THE PLANT AND ITS PHYSICAL ENVIRONMENT. 199 



influenced by the conditions of temperature, that they only proceed 

 at all between certain limits, but that within these limits the rate is 

 largely a function of the temperature. As the limits, however, are 

 approached, other factors become operative and arrest or even reverse 

 the reaction. A recognition of these facts enables us to appreciate 

 at once the vital importance of temperature in connexion with the 

 chemical changes that go on within a plant, and especially those which 

 are concerned with the elaboration or utilization of food substances. 

 This is equally true whether we are thinking of the complicated 

 chemical compounds which are built up within the plant from the 

 simpler bodies that are absorbed as food materials from the soil and 

 the air, or whether we are considering the processes by which the 

 food reserves of a seed begin to undergo those changes necessary to 

 enable germination to begin and to be carried to a successful issue. 

 In these and all similar cases we are dealing with chemical transforma- 

 tions, and the chemical transformations are dependent {inter alia) 

 on appropriate temperature conditions, though it is seldom, or perhaps 

 never, that temperature alone is concerned. But at any rate, in order 

 to ensure suitable development, this factor must really be properly 

 adjusted, otherwise what we call diseased or abnormal conditions of 

 growth supervene — if, indeed, any development takes place at all. 



But the influence of temperature does not stop short at permitting 

 various chemical changes in the not-living organic contents to go on 

 with greater or less rapidity, and subject to definite laws. I have said 

 the plant may be likened to a laboratory ; it is very well stocked 

 with delicate apparatus, and this apparatus, which as a whole constitutes 

 what we call the living stuff, provides the means for the carrying on 

 of the processes which I have just indicated under the general name 

 of chemical change. The apparatus itself — the framework or matrix 

 in which these changes are proceeding — is of a complexity at present 

 immeasurable. It is also, as I have said, itself affected by the con- 

 ditions, or at any rate by some of them, which build up or break down 

 the relatively simple substances within it. We are not dealing with 

 a simple machine like a steam engine, in which the coal, water, &c., 

 merely provide the energy for causing the mechanism to work. Vital 

 mechanism is a vastly more complex thing. For it is itself subject to 

 change, and any alteration in its own conformation will produce effects 

 as yet for the most part incalculable in the final result. We can 

 visuaUze, in a simple instance, how an alteration in the end products 

 may be brought about by a slight modification of the mechanism if 

 we remember the way in which an ordinary pendulum clock becomes 

 fast or slow as the temperature falls or rises. But vital mechanism 

 is more complicated than this. 



Let us consider, as an example taken from plants, the problem 

 of root pressure, or bleeding. Every gardener knows that if a vine or 

 other plant is cut in the early spring it is apt to continue to discharge 

 a watery sap from the cut surface. This is water which has been 

 forced, into the stem by the roots, which in their turn have absorbed 



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