428 



PLANT RESPONSE 



conditions has become uniform. This occurs, generally 

 speaking, after a period of variation which does not exceed 

 two or three minutes. I give here a table embodying the 

 results of the experiments on Balsam, and on Crinmn, which 

 show how increase of internal hydrostatic pressure increases 

 the rate of growth up to an optimum, after which there is a 

 diminution of growth. 



Table showing Effect on Growth of Increased Internal 

 Hydrostatic Pressure 



The curve seen in fig. 174 exhibits graphically the rela- 

 tion between these internal pressures and corresponding 

 growths in the case of Balsam seedling. It will be seen 

 that after a certain moderate rate of growth has been 

 attained by increase of pressure, the curve becomes a straight 

 line ; that is to say, after this point, equal variation of pres- 

 sure produces equal variation in the rate of growth. But in 

 the first part of the curve, where the rate of growth is feeble, 

 an equal increase of pressure causes a disproportionately 

 large increase in the rate of growth. This is still more 

 strikingly shown when the growth, to begin with, is zero — 

 that is to say, at standstill ; in such a case, by gradually 

 increasing the internal pressure, we arrive at a point where 

 growth begins abruptly, after which increasing pressure 

 causes an increasing rate of growth. But if the pressure be 

 now brought back to a point just below that at which growth 

 was initiated, it is found not to be arrested, but to persist. 

 Thus the curve does not here return upon itself 



Since the various growth-curvatures are brought about by 

 the variations of internal hydrostatic pressure caused by the 



