540 PHYSIOLOGY OF GROWTH. 



movements lies at about 22 C. I saw one complete circuit made, 

 at a high temperature, in a few minutes. The ascending journey 

 is somewhat more slowly accomplished than the descending one 

 (see Fig. 184). 



205. The Influence of External Conditions on Some Movements 



of Variation. 



The influence of external conditions on movements of variation 

 has already been frequently pointed out. We may here pursue 

 the subject further. In order to determine the influence on the 

 pulvini of Mimosa pudica of ether or chloroform vapour, it is 

 best in my experience to proceed as follows. A detached leaf 

 whose leaflets have been caused by gentle stimulation to lay 

 themselves together, is placed with its stalk in a small glass 

 vessel containing water. The glass stands in a dish into which 

 we have poured some ether or chloroform. The whole is covered 

 with a bell-glass, and exposed to direct sunlight. During the 

 narcosis, which is continued for a few minutes, the leaflets com- 

 pletely expand ; they now react neither to shock nor contact. 

 They recover their irritability, however, when transferred to a 

 moist atmosphere free from chloroform or ether. 



The following experiment is very instructive. The under 

 side of the pulvinus on the main leaf-stalk of a Mimosa growing 

 under very favourable conditions is repeatedly and at short 

 intervals stimulated by means of a small rod of wood. (In my 

 experiments the stimulation took place every half-minute for five or 

 six minutes, and the under side of the pulvinus received on each 

 occasion several blows.) If the plant is left to itself under a bell- 

 glass, the depressed main leaf-stalk rises after a few minutes, 

 but still it does not now react to contact. The organ of movement 

 thus, by often repeated contact, rendered insensitive to further 

 stimulation, again becomes irritable, however, after a short time. 

 It is of special interest that the Mimosa leaflets outspread after 

 narcosis with ether or chloroform, and the main leaf-stalks of 

 Mimosa elevated after the cessation of shocks, are not irritable 

 immediately, but only after lapse of time. It teaches, viz., that 

 the causes on which depends the return of the structures to a 

 condition of sensitiveness to stimulation on the one hand, and 

 those on the other on which depends the irritability itself cannot 

 be entirely the same. 



