INTERNAL ENERGY AND EXTERNAL STIMULUS 429 



mm-i 



action of external stimulus, this quantitative demonstration 

 of the effect of internal pressure on growth, is of much 

 theoretical importance. 



I have already shown the connec- 

 tion between mechanical response and 

 growth-response, and demonstrated 

 the fact that the erectile mechanical 

 response and growth-elongation are 

 but different expressions of increased 

 internal activity. I shall now show 

 how the suctional and growth re- 

 sponses are related to each other, and 

 in what manner the action of the 

 former affects the latter. 



(5) Effect of ascent of sap on 

 growth. — We have already seen that 

 the positive turgidity-variation on 

 which growth depends is brought 

 about, under normal conditions, by 

 the ascent of sap. As regards the 

 latter, we have seen that when the 

 root is subjected to the stimulating 

 action of warm water there is a 

 sudden augmentation induced in the rate of suction. The 

 application of cold water, on the other hand, induces the 

 converse effect. The mechanical response of the plant to 

 warm or cold water, applied at the base, was shown to be 

 manifested in the erection or depression of the leaves of 

 Mimosa or Biophytum (p. 400). 



These agents are seen in the following experiments to 

 produce parallel effects on growth. A growing Crinum Lily 

 was taken, and its normal rate of growth ascertained to be 

 •005 mm. per minute. Ice-cold water was now applied at its 

 base, and this was found to cause an almost immediate arrest 

 of growth. As the temperature, however, was gradually 

 restored to that of the surroundings, the rate of growth was 

 also slowly recovered. Five minutes after, the rate was only 



Pig. 174. Curve showing 

 Relation between Internal 

 Hydrostatic Pressure and 

 Rate of Growth {Crinum 

 Lily) 



