76 



PHYTOHORMONES 



cells. The longer a man has been without exercise the less 

 he is able to exercise, and correspondingly the longer a cell 

 has been without auxin, the less it can respond to applied 

 auxin. The loss of sensitivity may be clearly seen from 

 Table IV (p. 40), which shows that if the apical zones are 

 removed, the curvature obtained is less; this is because the 

 basal zones curve less, for a given concentration of auxin, 

 than do the upper zones. Du Buy showed that the same 

 phenomenon namely decreased sensitivity to auxin, occurs 



Fig. 32. Growth rates of basal zones of Avena coleop- 

 tiles. Ten hours after beginning of measurement growth 

 has ceased; at 12 hours the plants are decapitated 

 just above measured zones and auxin (10 mg. indole- 

 acetic acid per gram lanoline) applied to the cut surface. 

 Growth is resumed, increasing for the first 20 hours (re- 

 juvenation), then decreasing again. Ordinate, elonga- 

 tion in per cent per 10 hours. (After Went, 1935.) 



auxin 



J//////////////, 



24 



36 



48 60 hrs 



in old coleoptiles (more than 100 hours from planting). In 

 plants of the same age, physiological aging may be produced 

 by decapitating, and preventing auxin regeneration by rede- 

 capitating after suitable intervals. The first effect of this 

 treatment is to increase the response to applied auxin (see 

 III C 4) because the auxin content of the plant becomes very 

 low, but subsequently, if the lack of auxin continues, the 

 plants become less and less sensitive, — they are ''aged." 

 Apparently this aging is due to the thickening of the cell 

 walls, which continues independent of elongation. Du Buy 

 found that aged plants show a decreased extensibility, as 

 measured by their bending under applied lateral tension, 

 which would explain their decreased response to auxin. That 

 aging is due to thickening of the walls is confirmed by the be- 

 havior of de-seeded plants from which the materials for wall 

 thickening are removed, and which show correspondingly 



