PROPULSION OF SAP AND ITS VARIOUS EFFECTS 399 



as if no closing membrane were present. It is clear 

 that by the contracting expulsive action of excited cells, 

 transport of materials might be brought about more rapidly 

 than by mere osmotic action. 



Mechanical response to suctional activity. — The effect 

 of that internal rhythmic activity which, as we have seen, is 

 caused by the absorption of energy from various forms of 

 stimulus, is to induce an increase of turgidity throughout 

 the plant. I shall next proceed to describe a mechanical 

 means of obtaining some indication of this internal 

 activity. 



I have shown that owing to the dorsi-ventral differentiation 

 in the pulvinus of Mimosa, when the turgidity of the organ is 

 increased, the leaf is erected ; and when the turgidity is 

 decreased, as by the direct action of an external stimulus, the 

 leaf is depressed. Similar effects occur not in pulvinated 

 organs alone, but in all dorsi-ventral organs, in which the 

 lower half is more excitable than the upper. Thus, for 

 example, the lower half of . the petiole carrying the lateral 

 leaflets of Biophytum is more excitable than the upper half. 

 It is true that the leaf possesses a slightly developed pulvinus ; 

 the differential excitability on which the motile response 

 depends is not, however, confined to this pulvinus alone, but 

 extends throughout the length of the petiole. The petiole 

 thus acts like a diffuse pulvinoid. 



The action of stimulus on the diffuse pulvinoid of Biophytum 

 and on the pulvinus of Mimosa is to produce the fall of the 

 leaf. And just as the leaf of Mimosa was erected by increased 

 turgidity, so in Biophytum also we should expect to have, with 

 similar increase of turgidity, a similar responsive movement 

 of erection ; and this I find to be the case. The erection of 

 the leaf of Mimosa or Biophytum is therefore a mechanical 

 indication of an increase of turgidity or of positive turgidity - 

 variation, by whatever means this may have been induced. 

 Such an increase may be brought about by an augmentation 

 of suctional activity. And the up movements of dorsi-ventral 



