694 LECTURE XXXIX. 



root grows downwards perfecdy vertically, and the primary stem vertically upwards. 

 We now place the vessel in the middle of a room, so that the plant is lighted from 

 the left, as shown by the arrows, and preferably so that the rays of light meet the 

 root and shoot at right-angles or nearly so. Even after 1-2 hours it will be seen that 

 the growing parts of the plant have made the curvatures shown in the figure, and 

 these after a few hours are considerably more pronounced ; the plumule has bent over 

 towards the window whence the light comes, and the illuminated side has become 

 concave, i.e. it is positively heliotropic, and since at this time the upper two-thirds of 

 the length of the organ are still growing, the whole of this region has become curved : 

 the lower portion, already fully developed, has remained straight. 



The root of this plant is negatively heliotropic and has become so curved at d 

 (since growth was still proceeding at this part), that the convex side is turned towards 

 the source of light, and the free root-apex e accordingly away from it. The part 

 of the root 6 d, which was already fully grown before the experiment began, has 

 remained quite straight. 



The positive and negative curvatures of the heliotropic parts lie in the same 

 vertical plane, which may be supposed to extend from the source of light to the dark 

 side of the room. 



What has been said sufficiently describes the heliotropic behaviour of most 

 plants ; of course there are also other cases, just as with geotropism, where, in con- 

 sequence of the stimulus of light, the growing parts of plants tend to place them- 

 selves transversely or obliquely to the incident ray of light, but I shall take these 

 and other cases into account in the next lecture ; meanwhile, we here keep in view 

 only the typical case illustrated by the figure. 



In the plant experimented with in Fig. 391, however, neither the positive nor 

 the negative heliotropism can properly exert its full effect, since the organs con- 

 cerned are at the same time geotropically irritable also. If, then, in consequence of 

 the stimulus of light, the apical part at a assumes an oblique or even horizontal 

 position, the geotropic stimulation comes into play, in consequence of which 

 it strives to become erect. The curvature which it actually presents after 

 several hours, therefore, results from two tendencies — the inclination towards the 

 light, and the tendency to erect itself. The plumule is thus in a position of unstable 

 equilibriuin between two mutually opposing forces: on shading the plant, the 

 plumule becomes erect because the heliotropic curvature is then feebler ; on 

 illuminating it more strongly, the heliotropic curvature prevails over the tendency 

 to geotropic erection, and the same (but with the converse result) would apply to the 

 negative curvature of the root. 



If, therefore, for the purpose of more exact investigations, the heliotropic 

 curvatures are to be made visible in their pure form, the action of geotropism must 

 be eliminated. This can be accomplished by fixing the plant to the klinostat described 

 above (p. 684), and placing the axis of the instrument so that it stands at right- 

 angles to the window; on then fixing the plant so that it stands at right-angles to the 

 axis of the klinostat, and thus revolves in a plane parallel to the window-panes, 

 the effect of geotropism is eliminated, while the pknt is continually illuminated from 

 one side and can therefore show the heliotropic stimulation free from secondary- 

 influences. In this way Hermann MuUer (of Thurgau), who was then my assistant, 



