48 PLANT RESPONSE 



relatively slow, and, as has been said, definite, and charac- 

 teristic of the plant under normal conditions. This velocity 

 varies, as will be seen later, in the case of different plants, from 

 a rate of about *5 to about 15 mm. per second. And this 

 true excitatory response, mechanical or electrical, undergoes 

 appropriate modifications, according to the physiological 

 changes of the tissues, and is abolished at death. 



The hydrostatic effect, on the other hand, may be seen, 

 in the form of a preliminary twitch, when the specimen is 

 indirectly stimulated — that is to say, when the stimulus is 

 applied at a distance from the point where the responsive 

 effect is observed. The hydrostatic effect gives rise to posi- 

 tive turgidity-variation ; abnormal positive or * up ' mechanical 

 response ; and abnorma 1 gal vanometric positivity. The velocity 

 of transmission of this hydrostatic disturbance is relatively 

 very great, being about several hundreds of mm. per second. 



The conditions of exhibition of excitation by lateral 

 response. — We shall next consider the question of that 

 division of plants into sensitive and ordinary which has 

 led to the impression that only the former are excitable. 

 And, first, we shall study the conditions which are favourable 

 to the exhibition of the motile effect, according to which it 

 is customary to estimate the sensitiveness of the plant. We 

 have seen that in plants like Mimosa it is the difference in 

 excitability between the two halves of the motile organ 

 which makes it possible for it to exhibit the state of excita- 

 tion by means of lateral movement. If, then, through any 

 circumstance, this difference of excitability as between the 

 two halves of the organ be diminished or abolished, a plant 

 which is undoubtedly sensitive will appear insensitive, as 

 judged by the mechanical test. The reductio ad absurdum 

 is reached when the same plant is sensitive and insensitive at 

 the same time. 



As an instance of this, we may take the plant Biophytum. 

 In consequence of age, the differential excitability of the 

 pulvini of the leaflets disappears. Hence, when an old leaf 

 is excited, its leaflets give no motile indication, and we are 



