EXPERIMENTS UPON LLGHT-PEKCEPTK )\ 629 



Capsella Bursa pastoris (inflorescence-axis), 



Brassica Natpus (hypocotyl), 



Ipomoea purpurea - - 



Avena sativa (cotyledonary sheath), 



Lcpidium sativum (hypocotyl), - - J G 



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The corresponding value in the case of the human eye is originally 

 about yoo' ^ Llt r 'i ses U1 ^he course f life to J . It may thus be 

 assumed, that the more sensitive types of plants are possessed of a 

 capacity for perceiving differences of light-intensity which is not 

 inferior to that of the human eye. It follows, that any local differences 

 in the illumination of the inner epidermal walls that are detected by 

 an observer, in such cases, are likewise within the range of perception of 

 the sensitive protoplasm. 



In view of the measurements that have just been cited, it was not 

 to be expected that the experiments upon wetted leaves, carried out by 

 the author and other physiologists, should give similar results with 

 different plants. In the case of young plants of Begonia semperflorens, 

 leaves with their upper surfaces wetted are quite incapable of 

 perceiving the direction of the incident light ; consequently, when such 

 leaves are obliquely illuminated, they do not make the slightest attempt 

 to return to their heliotropically fixed position. In quite a number of 

 instances on the other hand, wetted leaves do return to their normal 

 position, though not so rapidly as they do when dry ; wherever this is 

 the case, it will be found that the inner epidermal walls are curved, or else 

 that reflection of light produces a differential illumination of the inner 

 wall analogous to, though less pronounced than, that which prevails 

 under normal conditions. 



It is evident that such crude experiments with wetted leaves do 

 not provide a satisfactory means of testing the importance of the light- 

 condensing action of papillose epidermal cells, in relation to the per- 

 ception of light by diaheliotropic leaves. For this reason the author 

 has carried out experiments of a somewhat different kind upon leaves of 

 Tropaeolum majus. A portion of the leaf surface is wetted and covered 

 with a sheet of mica, the rest remaining dry ; the two regions of the 

 leaf are separated by a screen of black paper. The petiole having been 

 darkened in a suitable manner, the dry and wet portions of the lamina 

 are illuminated obliquely from opposite sides. The result is always the 

 same ; the leaf moves towards the light that falls upon the dry surface. 

 This experiment succeeds even if the wetted area is from 2 '2 to 4*8 

 times as large as the dry, both being equally illuminated, or if the two 

 areas are equal, but the wetted one is caused to receive about twice as 

 much light as the dry one. This improved " wet-leaf experiment," 



