SYMBIOSIS 15 



carbohydrate (sugar) is derived from the radiant energy of light. The 

 green pigment, chlorophyll, which is associated in the green cells of the 

 plant with specialised, granular bodies called chloroplasts, absorbs light, 

 and in some way, as yet imperfectly understood, this radiant energy 

 is utilized by the protoplasm of the chloroplasts in the manufacture of 

 sugar. The plant possesses also the power of synthesising yet more 

 complex substances. Besides carbohydrates such 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 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 photo- 

 synthesised 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 besides 

 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 produced. The liberated energy serves for the performance 

 of the work which the living plant must do, and also, converted into 

 heat, contributes to maintain the temperature of the plant's tissues 

 at a proper level. The surplus of carbohydrate and of protein not used 

 for constructive or respiratory purposes the plant puts by for future 

 use. The starch, oil and nitrogenous substances contained in seeds, 

 tubers, and other storage-organs of plants respresent this reserve food- 

 material. 



The power possessed by the green plant of manufacturing food- 

 materials in excess of its immediate needs is the lever which makes the 

 whole world of animal life to move. For the animal has no such 

 synthetic powers, and yet it requires the same food-substances as the 

 plant. Hence it is constrained to take them from the plant. The 

 aphorism "all flesh is grass" is no mere figure of speech, but a terse 

 statement of truth. 



The facts of the wonderful and partly still secret powers 

 of manufacture possessed by plant protoplasm may be 



