PROPULSION OF SAP AND ITS VARIOUS EFFECTS 403 



We must remember, however, that the internal energy 

 which maintains the normal turgid condition is itself the 

 result of energy previously absorbed from external sources ; 

 and if the plant be cut off from these sources of external 

 energy, then its own tonic condition will fall below par. Thus 

 the leaves of many plants are seen to droop when kept too 

 long in darkness, and exposure to light makes them recover 

 their natural position of normal turgidity. Hence in the case 

 of a plant whose condition is sub-tonic the leaves may be 

 made turgid by exposure to light ; but after the attainment of 

 the normal tonic condition, exposure to strong light will 

 bring about the proper contractile response to stimulus, with 

 the characteristic external motile indication of diminished 

 turgidity. 



We have thus seen the various effects produced by the 

 internal or latent energy of the plant. We have seen it bring 

 about the ascent of sap by means of the increased activity of 

 the plant-cells. It was seen to produce exudation pressure, 

 and excretion of nectar from intact plants. We have traced 

 it out to its appropriate expression by the lateral movements 

 characteristic of positive turgidity-variation, in the case of 

 anisotropic or dorsi-ventral organs. We have seen, too, how 

 necessary it is to the production of recovery of an organ from 

 the action of an external stimulus, the effects of local ex- 

 ternal stimulation, and of this internal activity, being opposed 

 in character. Any increase of this internal energy is thus a 

 factor tending to hasten the recovery of the organ from 

 stimulation, and when it is sufficiently great, it may, by its 

 antagonistic action, reduce the amplitude or even abolish 

 response to external local stimulus. W r e have again seen 

 this internal activity finding motile expression in the auto- 

 nomous movements of leaflets of Desmodium ; but, for its 

 mechanical exhibition, we need not confine our attention to 

 the sensitive plants so called, for we shall find the same 

 internal activity exhibited mechanically by all plants, in their 

 rhythmic growth-responses, to be described in the following 

 chapters. 



