1)IA-1I1;LI()TK()I'ISM and DIA-GKOTKOI'IS.M 641 



Difficulty of distinguishing- between effect of light and 

 other reactions. — In the case of the response of pulvinated 

 organs, we had the advantage, owing to their great motility, 

 of being able to refer each particular responsive movement 

 to the immediate stimulating action of incident light. In 

 ordinary leaves, however, the movements being very sluggish, 

 it takes so long a time for any given responsive action to 

 attain the requisite magnitude for ordinary observation, that 

 other factors of variation intervene,. and it becomes difficult 

 to know how much of the resultant response is due to 

 heliotropic stimulus as such. It is this great difficulty of 

 disentangling the response due to light, which is the proper 

 subject of the inquiry, from numerous other subsidiary factors 

 that has led to the existing divergence of views among 

 observers. 



In the course of the present chapter, therefore, I shall 

 shortly enumerate those various agencies which are subsidiarily 

 instrumental in bringing about the ultimate position assumed 

 by the leaf I shall then describe a method by which helio- 

 tropic action proper can be discriminated with certainty from 

 other influences. The relation between the fundamental action 

 in response to light, which has already been demonstrated, and 

 the heliotropic response of the leaves will thus be made 

 apparent. But before entering upon these questions I shall 

 briefly allude to the principal existing theories on this 

 subject. 



Theory of Frank. — There are at present two main types 

 of opinion with regard to the question of the effect of light on 

 leaves. Frank and his school account for that action of leaves 

 by which they place themselves with their flat surfaces perpen- 

 dicular to the direction of incidence of the rays of light, or of 

 the action of gravity, by assuming that dorsi-ventral organs 

 possess a peculiar propert)' of sensitiveness to the directive 

 action of light and gravity. This they designate as Transverse 

 or Dia-heliotropism, and Transverse or Dia-geotropism. It is 

 supposed that the habit has been acquired for the advantage 

 of the. plant, inasmuch as the leaves, by placing themselves 



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