STIMULATION AND ITS RESULTS 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 tothe 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 themif itisintense. This behaviour 
is known as phototazxis ; it is exhibited by the zoospores of 
many of the Alge 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. 368). 
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 
Marchantta 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. , 
Graviration.—The force of gravitation exerts an influ- 
ence upon plants which somewhat resembles that of lateral 
illumination. Most stems grow vertically upwards into the 
