THE METABOLISM OF PLANTS. 29 1 



detailed consideration of these various phenomena ; it will 

 suffice for the present to state generally that the conditions 

 which are essential to destructive metabolism are also essen- 

 tial to the exhibition of these phenomena. We have seen, 

 for example, that temperature has an important influence 

 upon the processes of destructive metabolism, and we shall 

 find that it has a marked effect upon growth and movement. 

 Again, the absorption of free oxygen is of importance in 

 destructive metabolism, and we shall find that the evolution 

 of heat and light by plants, their growth, and their move- 

 ments, are likewise dependent upon the absorption of free 

 oxygen, excepting, of course, in those plants which, as we 

 have seen (p. 211), are capable of living in the absence of 

 oxygen. We will now consider these phenomena individually 

 and in detail, and we will begin with growth. 



Growth. Before we can enter upon a detailed considera- 

 tion of the relation of growth to destructive metabolism we 

 must understand what we mean by "growth." By growth we 

 mean permanent change of form accompanied usually by 

 increase in bulk. We must clearly distinguish between growth 

 and constructive metabolism. Growth can, it is true, only go 

 on when the necessary material is supplied by the processes 

 of constructive metabolism, but a mere increase in the dry 

 weight of the organic substance of a plant does not necessa- 

 rily imply that it is growing, for increase in weight may take 

 place without increase in bulk or change of form. When, for 

 instance, in the development of the seed, the cells of the 

 endosperm, or of the perisperm, or of the cotyledons, become 

 filled with reserve-materials in the form of aleurone-grains, 

 starch-grains, etc., we have a great increase in the dry weight 

 of the organic substance without a corresponding growth of 

 the organ. Nor does an increase even of the organised struc- 

 tures of an organ, that is, of the protoplasm and the cell- wall, 

 necessarily imply that it is growing. Thus, an increase of 

 the cell-wall may take place without any perceptible enlarge- 

 ment of the cell, as, for instance, when a cell-wall thickens : 

 in fact it appears from the researches of Strasburger that the 

 cell-wall does not become thickened so long as the cell is 



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