PERCEPTION OF LIGHT 613 



IV. OPTICAL SENSE-ORGANS. 



The effect of light in inducing paratonic movements may depend 

 either upon variations in the intensity of illumination, as in the case of 

 nyctitropic (more properly nyctinastic) movements, or upon unilateral 

 incidence of the light, as in all cases of heliotropic (phototropic) or 

 helionastic (photonastic) movement. 



Those foliar pulvini which respond to variations of light- 

 intensity, do not seem to be furnished with special light-perceiving 

 organs. In all probability, sensitiveness to the alternation of light and 

 darkness is a property common to all the elements of the motor-tissue. 

 The absence of intercellular spaces full of air from the peripheral regions 

 of pulvini, must be regarded as an arrangement which facilitates the 

 penetration of light into the interior of these motor-organs. 



The question as to whether heliotropically reactive parts of plants 

 are possessed of special optical sense-organs or not, is bound up with the 

 more fundamental physiological question of the nature of the helio- 

 tropic stimulus ; for it is not known with certainty whether heliotropic 

 stimulation depends upon an asymmetrical distribution of light-intensity, 

 or upon the direction of the incident illumination. Charles Darwin 

 and Oltmanns, among others, maintain the former view, while the 

 latter is supported by Sachs and by Noll. From the standpoint of 

 physiological anatomy, the first of these alternative theories seems the 

 more acceptable ; for, given unilateral illumination, there will in all 

 circumstances be appreciable differences in the intensity of illumination 

 at different points in the interior of a light-perceiving organ, whereas 

 the original direction of the incident rays must inevitably be more and 

 more altered by refraction and reflection, as the light penetrates further 

 into the organ under consideration. 



A. PERCEPTION OF LIGHT BY PARALLELOTROPIC ORGANS. 



For the great majority of positively heliotropic axial organs, the 

 region of heliotropic perception concides with the region of curvature 

 (i.e. with the zone of longitudinal growth), just as in the case of 

 geotropism. The question as to whether all the cells in this region 

 have the same capacity of light-perception, or whether sensitiveness is 

 more or less strictly confined to particular tissues, has not yet been 

 properly investigated. But if it be assumed that sensitiveness is 

 restricted to a single tissue, it is obvious that the epidermis possesses 

 special qualifications for the purpose in question. For the rays of 

 light that fall upon the epidermal protoplasts on the illuminated side 

 of an organ, have undergone relatively less deflection from their 

 original direction than those which reach the more deeply-seated cells ; 



