STIMULATION AND ITS EESULTS 379 



normally bending part was illuminated. Further, when 

 the region normally curving under the influence of the 

 stimulation is mechanically hindered from bending, the 

 curvature takes place at a part a little lower down, which 

 normally remains straight. 



When the lateral light is fairly intense the resulting 

 movement takes place uninterruptedly ; when it is only 

 weak the position is assumed by a series of zigzag move- 

 ments, indicating that the new movement is an exaggera- 

 tion of the ordinary circumnutation of the part. When 

 the final position is reached the organ is found to circum- 

 nutate about the new direction of the axis. 



A somewhat similar response to the influence of a 

 lateral light is exhibited by many unicellular organisms. 

 When these are exposed to oblique illumination they take 

 up a definite position with regard to the incident rays, 

 placing their long axis parallel to them if the light is weak 

 and at right angles to them if it is intense. This behaviour 

 is known as phototaxis ; it is exhibited by the zoospores of 

 many of the Algae and by certain Desmids. 



Before leaving the subject of the effect of a lateral 

 light in inducing these movements we may point out that 

 the phenomena of heliotropism and apheliotropism must 

 be distinguished from those of photo-epinasty and photo- 

 hyponasty, which were alluded to in the last chapter (p. 363). 

 The difference is easily seen, for in the latter cases the 

 result of the access of the light is the same, whatever 

 be the portion of the organ stimulated. The thallus of 

 Marchantia becomes convex on the dorsal and concave on 

 the ventral side, whether the light impinges on the one 

 or the other. In the case of a heliotropic curvature 

 the side which is stimulated always becomes concave ; in 

 that of an apheliotropic one the stimulated side becomes 

 convex. 



GRAVITATION. The force of gravitation exerts an influ- 

 ence upon plants which somewhat resembles that of lateral 

 illumination. Most stems grow vertically upwards into the 



