2i6 THE LIFE OF THE PLANT 



somehow arrested ; for instance, if during the process 

 of absorbing water the cell should lose it by evaporation, 

 the cellulose formed from the protoplasm will go on 

 forming on the inner side of the undistended wall and 

 cause it to thicken. This thickened wall, in its turn, 

 yields still less to the pressure of the sap, and retards the 

 growth of the cell still further. Thus we may explain 

 the above-mentioned fact that checking the growth of 

 straw goes hand in hand with the thickening of its cell- 

 walls. We notice at the same time that this retardation 

 in the growth of organs and the simultaneous thickening 

 of their cell -walls must happen whenever there is a 

 deficiency of water. Probably the inhibitive influence 

 of light upon growth, which we have already studied, 

 depends upon the fact that plants evaporate more water 

 in the light ; and therefore the pressure of sap upon the 

 cell-wall, which causes growth, will not be so great as 

 when there is an abundance of water, in the shade or in 

 the dark. If the phenomena of heliotropism may thus 

 be connected with the phenomena of the evaporation of 

 water, perhaps we may also explain in the same way the 

 particular case of the transmission of the effects of 

 heliotropism in Darwin's experiments upon the seedlings 

 of cereals. You remember the energy with which these 

 organs exude water at the tips — from which we may 

 conclude that these same tips give off water vapour at a 

 similar rate. This loss of water must be made up by 

 the lower girdle of growth. Hence one-sided illumination 

 will cause one-sided transpiration and growth, as the re- 

 sult of which there will be a curvature towards the light. 

 We have already seen that the growth of tissues can 

 be retarded by direct mechanical pressure (as in the 

 growth of the wood) ; now we notice that it can be also 

 retarded by the weakening of the internal pressure, and 

 this retardation is accompanied in both cases by greater 

 thickening of the cell-walls. Under the influence of a 

 one-sided action of external factors, the growth of 



