THE PLANT BODY 85 



fied for reproduction. Before turning to the flower which 

 in its function has to do with the race rather than the 

 individual, it is important to consider the organism as a 

 whole how the various cells, tissues, and organs cooperate 

 in the nutrition of the living plant; for nutrition, it will be 

 recalled, is the function of primary iiriportance to the indi- 

 vidual. 



The essentials of nutrition were readily described in the 

 simple green plant Sphaerella, because the whole organism 

 comprises but a single cell which directly interchanges matter 

 and energy with its environment. But with the establish- 

 ment of the complex plant body, an organization of many 

 millions of highly specialized cells, the intricate interrelation- 

 ship of these various parts to the nutrition of the whole 

 the mechanical engineering of the plant becomes a problem 

 in itself. 



1. Circulation Paths 



The green plant, as we know, takes in the raw materials 

 and builds them up into its foodstuffs. In the case of the 

 higher plants, water in large amounts is taken in through the 

 root hairs. Dissolved in this water are various substances 

 nitrates, phosphates, sulfates, etc. supplying most of the 

 elements that are necessary for the make-up of protoplasm. 

 The leaves admit carbon dioxide through the stomata. Thus 

 the substances which are to be built up into foodstuffs enter at 

 the opposite ends of the plant, and must be brought together 

 in a chemical laboratory, as it were, in order that their union 

 may be effected. The organ in which food construction takes 

 place is the leaf, and, specifically, in the chloroplastids of the 

 chlorenchyma cells. Accordingly we must consider the high- 

 ways which bring the raw materials from the root hairs to 

 the leaves and those which distribute the finished products 

 to the various parts of the plant for their use. (Fig. 48.) 



