PHOTOTROPISM 121 



communities where much mutual shading occurs. The 

 relation to diffused light is thus a generally important one. 



The phototropic reaction is seen at its simplest in organs 

 with radial symmetry. A young stem, when lit from one 

 side, curves, as the result of unequal growth rates on the 

 two sides, till its axis Hes in the direction of the light-rays, 

 and so is uniformly illuminated. Most often the shoot, if 

 the zone of growth is long, bends somewhat past the position 

 of equilibrium and then reverses the movement, till finally 

 adjustment in the direction of the light is attained. Such 

 curvatures are frequently seen in potted geraniums and 

 fuchsias grown in windows, and by this means, even without 

 adjustment of the leaf, better illumination of the more 

 shaded leaves is secured. 



It is with such orthotropic organs (especially with the 

 coleoptiUy or first sheath-like leaf, of the oat, which is 

 peculiarly sensitive) that the most exact investigations on 

 the mechanism of the phototropic reaction have been made. 

 From these we know that the stimulus may be perceived 

 in one region of an organ and the differential growth reaction 

 take place in another. The conduction of the excitation 

 is in all probability due to the transport of some chemical 

 substance, a mode of conduction which finds its analogy, 

 not in the nervous system, but in the transference of 

 hormones in the higher animals. This is the general mode 

 of the conduction of excitation in plants. For orthotropic 

 organs, at least, it is well established that the essence of the 

 stimulus lies in the difference in intensity of the illumination 

 on the two sides, and not in the oblique direction of the 

 rays ; although the response results in the assumption by 

 the organ of a definite relation to the direction of the rays. 

 The blue end of the spectrum is active in inducing 

 phototropic response ; red light has practically no effect. 



The reaction of the leaf is much more complex. As we 

 have seen its final position is often attained by torsions, 

 and the changes in growth rate leading to a torsion are 

 obviously more intricate than are those leading to a simple 



