HEL10TROPISM. 193 



grow either in light or darkness. It must not be inferred, 

 however, that there is a complete indifference to the presence 

 or absence of light, for careful experiments show that light 

 favors some metastatic changes, while in many .cases it actu- 

 ally exerts a retarding influence. Thus if all other condi- 

 tions, as temperature, moisture, etc., are made constant, the 

 rapidity of growth of most aerial stems is considerably greater 

 in darkness than in light ; while under similar conditions 

 the growth of the leaves of most plants is less. Experiments 

 show that the retardation of growth is due to the rays of 

 high refrangibility, blue, indigo, violet, and ultra violet, and 

 that, so far as the metastatic changes under consideration 

 are concerned, the less refrangible rays are equivalent to 

 darkness. 



III. HELIOTROPISM. 



263. The retarding influence of light upon the growth 

 of stems gives rise to a curvature when the illumination is 

 stronger upon one side than upon the other. Thus, as is 

 well known, most plants, when grown in windows, bend 

 strongly toward the light, and if their position be afterward 

 reversed they soon bend again toward the side of greatest 

 illumination. To this phenomenon, which is an exceedingly 

 common one throughout the vegetable kingdom, the name 

 Heliotropism* has been given. The explanation which is 

 commonly given is that the light retards the growth on the 

 illuminated side, while the shaded side elongates, resulting 

 in a tension which necessarily produces a curvature. 



254. Evidently allied in some way to heliotropism is the 

 bending of certain organs away from the light. Thus the 

 leafless stems (runners) of Saxifraga sarmentosa, when grown 

 in a window so that they are illuminated upon one side more- 

 strongly than upon the other, curve toward the darker side. 

 This opposite bending has been called Negative Heliotro- 

 pism, and is supposed to be caused by light in some way not 

 yet understood. The tendrils of the Vine and Virginia 



* From the Greek #/UoS, the sun, and Tpeirsiv, to turn. 



