706 PART IV. THE PHYSIOLOGY OF PLANTS. 



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. 



§ 9. Metabolism. This subject will be subdivided into: 1, 

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

 bolism ; 5, Products of Metabolism ; 6, Supply and Expenditure of 

 Energy. 



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 

 air contain from 12 to 15 per cent, of water, herbaceous plants 60 

 to 80 per cent., and many water-plants and Fungi as much as 

 95 per cent, of their whole weight. 



The residue, which gives off no more water at a heat of 100° C, 

 the dry solid, consists of a great variety of chemical compounds ; 

 these are partly organic, that is to say, combinations of carbon 

 with other elements, and partly inorganic. These organic sub- 

 stances which occur in the living plant (with the exception of 

 salts of oxalic acid) all contain hydrogen. Some of them, such as 



