158 CHAP. XV. PHOTONASTIC PHENOMENA 



Response of the leaflet of Mimosa. — Equally anomalous 

 appears to be the response of the Mimosa leaflet. Strong 

 light acting from above induces a positive phototropic 

 movement upwards. But when the direction of the incident 

 light is changed so as to act from below, the responsive 

 movement is still upwards, that is to say, a negative photo- 

 tropic movement away from light. 



Such paradoxical reactions led to the employment of 

 the term photonasty to describe this class of phenomena, 

 supposed to be totally unrelated to normal phototropic 

 action and due to a different kind of irritability and a 

 different mode of response. Is there really a hiatus between 

 phototropic and photonastic reactions, or is there a possi- 

 bility of discovering continuity between them ? Investi- 

 gations on the effect of light on the main pulvinus of Mimosa, 

 acting alternately from above and below, gave a valuable 

 clue to the solution of the problem. 



It is necessarv in this connection to bear in mind the 

 anatomical and physiological characteristics of the two 

 halves of the pulvinus, the upper and the lower. The 

 pulvinus may summarily be described as consisting mainly 

 of two masses of cortex, separated by a thin flexible vascular 

 strand. The relative moto-excitabilities of the two halves 

 of such an anisotropic dorsiventral organ are easily demon- 

 strated by the application of diffuse stimulation, which causes 

 an impulsive fall of the leaf, proving the predominant 

 excitability of the lower half. The excitability of the upper 

 half of the pulvinus is, however, not altogether absent, 

 but relatively feeble, as will be presently demonstrated 

 by local application of stimulus on that half of the 

 organ. 



I now describe the characteristic effects of local 

 application of : 



1. Feeble stimulus on the upper half ; 



2. Strong stimulus on the upper half ; 



3. Feeble stimulus on the lower half ; and 



4. Strong stimulus on the lower half. 



