34 PRACTICAL BOTANY. 



to light until the whole of the chlorophyll is dissolved out ; it 

 must then be treated for several hours in strong solution of 

 potash ; after neutralisation with acetic acid the tissue may be 

 treated with iodine. 



d. Oil-drops. 



i. Coloured black on treatment with osmic acid, 

 ii. Soluble in alcohol, in ether, and in potash (sapo- 

 nified). 



e. Mineral substances. 



i. Calcium oxalate : occurs with two molecules of 

 water of crystallisation in crystals belonging to the 

 clinorhombic system (including raphides), or with six 

 molecules in crystals belonging to the quadratic system. 

 Clusters of crystals and sphsero-crystals may consist of 

 crystals belonging to either system. Insoluble in acetic 

 acid ; soluble in nitric acid, without evolution of gas. 



ii. Calcium carbonate : occurs usually in small crystals, 

 the crystalline nature of which can only be ascertained 

 by means of the polariscope. Soluble in acetic acid, 

 with evolution of bubbles of gas (CO 2 ). 



iii. Calcium phosphate (also magnesium phosphate) : 

 occurs in the form of granules (e.g. the globoids). 

 Soluble in acetic acid without evolution of gas. 



iv. Calcium sulphate : occurs in the crystalline form. 

 Soluble with difficulty in acetic or nitric acid. 



/. Crystalloids : may be seen in the more external 

 cells of potato-tubers, in the form of cubes. 



i. They give the reactions characteristic of proteids. 

 ii. Soluble in potash. 



iii. Soluble in saturated solution of common salt. 



g. Aleurone-grains : occur most prominently in oily 

 seeds. 



