96 LIFE IN THE SEA [CH. 



and the sugars either pass directly into the blood 

 stream without further change or they are transformed 

 into compounds which are capable of assimilation. 

 But and this is the aim of the whole series of 

 digestive actions the reconverted proteid and fats 

 are different from those which were taken into the 

 alimentary canal and are now similar to those which 

 make up the tissues of the animal. They are the 

 true foods, and as such they are taken into the blood 

 stream and are carried over the body to be assimilated 

 by the cells of the tissues. 



The nutritive processes of plants are profoundly 

 different from those of animals. The proximate food- 

 stuffs of the plant organism (or the raw materials of 

 the food-stuffs, as they are sometimes called) are 

 salts of nitric acid or nitrous acid, salts of ammonia, 

 and carbonic acid. These are exceedingly simple 

 substances, and though the plant is able to make use 

 of more complex nitrogenous stuffs than the salts of 

 nitric acid or ammonia, yet the latter are a sufficient 

 source of food. The mineral salts are taken up from 

 solution in the water of the soil by the root hairs, and 

 the carbonic acid is taken in from the atmosphere 

 through the stomata of the leaves. If we are dealing 

 with a marine plant these substances are simply 

 absorbed from solution in the sea-water over the 

 entire surface of the organism. The carbonic acid 

 and water taken into the plant tissues are combined 



