CHAPTER XV. 



THE ABSORPTION OF FOOD-MATERIALS. 



I. Food and food-materials. — The protoplasm or the living 

 material within actively growing plants and animals is continu- 

 ally undergoing chemical changes which result in its destruction 

 and the formation from it of simpler compounds. To repair its 

 waste and to enable it to carry on the work of constructing new 

 parts, food is necessary. 



The nature of the food of a plant, or the substances which are 

 utilised by the protoplasm for the formation of new organs and for 

 its own nutrition, is most readily understood after a consideration 

 of the materials which are consumed during the growth of an 

 embryo plant from a seed. 



The substances stored by the parent in the endosperm or within 

 the tissues of the embryo for the nutrition of the latter are chiefly 

 complex organic compounds such as starch, fats, and proteids, 

 and it is these substances, or very slightly altered forms of them, 

 which are consumed in the processes of nutrition and growth 

 which occur when germination commences. 



Similarly, the substances upon which the young shoots of a 

 sprouting potato tuber or the young leaves and flowering shoots 

 of a growing bulb are fed, are carbohydrates, fats, and proteids 

 or organic compounds of analogous complex constitution. 



The developing buds of a tree in spring are also nourished by 

 similar compounds, and there is every reason to conclude that 

 the protoplasm in plants and animals alike, depends at all 

 times for its immediate nutrition upon organic materials of this 

 character. 



