500 PLANT RESPONSE 



must remember that the movement in response to gravitation 

 has to overcome opposite mechanical movements before it 

 can be made perceptible at all ; for we have, firstly, the 

 weight of the organ, which tends to make it bend down ; and 

 secondly, on account of this bending, we have greater tension 

 on the upper surface, which, as we have seen, increases the 



Fig. 207. Record of Apogeotropic Response in Scape of Uriclis Lily 



The up curvature due to apogeotropic action proper commenced forty 



minutes after the specimen was laid horizontally. 



rate of growth, tending to make that side convex. The 

 differential effect due to gravity has to overcome all this, and 

 becomes visible only when it has done so. After this stage 

 has been reached, the rate of movement upwards goes on 

 increasing until a fairly constant rate is attained. 



Record of different rates when specimen is held at 

 angles of 45 and 135°. — Czapek has found that the effective 

 stimulus of gravitation is greater when the organ is held at 

 1 35 to the vertical than when held at 45 . This difference 

 of effect can be obtained quantitatively with great accuracy 

 by taking successive records with the specimen in the two 

 positions ; and in order to eliminate the effect of any chance 

 disturbance, or of spontaneous variation, I took four alternate 

 records, first with a specimen at 45 , then at 135 , then back 

 once more at 4$°, and then again at 135 , each position 

 being maintained just long enough for the attainment of 

 the permanent effect. The change from one position to the 

 other was made quickly, in the course of less than a minute. 

 The experiment was carried out with the unopened flower of 

 Crinum Lily, in which we have already found that the rate of 

 growth is very regular and considerable. 



