344 LIFE MOVEMENTS IN PLANTS 



enable us to find out, whether any variation of excit- 

 ability occurred during the course of the experiment and 

 make allowance for it. The records show that stimula- 

 tion did enhance the excitability of the organ to a small 

 extent. Thus the first stimulation at 45° induced an 

 amplitude of response of 5 mm. ; the sacond stimulation 

 at 45° i. e. the third response of the series, induced a 

 slightly larger response 7 mm. in amplitude. Similarly 

 the two responses at 90° gave an amplitude of 9 mm. 

 and 11 mm. respectively (Fig. 128). Taking the mean 

 value of each pair, the ratio of tropic effects for 90° 

 and 45 ' is = 10/6 = 1*7 nearly. 



EFFECT OF DURATION OF EXPOSURE. 



Experiment IBS. — The specimen employed for the experi- 

 ment was a flower bud of Crinum in a slightly sub-tonic 

 condition. Successive responses exhibited on this account, a 

 preliminary negative* before the normal positive curvature. 

 The successive durations of exposure were for 1, 2, and 



Fig. 129.— Effect of increasing duration of exposure 1:2: 3 minutes, on photo- 

 tropic curvature. Note preliminary negative response. {Crinum). 



3 minutes. The amplitudes of responses (Fig. 129) are in 

 the ratio of 1 : 2*5 : 5. 



♦ An explanatiop of this preliminary effect wjll be found in the nest chapter. 



