Chap, xi Sensitivity and Stimulation 469 



features of heliotropic curvature, which have a bearing on 

 the general phenomena of irritability, will be noticed later. 



The influence of light upon organisms which are free to 

 move is very similar to the heliotropic influences affecting 

 fixed plants. This phenomenon, to which the name photo- 

 taxis was given by Strasburger in 1878, was observed by 

 Braun in 1851, but little attention was given to it till 

 some years later. Cohn found in 1867 that the rays con- 

 cerned are those of high refrangibility ; an observation 

 confirmed by Strasburger in 1878. The general effect of 

 light on the movements of locomotion was noticed by 

 Hofmeister in 1867, in the course of his study of the 

 behaviour of the Myxomycetes ; and Famintzin showed in 

 1867 and 1868, that a similar effect is produced by light 

 on certain Oscillatoriae. Baranetzky confirmed Hofmeister's 

 observations in 1883. In the same year Engelmann found 

 that this susceptibility is shared by certain Bacteria, B. 

 photometricum especially being quiescent in darkness, but 

 commencing to move after a short latent period when 

 illuminated. 



The nature of the phototactic movement was studied 

 by Strasburger in 1878 ; in the paper alluded to he says 

 that certain zoospores place their axes parallel to the direc- 

 tion of the incident rays, a statement confirmed by Stahl 

 in 1880. Strasburger further pointed out that the influence 

 is not confined to the visible spectrum, but that it is excited 

 also by the rays of the infra-red, thus showing a corre- 

 spondence between the phototactic stimulation of freely 

 moving organisms and the stimulation leading to helio- 

 tropic curvatures as established by Wiesner. 



Geotropism 

 The introduction of this term is due to Frank, who applied 

 it to the phenomena of curvature under the influence of 

 the attraction of the earth, which had long been observed 



