THE RECORD OF GROWTH-RESPONSE 42 1 



3 mm. from the responsive pulvinoid. I then repeated the 

 experiment with the same leaf, but applying stimulus at the 

 greater distance of 5 mm. The responses now consisted of 

 a series of up movements of the leaf, indicative of positive 

 turgidity-variations (fig. 170, b), the direct effect of stimulus 

 not reaching the organ. 



Now, turning our attention to the growing organ, we find 

 that the fibro-vascular element, which possesses the power of 

 conduction to a high degree, is not yet fully established in 

 the zone of growth. If, then, contiguous to the growing 

 zone there be a mass of active tissue thrown into a state of 

 rhythmic excitation, it is to be expected that the indirect 

 effect of such stimulation will alone act, and give rise in the 

 region of growth to pulsations of increased turgidity. It will 

 be remembered from the last chapter'that water is conducted 

 by preference along the fibro-vascular elements ; and since 

 these strands end below the zone of growth, it is clear that 

 there must be in this region an accumulation of water, and 

 consequent over-turgidity of the tissue ; a condition which is, 

 as we know, sufficient to initiate rhythmic excitation. 1 This 

 region, then, acts like the actively excitable tissue of Colocasta, 

 which, as we saw, gives rise to spasmodic expulsions of water. 

 In the latter case there is, however, a channel by which the 

 water escapes, thus relieving the pressure on the tissue ; but 

 the growing organ offers only a cul de sac, and the constant 

 repetition of hydrostatic blows thus effects those positive 

 turgidity-variations that are to result in the responsive 

 elongations and incomplete recoveries of the tissue, bringing 

 about growth-movements. 



* Inner stimuli.' — It is thus seen that growth represents 

 the indirect effect of stimulus ; its motive power residing 

 in the rhythmic activity of the internal tissues of the plant. 

 This rhythmic activity has been shown to be, in its turn, 

 the result of the tonic condition of the plant, that is to say, 

 of the sum total of energy previously absorbed, and held 



1 Or the over-turgidity of the growing region may be sufficient of itself to 

 initiate rhythmic activity. 



