CHAP, xi Sensitivity and Stimulation 467 



This was first shown by Charles Darwin in 1880 in the 

 case of young seedlings. 



Hofmeister found, in 1863, that the region of curvature 

 is the most actively growing zone of the axis. Miiller- 

 Thurgau, in 1876, and Wiesner, in 1878, came to similar 

 conclusions, though they did not altogether agree as to 

 the position of the maximum points. 



In Darwin's work he brought to notice for the first time 

 the extraordinary degree of sensitiveness which particular 

 organs possess ; certain seedlings bending their stems towards 

 a light which was not sufficiently intense to cause them 

 to cast a shadow on a piece of white paper held behind 

 them. He showed, too, how greatly the degree of sensitive- 

 ness varies in different cases. 



The way in which light sets up stimulation has been the 

 subject of some controversy. Sachs, in 1873, and Miiller- 

 Thurgau, in 1876, concluded that the prime factor is the 

 direction of the rays, or the angle of their incidence upon 

 the plant, while Darwin attributed it to the difference of 

 intensity of the illumination received by the opposite sides 

 of the organ. Wiesner regarded both factors as co-operating. 

 Oltmanns' researches in 1897 showed that intensity is very 

 important. 



The particular rays concerned in stimulation excited 

 much attention, but no accurate views were obtained till 

 Wiesner took up the question. In his important memoir 

 of 1878, Die heliotropischen Erscheinungen, he showed that 

 all the rays of the visible spectrum except the yellow cause 

 heliotropic curvature, but that those which produce the 

 greatest effect include the violet and the ultra-violet. He 

 constructed a curve showing the relative influence of the 

 different rays, and ascertained that while the maximum 

 lies there a second crest of the curve can be detected in 

 the infra-red region. The rays of this part of the spectrum 

 are effective after passing through a solution of iodine in 



G 2 



