INTERNAL ENERGY AND EXTERNAL STIMULUS 427 



growth-response was initiated after a latent period of eleven 

 seconds (fig. 173). 



It will be remembered that the positive turgidity-variation, 

 on which growth depends, is hydrostatically transmitted at a 

 much quicker rate than the state of excitation itself. In the 

 present case, the responsive elongation in the growing region at 

 a distance of about 12 cm. took place eleven seconds after the 

 application of water at the root. There was a certain loss of 

 time before the cells in the growing region became of sufficient 

 turgidity to initiate growth. The small supply of water which 

 had been given was enough to maintain growth for three minutes 

 only, after which the plant came again to a standstill. Another 

 2 cc. of water was now applied, and the latent period was 

 found reduced, as we should have expected, being now three 

 seconds only. This was due to the fact that the cells had not 

 now to absorb water before they could be sufficiently turgid. 

 This renewed growth-activity was again, however, exhausted 

 after about three minutes ; and it was very interesting to 

 observe how the response of growth followed, for a little while, 

 after each such doling out of water. 



(4) Effect of increased internal hydrostatic pressure. — 

 We found, in the case of Desmodium, when the cut petiole, carry- 

 ing the motile leaflets, was subjected to increased hydrostatic 

 pressure, applied by means of the U-tube, that the rhythmic 

 activity of the leaflet, as shown by its quickening, was thereby 

 increased (p. 320). We saw also in that case that this increased 

 frequency, due to increased pressure, reached an optimum, 

 and that beyond this, under excessive pressure, the pulsations 

 became irregular, or even came to a stop. 



In order, then, to study the effect of hydrostatic pressure 

 on growth, I mounted the specimens — in this case an entire 

 seedling of Balsam and a cut flower of Crinum—m U -tubes 

 supplied with water, and proceeded to take records, first 

 under normal conditions— z>. the level of water in the two 

 limbs being the same— and then under a gradually increasing 

 hydrostatic pressure. These records are made, it should be 

 said, only when the rate of growth under the changed 



