REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 705 



to show, moreover, how the latter may be derived from the 

 former. This may be clearly and easily seen by taking a simple 

 case, in which we excite the stem by the application of external 

 stimulus, on a zone a few centimetres below its upper end. 

 As the direct effect of this stimulation, a contraction takes 

 place in that zone. By this contraction an active expulsion 

 of water, with local negative turgidity-variation, is brought 

 about, and the expelled water is driven outwards in both 

 directions from the excited zone. Following the course 

 of the water which is thus forced upwards, we have found 

 that it produces an increase of turgidity, or positive turgidity- 

 variation, with consequent distension or expansion of all the 

 cells above the stimulated area. Work is thus performed on 

 these cells, in consequence of external stimulus, by which 

 their latent energy is increased. 



The energy of the stimulus applied in one has thus been 

 conveyed to another region, hydraulically, there to give rise 

 to an effect of increased turgidity and expansion, which was 

 designated as the indirect effect. Thus the effect of external 

 stimulus is seen to be twofold — namely, first by its direct 

 action to induce local contraction, and secondly by its indirect 

 effect, of increasing the internal energy, to bring about an 

 expansion. The expressions of direct and indirect stimula- 

 tion are thus seen to be opposite in character, and we have 

 seen how they find opposite modes of expression in the case 

 of each form of response. 



Various forms of responsive expression. — The next 

 question to be passed in review is that of the various modes 

 in which the response of the plant was seen to find expression ; 

 and here we found various responsive phenomena which are 

 characteristic of life, and apparently entirely unrelated, to be 

 ultimately dependent upon this fundamental inter-action 

 between, on the one hand, the contraction due to external 

 stimulus, and on the other the expansion due to internal 

 energy. 



{a) Lateral motile response by differential contraction. — 

 Taking first the responsive mechanical movement of motile 



z z 



