in] GREEN CELLS OF CONVOLUTA 79 



as sugar, which consists of carbon, hydrogen and 

 oxygen, the plant prepares synthetically its own 

 nitrogenous food-substances, the proteins. Though 

 next to nothing is known of the details of protein- 

 synthesis as carried on by the plant, this much is 

 known, that the nitrogen contained in the proteins 

 is derived by the green plant from inorganic sources, 

 chiefly from nitrates which are absorbed in solution 

 from the soil or water in which the plant is growing. 

 Having thus manufactured its food-substances from 

 raw, inorganic materials, the plant is free to feed 

 upon them, that is, to use them either to build up 

 and repair its living substance (protoplasm) or to 

 convert them directly or indirectly into substances 

 (secretions) which enter into the composition of its 

 tissues. Thus, for example, from the photosynthesised 

 carbohydrate, are derived the cellulose substances 

 which form the enclosing shell or cell-wall within 

 which is contained each individual mass of protoplasm 

 which we call a cell or protoplast. But beside serving 

 such constructive purposes, much of the manufactured 

 food-substance, particularly the carbohydrate material, 

 is used for respiratory purposes, that is, for supplying 

 the energy wherewith the plant does the work of 

 living. By inducing compounds like sugar to unite 

 with oxygen, their decomposition and oxidation are 

 effected, with the result that energy is liberated and 

 simpler substances, e.g. carbon-dioxide and water, are 



