TYPES OF RESPONSE I7I 



1. Radial Organ : 



{a) Thick shoot ; transverse conduction of excita- 

 tion neghgible ; positive phototropic response, 

 e.g. positive curvature. 



(b) Thin shoot or root ; transverse conduction of 

 excitation possible ; sequence of responses : 

 positive, neutral, and negative, e.g. reversal of 

 positive into negative in stem of Oryza and in 

 root of Sinapis. 



2. Pulvinated Organ : 



(a) Pulvinus thick ; transverse conduction of excita- 

 tion negligible ; positive response ; pronounced 

 concavity of the excited side, e.g. midday sleep 

 or para-phototropism of Erythrina indica, 

 Clitoria Tcrnatea, and others. 

 (h) Pulvinus thin ; transverse conduction of ex- 

 citation pronounced ; transient and hitherto 

 unnoticed positive followed by predominant 

 negative ; application of stimulus on the opposite 

 and more excitable side produces movement 

 in the same direction, now positive response. 

 The result would thus appear to be independent 

 of the direction of light. Examples are found 

 in the photonastic movements of the leaflets 

 of Mimosa pudica and of Averrhoa and Bio- 

 phytum. 

 Owing to varying combinations of numerous unknown 

 factors the phenomena of growth and its responsive varia- 

 tions under stimulation present many perplexities. For 

 instance, take the effect of external stimulus on growth. 

 Here subminimal stimulus induces one effect, and moderate 

 stimulus the very opposite. Should the tonic condition 

 of the plant happen to be below par, the effect of stimulus 

 will be an abnormal acceleration of growth, but during the 

 course of the experiment (owing to the continued action of 

 stimulus) the effect will mysteriously revert to the normal 

 retardation. The point of application of stimulus will 



