THE TRANSPORT OF GROWTH SUBSTANCES 79 



movement of dyes depends upon the charge of the dye and the 

 potential gradient in the plant. It was concluded that growth 

 substance is transported from the aerial portion of a plant toward 



the root. 



The fact that growth substance may be transported both 

 upward (transpiration stream) as well as downward in the axes of 

 tomato, tobacco, artichoke, and other plants (Zimmerman and 

 Wilcoxon, 1935; Zimmerman and Hitchcock, 1935) sheds no 

 particular light on the question of electrophoresis in living tissues. 

 Ramshorn (1934) pointed out that variations in growth brought 

 about by modification of the supply of growth substance might be 

 accounted for by changes in electrical potential. It is difl&cult 

 to say in all cases whether the observed changes in electrical 

 polarity are the cause or the result of the distribution of growth 

 substance (Czaja, 1935a). In view of the few and diverse 

 reports on the subject, it is not possible at present to present a 

 complete explanation of the longitudinal transport of growth 

 substance. 



Movement of Growth Substance from Agar Blocks into 

 Decapitated Avena Coleoptiles. — The manner of entry of growth 

 substance into plants from the outside has been studied by 

 Thimann and Bonner (1932). By applying an agar block 

 containing growth substance to each of a series of Avena test 

 plants (for a standard time) and measuring the resulting curva- 

 tures, it was found that a constant fraction (13 to 21 per cent) of 

 the growth substance present in the block passes into a given 

 coleoptile. It was calculated that the rate of growth-substance 

 movement from the block into the plant is proportional to the 

 concentration of the substance in the block. 



SUMMARY 



It is difficult to explain the transport of plant-growth hormones 

 in the higher plants from the place where they are formed to 

 the place where they become functionally active. Movement 

 apparently takes place either through the nonliving xylem cells 

 in the vascular bundles or through the living cells of the phloem 

 and parenchymatous tissues. Translocation through young, 

 living tissues is polar. The substances move in a morphologically 

 basipetal direction, for example, prevailingly from the tip region 

 downward in the Avena coleoptile and from the embryonic 



