45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 185 



This layer is not the endodermis, but lies externally to it ; the 

 endodermis frequently contains starch-grains, and is sometimes 

 termed the starch-sheath, but it is rather a depository than a con- 

 ducting-tissue. 



The direction in which organic substances travel in the plant 

 seems to be determined simply by the demand for them. Just as 

 the water and the substances in solution absorbed by the roots 

 travel to the transpiring and assimilating organs, so the organic 

 substances produced in the assimilating organs travel in the 

 plant to those parts in which organic substance is either being 

 used in growth, or is being stored up as reserve material. In a 

 Potato-plant, for example, part of the organic substance formed 

 in the leaves travels to the growing-points of the roots and of the 

 shoots, where it is required for the development of new leaves, 

 flowers, branches, etc., whilst the residue travels to the under- 

 ground shoots which are developing into tubers and are storing up 

 quantities of starch. Similarly, these organic substances travel 

 apparently by the same channels and in the form of the same 

 chemical compounds, from organs which serve as depositories of 

 reserve material, when these stores are drawn upon to supply the 

 growth of developing parts. For instance, when a Potato-tuber 

 begins to sprout, the starch, which is the principal reserve 

 material, is drawn upon, being gradually converted into sugar, 

 in which form it travels to the growing-points of the young shoots 

 and supplies a large proportion of the plastic material necessary 

 for their growth. 



45. Metabolism. This subject will be subdivided into : 1, 

 Chemical Composition ; 2, Food of Plants ; 3, Anabolism ; 4, Cata- 

 bolism ; 5, Products of Metabolism. 



1. Chemical Composition. As a preliminary, a general account 

 of the chemical composition of plants will be given. 



All parts of living plants contain a considerable quantity of 

 water : this forms not merely the principal constituent of the cell- 

 sap, but also saturates the cell-walls, the protoplasm, in short, all 

 organised structures ; it is, in fact, one of the peculiarities of or- 

 ganised structures that minute particles of water are interposed 

 between the particles of solid matter of which they consist. By 

 heating to 100 or 110 C., all the water contained in any part of 

 a plant is expelled, and in consequence it will naturally lose 

 weight. The amount of this loss, that is, the quantity of contained 

 water, is very different in various plants ; ripe seeds dried in the 



