diatoms. On examination with the microscope, 

 the diatoms will then be found to have lost their silicious skeletons. In some 

 instances a thin exterior membrane which is stained brown by iodine is to be ob- 

 served; but in other instances this membrane has been a too insignificant part of 

 the skeleton to retain its identity after the removal of the silica. 



Sinapine, Ciel^NOs. This is an alkaloid occurring in the seeds of the white 

 mustard. When sections of these seeds are placed in a concentrated solution of 



