l66 CHEMICAL COMPOSITION OF PLANTS 



complex compounds to which no chemical formula can yet be 

 given. They are generally slimy like the white of an egg, and 

 like the latter substance many of them coagulate on heating; 

 some of them are soluble while others are insoluble in water. 

 The simplest proteins are composed of carbon, hydrogen, oxygen, 

 nitrogen and sulphur; they contain from 15 to 17 per cent, of 

 nitrogen and from J to 3 per cent, of sulphur. 



As protoplasm consists largely of proteins, they are met with 

 in all living parts of plants : moreover some of these compounds 

 are found dissolved in the cell-sap. 



Certain proteins are stored in the vacuoles and cell-sap of seeds 

 and other resting-organs as nitrogenous reserve-food in the form 

 of round or irregularly-shaped solid grains; such grains are 

 termed Aleuron- ox protein-grains. In cereals the aleuron-grains 

 are very small and round, and are chiefly stored in the outermost 

 cell layers of the endosperm (Fig. 81). In other starchy seeds 

 such as beans and peas they are small, but in many oily seeds 

 such as those of the castor-oil plant and Brazil-nut, the aleuron- 

 grains are large, and generally contain a small round particle or 

 globoid of calcium and magnesium phosphates, together with a 

 larger or smaMei proiein-crysfa/ or crystalloid. 



The seeds of the lupin contain on an average about 34 per 

 cent., beans 24, wheat-grains 13, barley-grains 10, straw 3, 

 potatoes 2, and turnips about i per cent, of proteins. 



Solid proteins stain a yellow colour with iodine. 



Ex. 94. — [a) Divide a wheat-grain in two transversely : then cut a thin 

 section to include a small portion of the pericarp and aleuron-layer as in Fig. 81. 



Mount in dilute glycerine and run a drop of iodine solution untfer the cover- 

 slip : note the colour of the starch-grains and the aleuron-grains. 



(fi) Cut a similar section of a barley and an oat grain. Are the aleuron- 

 layers in these grains the same as in wheat ? 



Ex. 95. — Cut sections of the cotyledons of beans and peas: mount and 

 examine in dilute glycerine ; note the small aleuron-grains in the cells along 

 with large starch-grains : stain with iodine and re-examine. 



2. Amides. — These are soluble crystalline nitrogenous com- 



