LECTURE VII 

 HELIOTROPISM 



i. THE HELIOTROPISM OF SESSILE ORGANISMS 



MACHINES which are constructed artificially are arranged in such 

 a manner that the energy which they require is provided by the hand 

 of man. Through the blind play of the forces of nature durable 

 machines can be created only if their supply of energy is regulated 

 automatically. As an example of that type of machine we may men- 

 tion the waterfall. The waterfall is a machine which transforms dis- 

 tance energy into kinetic energy and heat, and the permanency of this 

 machine is guaranteed by the physical conditions that determine the 

 continued flow of water to the cataract. The green plants represent 

 another type of such machines ; namely, machines which, among others, 

 transform radiating energy into chemical energy. The permanency of 

 this kind of machines is guaranteed by the presence of an automatic 

 arrangement in such plants, whereby their stems grow toward the light. 

 The automatic turning of the stems of many plants toward the light is 

 called heliotropism. We shall go a little deeper into the analysis of these 

 phenomena, inasmuch as heliotropism and similar phenomena give, 

 to a large extent, an insight into the mechanism of automatic self- 

 preservation of organisms. 



The stems of many plants in the open grow vertically upward, 

 while the same stems when raised in a room which receives light from 

 only one side grow toward the window. Roots which contribute toward 

 the maintenance of the plant by absorbing the necessary salts from the 

 soil show very frequently (though not always) the opposite behavior. 

 When exposed to light they bend and grow away from the source of 

 light. This behavior is determined only by rays of a certain wave 

 length of the visible spectrum and possibly by some ultraviolet rays. 

 The dark heat rays have no such effects. 



We do not yet know with the same degree of certainty, as in the case 

 of the process of assimilation, the relative heliotropic efficiency of each 

 part of the spectrum; but from experiments with colored screens it 

 appears that the more refractive green, blue, and violet rays of the 



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