THE KINDS OF STORED FOOD l8l 



almonds. The enzyme emulsin, occurring with it, splits it 

 into oil of bitter almond, prussic acid and glucose. Other 

 glucosides are potassium myronate in mustard seeds, solanin 

 in many Solanaceae, such as bittersweet and Irish potato, sali- 

 cin in the bark and leaves of willows, caniferin in the wood and 

 cambium of Conifers, digitalin, the poisonous substance in 

 Digitalis purpurea, indican, occurring in species of Indigofera, 

 which by its enzyme is broken down into a kind of sugar and 

 indigo-blue. 



The full list of known glucosides would be a long one, and 

 there is a multitude of bitter products in plants apart from the 

 alkaloids, many of which will yet be found belonging to the 

 group of glucosides. 



Mucilage. — This is characterized by its swelling enormously 

 in water. It is known to be stored as reserve food in the tubers 

 of some orchids and in the seeds of a few Leguminosae. It is 

 clearly related to reserve cellulose in its chemical nature, and 

 like the latter is transformed to sugar in its digestion. 



Proteids. — Proteids are the most complex of plant foods 

 and at the same time the most important, since they are the 

 chief constituent of the living protoplasm itself. Their exact 

 chemical constitution has not been determined, but it is known 

 that they contain carbon, hydrogen, oxygen and nitrogen, and 

 many contain in addition sulphur, and a less number have 

 phosphorus also. 



The food substances here enumerated before the proteids 

 contain no nitrogen with the exception of some of the gluco- 

 sides, and it is the proteids chiefly that contain the nitrogen 

 supply of plants, 50 to 90 per cent, of the nitrogen in the vege- 

 tative parts being held by proteids, and in seeds and spores 90 

 to 98 per cent, of the nitrogen is contained in the proteid reserves. 



The proteids occur in three distinct conditions: as definite 

 rounded granules known as aleiirone grains, as smaller amor- 

 phous proteids, and as soluble proteid, which under normal 

 conditions is in solution in the cell-sap. 

 [^Aleurone grains occur most abundantly in seeds associated 



