4 1 PLANT RESPONSE 



it is manifested in a rhythmic manner, by periodic variations 

 of turgidity, bringing on responsive movements. It was 

 also stated that responsive movements under the action of 

 stimulus took place in ordinary young tissues, these move- 

 ments being lateral when the tissue was anisotropic, and 

 longitudinal when it was strictly radial. It would follow, 

 then, that when the sum total of the latent energy in such 

 tissues was above par, they might be expected — like the 

 leaflet of Biophytum or Desmodium under similar conditions — 

 to exhibit their rhythmic excitation by repeated lateral or 

 longitudinal movements. I shall now proceed to show that 

 in the case of these young tissues, under favourable circum- 

 stances, this multiple rhythmic excitation finds expression in 

 the responsive movement known as growth. 



In the majority of instances an organ is not absolutely 

 radial ; hence, during growth, we obtain the lateral respon- 

 sive movements which are known as circumnutation. In 

 bilateral growing organs, these movements are to and fro, 

 in strict parallelism to the to and fro movements of the 

 leaflet of Biophytum. In such instances the axis of bilate- 

 rality is fixed, and the responsive movement takes place in 

 a definite plane. In the leaflet of Desmodium also rectilinear 

 movements are often observed ; but as a rule, owing to the 

 revolution of the bilateral axis, the movement of the leaflet is 

 circular or elliptical, and in the case of growing organs, from 

 the same cause, circular or elliptical movements of nutation 

 are common. The ideally simple and most interesting 

 example of this multiple rhythmic activity is seen, however, 

 in the growth-movements of radial organs, these being longi- 

 tudinal ; for there is not in this case that complication which 

 arises from the gradual shifting of the bilateral axis seen in 

 the growth of anisotropic organs. 



In order to prove the identity of these rhythmic growth- 

 movements with multiple response, we have to show 

 (i) that such rhythm is a characteristic of growth; (2) that 

 each pulsatory growth-movement of the series exhibits all 

 the characteristics of true response ; (3) that the series itself 



