CHEMO-TROPISM AND GALVANO-TROPISM 



551 



Excitatory versus plasmolytic reaction in pulvinus of 

 Mimosa: (i) Favourable tonic condition. — I have already 

 stated that a moderately strong solution of sodium chloride 

 is a stimulating reagent. It is therefore to be expected that 

 its diffuse application on the pulvinus will bring about a fall 

 of the leaf ; and this, as we shall presently see, is found to be 

 the case. We saw, however, in studying the effect of sodium 

 chloride on growth-response, that under the favourable tonic 

 condition induced by the optimum temperature of 34° to 

 35° C. it gave rise to a preliminary 

 relaxation, followed later by contrac- 

 tion. From this it occurred to me, 

 that in the parallel response of the 

 pulvinus of Mimosa we might expect, 

 under similar favourable conditions, 

 to observe two opposite responsive 

 movements : first, a preliminary ex- 

 pansion, exhibited by erection of the 

 leaf, and afterwards a contraction, 

 exhibited by the depression of the 

 leaf. 



In carrying out the experiment, 

 I cautiously raised the temperature 

 of the pulvinus to 34° C. and then 

 applied to it a 3 per cent, solution of 

 sodium chloride at the same tem- 

 perature. Tiie responses were auto- 

 matically recorded by the leaf on 

 smoked paper wrapped about the 



revolving drum. It will be seen (fig. 228) that the preliminary 

 effect was a movement of relaxation or erection, which was 

 completed in the course of one minute, and was followed by 

 the opposite movement, or fall of the leaf. In these two 

 opposite responsive movements, then, occurring in succession 

 to each other, we have an analogous case to that of the two 

 opposite and succeeding modifications of curvature, observed 

 by Noll in a curved organ, immersed in salt solution. 



Fig. 228. Response of 

 Leaf of Mimosa in 

 Favourable Tonic Con- 

 dition to Chemical .Sti- 

 mulus of 3 per Cent. 

 Salt Solution 



The preliminary response 

 is here seen to lie 



