CHAPTER VII 



THE FOOD OF PLANTS 



PART I 



GENERAL VIEW 



SECTION 50. Nutritive Metabolism. 



A SEED or a spore contains but a small portion of the food material 

 which the plant will require during the course of its existence, and hence 

 .an additional supply of nutriment must be obtained from without. The 

 amount of food material required may be very great in comparison with 

 that contained in the original germ, as when an oak develops from an acorn, 

 or when a few fungal spores produce in a few days a mycelium many 

 thousand times heavier than their original weight. The percentage com- 

 position of a dried plant is such as to indicate that it is carbon and its 

 compounds, i.e. organic substances, which are of primary- importance to it. 



As a general rule nutrient material becomes of use to the plant only 

 after it has undergone various chemical changes, frequently extremely 

 complex in character, which are included under the head of metabolism. 

 The plant is able to assimilate absorbed food materials into its own living 

 substance, and so to obtain a continual supply of plastic material, while 

 at the same time various destructive processes of metabolism afford the 

 supply of energy necessary for continued existence. In the plant, just as 

 in the animal, both constructive and destructive metabolism are always 

 active, while the fact that a portion of the food must always be used to 

 provide the necessary supply of energy, prevents a plant ever containing 

 the whole of the material which has been absorbed in the process of 

 development. Indeed, it is of the utmost importance that plants should 

 be able to excrete those metabolic products of which no further use can 

 be made. 



The fact that a fungus or a higher plant may grow when fed with 

 sugar or glycerine shows that it is able to form from these substances all 



