202 ABSORPTION OF FOOD-MATERIALS 



Animals and parasitic and saprophytic plants obtain these 

 compounds directly or indirectly from the bodies of other living 

 or dead organisms, and without a supply of such substances they 

 soon die. Green plants likewise need food of a similar complex 

 nature for development and growth ; they are, however, not 

 generally adapted to obtain compounds of this character from 

 their surroundings, but are able to manufacture them from 

 inorganic compounds such as carbon dioxide, water, and various 

 salts which they derive from the atmosphere and the soil. 



Although these simple inorganic materials absorbed from the 

 air and the soil are frequently spoken of as the food of plants, 

 it is better perhaps to speak of them as food-materials, for the 

 living substance of a plant cannot directly nourish itself upon 

 them. It is only after they have been elaborated or built up 

 into more complex compounds that they become food which 

 can be used for the nutrition of the protoplasm and the formation 

 of the tissues of growing organs. 



A seedling after it has consumed the food stored for its use 

 by its parent, is unable to make use of carbon dioxide and simple 

 salts supplied to it until it is exposed to light under certain 

 conditions which allow it to elaborate and synthetically build up 

 from these inorganic materials compounds similar to those 

 which it has already consumed, and which were supplied and 

 manufactured previously by its parent. 



2. Food-materials and their absorption. — The food-materials 

 absorbed by ordinary green plants are derived from the sur- 

 rounding atmosphere and soil upon which the plants grow. 



By the methods of sand-culture and water-culture it has been 

 proved that for complete and perfect nutrition, green plants 

 must be supplied with food-materials which contain collectively 

 some ten or eleven elements as explained in chapter xii. 



It has also been determined by the same experimental 

 methods that plants are by no means indifferent as to the form 

 in which any particular element is presented to them. For 



