Researches on Growth and Movement in Plants. 



391 



Hence, unilateral stimulation of an anisotropic organ will appear to give 

 different results depending on the particular flank that is excited. Stimulation 

 of the less excitable side will give the sequence of positive, neutral, and 

 negative response ; stimulation of the more excitable side will give only the 

 positive. The question as to which side is the more excitable can easily be 

 determined by applying [to the organ a diffuse electric shock. The more 

 contracted and concave side is the more excitable. 



I may now classify some of the principal types of response that will be met 

 in practice. In anisotropic organs, stimulus is supposed to be apphed on the 

 less excitable side. 



I. Eadial organ — 



(a) Thick stem, transverse conduction negligible : positive phototropic 

 response. 



(&) Thin stem, transverse conduction possible : sequence of responses — 

 positive, neutral, and negative. 



II. Pulvinated organ, motile or non-motile — 



(a) Transverse conduction negligible : positive response ; pronounced 

 concavity of the excited side, e.g., mid-day sleep or paraphototropism of 

 Erythrina indica, Clitoria ternatea, and others. 



(6) Transverse conduction moderate : sequence of response positive, neutral, 

 and negative, e.g., main pulvinus of Mimosa. 



(c) Pulvinus thin and transverse conduction pronounced : transient and 

 hitherto unnoticed positive followed by predomiuant 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 lateral leaflets of Mimosa pudica and leaflets of Biophytum 

 semitivum. 



The following is a diagrammatic representation of the typical cases : — 

 (Arrow represents direction of incident light.) 

 a. h. 



I I 



+ ++ + + 



