400 PLANT RESPONSE 



petioles in which the lower surface is the more excitable may 

 thus be taken as mechanical indications of such added 

 activity. 



Effect of warmth. — We have already seen that applica- 

 tion of warm water to the root of a plant, increases its suctional 

 activity. I was able to demonstrate this fact by means of the 

 mechanical response of Mimosa and Biophytwn. On making 

 the application, the leaves of both plants responded by erec- 

 tion to a position which in the case of Mimosa was 9 mm. 

 and in the case of Biophytum 12 mm. above normal. A rise 

 of temperature we have also seen to have the effect of enhanc- 

 ing the internal energy of the plant. By doing this, it brings 

 about an erection of the leaves. 



Effect of cold. — We have seen, on the other hand, that 

 the effect of cold is to stop rhythmic activity, and thereby to 

 arrest the ascent of sap. The motile indication given by the 

 leaf would in this case be the opposite to that of positive 

 turgidity-variation, that is to say, would consist of a fall or 

 droop. On applying ice-cold water to the root of Mimosa 

 I found that the effect of cooling was to induce a lowering or 

 drooping of the leaf, to a position of 3 mm. below the normal. 

 In Biophytum the corresponding fall was through 5 mm. 

 Lowering of temperature, by depressing the internal energy 

 of the plant, has the same effect. 



Explanation of the drooping of leaves during frost. — 

 These experiments offer an explanation of that drooping of 

 leaves which is observed in frost, and of the disappearance of 

 this drooping when a plant is restored to a warmer atmosphere 

 indoors ; for we have seen that when the internal energy of 

 the plant is normal, or above par, its suctional activity and 

 consequent turgidity are high, this favourable internal condi- 

 tion being outwardly exhibited by the erection of the leaves. 

 But when the internal energy is below par, the reverse effect 

 is seen in their droop or fall. 



Explanation of response and recovery. — It has been 

 stated, when describing the effect of stimulus in inducing a 

 fall of the leaf, and its subsequent erection, that the latter 



