PHYSIOLOGY 253 



Heliotropic CURVATURE of an organ is accomplished, it is found THAT 



THE SIDE TURNED TOWARDS THE LIGHT GROWS MORE SLOWLY, THE SIDE 

 AWAY FROM THE LIGHT MORE RAPIDLY THAN WHEN ILLUMINATED 



from all sides. This may be readily shown by previously marking 

 with Indian ink regular intervals from one to two millimetres apart 

 on the opposite sides of the organ. After the curvature has taken 

 place the intervals between the marks will be found to be much 

 farther apart on the shaded side than on the side turned to the 

 light. As compared with the elongation under normal conditions 

 of growth, the marks on the illuminated side have remained nearer 

 together, while those on the shaded side have drawn farther apart ; 

 that is, the growth in the case of a positive heliotropic curvature has 

 been retarded on the illuminated side and promoted on the shaded 

 side. It also becomes evident, from observation of the ink -marks, 



THAT CURVATURE TAKES PLACE ONLY IN THE PORTIONS OF STEMS 

 STILL IN PROCESS OF GROWTH, AND THAT THE CURVATURE IS GREATEST 



where the growth is most vigorous (Fig. 201). The curvature is 

 then only a result of unequal growth induced by one-sided illumination. 



It was formerly believed that the increased growth of the shaded side was pro- 

 duced by the beginning of etiolation, and that the diminished growth on the illumin- 

 ated side was due to the retarding effect which light exerts upon growth in length 

 (p. 234). Other heliotropic phenomena were found to be at variance with this 

 explanation of heliotropism. Unicellular perfectly transparent Fungus hyphse are 

 also subject to positive heliotropic curvature, although in this instance there can 

 be no shaded side ; on the contrary, the side of a hypha turned away from the 

 light is especially illuminated on account of the refraction of the light rays. The 

 fact, too, that negative heliotropic curvatures also take place renders it evident that 

 heliotropism cannot be due to one-sided etiolation ; for in negative heliotropism 

 the side most directly illuminated is the one that grows more rapidly, although 

 the retarding effect of light on the normal growth in length of negatively heliotropic 

 organs is equally operative (roots, rhizomorpha). 



It is evident from these considerations that it is not the difference 

 in the intensity of the light which causes the heliotropic curvatures, 

 but the direction in which the most intense light rays enter the organs. 

 Light acts as a motory stimulus when it penetrates an organ 

 in any other direction than that which corresponds with the 

 position of heliotropic equilibrium. 



The heliotropic curvatures are most strongly produced, just, as 

 in the case of the heliotactic movements of freely moving swarm- 

 spores, by the blue and violet rays, while red and yellow light exerts 

 only an extremely slight influence, or none at all. It is due to 

 the fact that the red-yellow and blue-violet rays are always present 

 together in daylight, that the heliotropism of the leaves is of advantage 

 to their assimilatory activity. 



Sensibility to heliotropic influences is prevalent throughout the 

 vegetable kingdom. Even organs like the roots of trees, which are 



