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TEXTBOOK OF PLANT PHYSIOLOGY 



Wendt) may be observed. Under the influence of the stimulus of 

 light, the total amount of the growth hormone in the sensitive 

 apex, as well as its distribution, is altered. As the exact quantita- 

 tive determinations by Wendt have shown, light in itself decreases 

 the amount of the growth-promoting substance in the apex. This 

 explains the delaying influence of light on growth, discussed in 

 Art. 90. Moreover, under the influence of a unilateral illumina- 

 tion there takes place an electrical polarization of the cells, which 

 produce and transmit the growth-promoting substance. The 

 illuminated side receives a negative, the shaded side, a positive 



charge. Under the influence 

 of this polarization, the current 

 of growth-promoting substance 

 is shifted to the shaded side. 

 The cells of this side elongate 

 more rapidly and more strongly 

 than do those of the opposite 

 side and as a result a curva- 

 ture of the seedling towards 

 the light appears. As the 

 growth-promoting substance 

 is universal and not specific for 

 every plant, it is understand- 

 able why an alien tip may 

 induce almost the same curvatures as the proper one. Other 

 authors (Boysen-Jensen, Stark, Gradmann), however, do not 

 accept this idea and suppose that under the influence of light there 

 arise special growth-checking hormones which differ from the 

 growth-promoting substance. 



A similar transmission of excitation takes place in many 

 leaves. As a rule, the leaf blade is the organ of perception, while 

 the petiole or the pulvinus is the seat of movement. In leaves, 

 however, the process, is much more complex, as their adjustment 

 to a position parallel to the direction of light rays is not merely 

 attained through unequal growth, but through complex twisting 

 curvatures. 



Until quite recently much discussion has centered around the 

 question as to precisely what is perceived by the sensitive organs 

 of a plant : the direction of light or the difference in the intensity 

 of illumination to which the two sides of an organ are subjected. 

 The question may be considered as finally solved in favor of the 



Fig. 129. — Sprouts of oats: a, before the 

 experiment; b, decapitated; c, the removed 

 tip is attached by means of gelatin; 

 d, phototropic curvature of a sprout with 

 a fixed tip, the growing side of which is 

 shaded (after Molisch). 



