LONGITUDINAL RESPONSE OF RADIAL ORGANS 69 



stimulus being partially fixed by growth. The responses, 

 moreover, afford some slight indication of fatigue. 



But though there is thus no lateral movement, owing 

 to the antagonistic character of the simultaneous longi- 

 tudinal contractions on all sides, we might nevertheless 

 hope to detect some responsive contraction in the length of 

 the organ as a whole. And perhaps it will be well to con- 

 sider at this point what would be the most favourable con- 

 dition for the exhibition of such contraction. Taking the 

 parallel instance of contractile animal tissue, namely muscle, 

 we can easily see that if this were attached throughout its length 

 to a rigid structure such as bone, contractile movement would 

 have been an impossibility. Similarly, contractile vegetable 

 tissues when attached to hard elements, such as wood, must 

 be prevented from exhibiting contractile movements. The 

 vegetable tissues, therefore, which ought to be best fitted to 

 exhibit this effect, will be comparatively deficient in hardened 

 fibro-vascular elements, and will consist largely of prosenchy- 

 matous cells, relatively longer in one direction, and very 

 elastic and highly extended by turgidity. The diminution 

 of this turgidity by stimulus might then be expected to 

 produce a relatively large degree of longitudinal contraction. 1 



Excitatory contractions in Cynereae. — This shortening 

 in length is most strikingly exhibited by the filaments of the 

 stamens of Cynerece. This subject has been specially studied 

 by Pfeffer, and the account which I give here is epitomised 

 from Sachs' ' Physiology of Plants.' 2 The filaments of the 

 stamens of Centaurea jacea are free from each other, but the 

 anthers cohere, forming a tube. The stamens are, in the 

 unexcited state, strongly curved convexly outwards. If now 

 the filaments are all excited simultaneously, say by mechanical 

 touch, there is produced a downward withdrawal of the 



' I have shown that galvanometric negativity is an unmistakable indication of 

 the excitatory contraction of a vegetable tissue. Employing this test, I find that 

 all living vegetable tissues are excitable. But the mechanical contraction pro- 

 duced will, other things being equal, be very marked only in long prosenchymatous 

 tissues, and in a mass of parenchyma relatively less marked. 



• Sachs' Physiology of Plants, Engl. ed. 1887, pp. 651 -2. 



