71 8 PLANT RESPONSE 



neither is any a perfect conductor, the difference between 

 extreme examples being one only of degree. 



The true excitatory effect, whether due to direct or trans- 

 mitted excitation, consists, as has been shown, of contraction, 

 with concomitant negative turgidity-variation. The- result 

 of this contraction and concomitant expulsion of water is, 

 however, the sending out of a wave of positive turgidity- 

 variation. Thus, up to the point reached by the true 

 excitatory effect, we obtain contraction, with negative 

 turgidity-variation; and beyond this point, a positive 

 turgidity-variation, with consequent expansion. This latter 

 effect we have designated as the indirect effect of stimulus. 

 It is thus seen that, whereas the direct effect of unilateral 

 stimulus is a concavity, its indirect effect is a convexity. 



We have also seen that it is possible by electrical means to 

 determine whether it is the direct or indirect effect of stimu- 

 lus which has in any given instance reached a point from a 

 distance ; for the indirect effect of stimulus, with its positive 

 turgidity-variation, is always attended by galvanometric posi- 

 tivity, whereas the true excitatory effect with its negative tur- 

 gidity-variation is characterised by galvanometric negativity. 



A tissue may conduct without exhibiting any motile 

 indication of its state of excitation. With reference to this it 

 is to be borne in mind that certain advantageous circum- 

 stances are necessary for the display of motile response ; for 

 since the fall of an excited leaf, such as that of Mimosa, 

 takes place in consequence of the expulsion of water, it 

 follows that when this is in any way impeded, as by over- 

 turgidity of the tissue, there may be excitation without any 

 responsive movement ; for this reason, the leaflets of Bio- 

 phytum, in the morning, when they are most tense, are not 

 so sensitive as later in the day. Motile excitability is as a 

 rule found to be abolished earlier than conductivity ; hence 

 a strong stimulus may be conducted through a region which 

 exhibits, through narcotisation, no motile excitability (p. 229). 



Polar effects of currents. — Another observation by 

 which the fundamental identity of excitatory phenomena in 



