288 THE FOOD OF PLANTS 



the numerous carbon compounds which take part in metabolism, and by 

 the oxidation of which a supply of energy is obtained. Even when a cell 

 or organ becomes adult a continuance of metabolism is essential so long as 

 life remains, although as a matter of fact the energetic metabolic changes 

 which may still proceed are mainly directed towards the liberation of an 

 adequate supply of energy. Indeed, only a small portion of the organic 

 food-supply may be used as constructive material even by an organ which 

 is growing rapidly, for without the continual supply of energy which the 

 katabolic processes afford, the vital mechanism must come to a standstill 

 as certainly as a steam-engine must cease working when the fires are 

 extinguished. Hence, both animals and plants die of hunger as soon as all 

 the reserve supplies which can be made available as food are consumed, 

 and no further supply of energy or food can be obtained. Though 

 dependent for its existence upon its internal metabolism, the plant or the 

 protoplast can exercise a certain regulatory control over its own metabolic 

 activity. 



Nutrition involves chemical metamorphoses of extremely varied 

 character, and hence a few general remarks are necessary upon the nature 

 of the changes which the food material may undergo. Nutrient substances 

 are frequently rendered capable of absorption by extracellular agencies (as in 

 digestion, &c.), while stored food, by undergoing intracellular metamorphosis, 

 may be transformed into a mobile product which can be translocated to the 

 points where it is required. Moreover, chloroplastids are able to produce 

 sugar or other food material by the remarkable synthetic metabolism of 

 which they are capable under appropriate conditions, and the sugar thus 

 produced plays precisely the same part in the general metabolism of the 

 plant as it would if it were absorbed directly from without. Similarly, all 

 plants appear to be able to form proteicl substances, although these may 

 be entirely absent from the food material. The production of proteids 

 by synthesis is a step towards the formation of living protoplasm, and 

 hence is to be regarded as the means by which the latter is constructed 

 and nourished. It is, however, frequently impossible to distinguish clearly 

 between those changes which serve to provide energy and those which 

 render the nutrient material available for use. Indeed, it is probable 

 that the two processes are inextricably associated, owing to the intimate 

 relationships which exist between all vital processes, and their mutual 

 dependence upon one another. 



Under the heading of ' metabolism ' all those chemical changes are 

 included which are produced by the organism and are of service to it, 

 no matter whether the changes are intracellular or extracellular in origin, or 

 what may be the nature of the causes to which they may be due, or of the 

 means by which they may be performed. The processes occurring in 

 a living organism cannot possibly be expressed in the form of a chemical 



