80 GROWTH HORMONES IN PLANTS 



regions downward in the stems and hypocotyls of dicotyledonous 

 seedlings. This polar transport may be reversibly abolished by 

 anaesthesia. 



It has been discovered recently that 3-indole acetic acid and 

 other growth substances when added to the soil can be absorbed 

 by the roots and carried upward (apparently in the transpiration 

 stream) into all parts of the plant. This upward movement 

 through dead xylem cells does not necessarily contradict the 

 facts embodied in the foregoing statement concerning prevailingly 

 downward movement through living tissues. 



The characteristic migration of growth substance through 

 excised portions of the Avena coleoptile has been studied by 

 applying it in an agar block at one end of the coleoptile cylinder 

 and recovering it in an agar block at the other end. From 

 experiments of this type it has been possible to distinguish two 

 important characteristics of hormone movement, viz., velocity 

 and amount of transport. The rate of movement (velocity) 

 is about 10 mm. per hour, which is considerably more rapid than 

 could be accounted for by diffusion; it is not affected by tempera- 

 ture and apparently is independent of the concentration gradient. 

 The amount moved in unit time (capacity, or intensity of trans- 

 port) is inversely proportional to the length of the organ at 0°C. 

 At higher temperatures the amount of movement seems to be 

 independent of the length of the pathway. The amount of 

 substance transported is increased by raising the temperature 

 to an optimum in the region of 30 to 40°C. 



Attempts to explain the mechanism of transport have been 

 based upon diffusion, protoplasmic streaming, interfacial tension, 

 and electrophoresis. Of these, the last seems to have much in its 

 favor. The acid character of the auxins would suggest that their 

 active radical might be moved toward the positively charged 

 pole in an electrical circuit. This prediction has been verified 

 experimentally in agar as well as in plant tissues under conditions 

 of controlled electrical potential. It is difficult to conclude 

 whether in all cases the observed changes in electrical polarity are 

 the cause or the result of the presence of growth substance. The 

 recent discovery of movement upward in the transpiration 

 stream makes it impossible, at present, to account for all the 

 phenomena of hormone transport on the basis of a single compre- 

 hensive mechanism. 



