650 PLANT RESPONSE 



angles. Movement in the other direction — that is to say, 

 towards the decrease of the angle — would, on the other hand, 

 indicate that the effect of light was to induce the normal 

 positive heliotropic response. From the record (fig. 261) it 

 will be seen that the latter was the case — that is to say, 

 the leaflet moved continuously, tending to become parallel 

 to the light, having in the course of twenty-five minutes 

 moved from its position at an angle of 45 s to one at 13 to 

 the horizon — that is to say, at an average rate of angular 

 movement of about i - 3° per minute. 



The lamina not the perceptive organ. — If, again, the 

 object of the so-called dia-heliotropic movement had been 

 the absorption by the upper surface of the lamina of the 

 largest possible amount of light, it would have been neces- 

 sary for the lamina to be the perceptive organ, determining 

 its movement according to the direction of stimulus ; but 

 that this is not the case may be demonstrated by subjecting 

 the lamina alone to the action of light, and covering the 

 pulvinus with a small opaque shield of black paper. On 

 doing so all movement is found to be arrested. If now, on 

 the other hand, the leaflet be covered with opaque paper, and 

 the pulvinus be left exposed, the usual heliotropic movement 

 is found to take place. This conclusively proves that, as 

 regards the response of the leaf to light, the lamina is not the 

 perceptive organ, and therefore that a supposed advantage to 

 itself cannot be the important factor in determining its move- 

 ment. That the lamina is not the perceptive organ has 

 indeed been shown already in the case of ordinary leaves, 

 when subjected to forms of stimulus other than light (p. 60). 

 We saw this, for example, in the case of a leaf of Artocarfms 

 when subjected to electrical and thermal stimulation. It was 

 in that case shown that since the lamina consisted of a mass 

 of non-conducting parenchymatous tissue, local excitation 

 might be caused by the incidence of stimulus, but could not 

 be effectively transmitted to the distant pulvinus or pulvinoid 

 by which the movement of the leaf was brought about. The 

 only case in which such transmission is possible to some 



