183 



B Stiiiui.ation by Gravitation and Light. 



Manv plant-organs curve under the influence of an unequal 

 illuminalion, as tliis causes an uneqnalitj in tlie longitudinal growth 

 at (iitferent sides of the organ (heliotropism). Since this depends 

 thus entirely on increase in lengtli, these movements must be 

 regarded here as being brought about both by gravitation and by light. 



It is known that light can cause certain movements of protoplasm: 

 tiie swarm-spores move towards light (positive phototaxis), whereby, 

 according to the experiments of Engelmann, it is the uncoloured 

 portion of the swarm-spore which receives the stimulus; if green 

 cells are exjiosed to the light 'after having been kept in the dark, 

 the chlorophyl-graius undergo a definite change of position, but 

 resume their original place when withdi'awn from the light. 



These reasons would already be sufficient to assume that I he 

 position of the static apparatus also can undergo the influence of 

 light, but such an assumption will become still more probable 

 when it can be shown by dilfereiit exain|)les that a similar shifting 

 of ihe apparatus, i.e. toirard.f the light, could furnish us with a 

 rather simple explanation of very different familiar phenomena. 



a. Positive and negative heliotropism. 



In the vertical position of main-axis and radicle, as was said 

 above, the middle-field of the static apparatus should lie against the 

 basal transverse wall of the statocyst ; if these organs receive light 

 from the side, and the apparatus, as we have just supposed, moves 

 towards the source of light, these organs can no longer be in 

 rest, and they can find the new equilibrium, i.e. the starch- 

 grains will come to rest again on the most sensitive middle-field of 

 the apparatus, only if the stem moves towards the light, and the 

 root on the contrary from the light; thus the familiar positive and 

 negative heliotropic curvatures. 



If the plant is replaced in the dark the organs return to their vertical 

 position, from which we should have to infer that after cessation 

 of the light-stimulus, the apparatus of itself returns to their former 

 place at the basal transverse wall. Consequently this is the same thing 

 observed with the chlorophyl-grains in the above-mentioned cases, 

 namely, that they are brought out of their position of equilibrium 

 by light and return to it when replaced in the dark. Pfeffer ') 

 considers this a matter of course. 



b. It is known that certain rhizomes react to light in such a 



Pflanzenphysiologie, 1904, Vol. 2, p. 780. 



