708 PLANT RESPONSE 



stimulus is opposite in character to that of the direct effect. 

 The indirect effect of stimulus, or the increase of internal 

 energy, thus results in an erectile response, as seen in the 

 erection of the leaf in Mimosa, or Biophytum, when their 

 internal energy is in any way enhanced, say by rise of 

 temperature (p. 400). Taking all kinds of response, it will be 

 found universally true that if the increase of internal energy 

 give rise to one form of expression, the impact of external 

 stimulus evokes the opposite. 



(&) Suctional response. — It has been shown that con- 

 tractile response gives rise to a forward impulsion of water ; 

 hence by the excitatory contraction of the root-cells the 

 movement of water upwards is initiated. When such con- 

 tractile movement is not single, but repeated or multiple, 

 continuous propulsion of water is maintained. The rate of 

 this propulsion therefore affords us a means of measuring the 

 rhythmic activity of the plant-tissue. 



(c) Growth response. — This pumping-in of water causes 

 the transmission of energy to the distant growing region, and 

 as by this the internal energy of the plant is increased, it 

 finds expression there in a pulsatory expansion, which is the 

 movement of growth. Following each pulse of expansion, 

 there is a recovery which is incomplete, owing to fixation of 

 growth-material. The resultant growth is thus the irreversible 

 effect of the entire process. This growth-movement is another 

 expression of that indirect effect of stimulation which we 

 have already considered, by which a distant excited point 

 gives rise to a progressive train of waves of positive turgidity- 

 variation. The action of these waves may be seen not only 

 in the movement of expansion at the growing point, but also 

 by the erectile response of an intervening motile organ. This 

 will be understood from the following diagram of an artificial 

 plant (fig. 277), which shows how contractile action at the 

 base, giving rise to an hydraulic wave, causes two different 

 expressions of (a) motile response of the lateral organ, or leaf, 

 and (b) growth-expansion of the terminal growing point. 

 The pulvinus of this artificial motile organ consists of an 



