JOHN W. MITCHELL 145 



pounds are now available, a means of testing this proposition is at hand. 

 Regarding the absorption of growth regulators, we can conclude 

 that the uptake of these substances by plants is greatly influenced by 

 such factors as the age of the tissues to which the chemical is applied, 

 light, temperature, and the presence of surface-active substances which 

 tend to increase the rate and extend the period of absorption. 



Translocation 



Two methods have been used to measure translocation of synthetic 

 growth-regulating substances in intact plants: first, evidence of a growth 

 response some distance from the treated area has been used as an indica- 

 tion that the growth regulator was translocated within the plant; second, 

 some growth-regulating substances have been tagged with radioactive 

 isotopes and the course of their movement through the plant followed 

 by means of usual tracer techniques. The path of translocation of growth 

 regulators depends to some extent upon the way the chemical is ab- 

 sorbed by the plant. Taken in through the roots the growth regulator 

 in most instances is moved rapidly upward through the water-conducting 

 tissues (22,35). Absorbed by leaves, however, the chemical is trans- 

 located mainly in living cells of the phloem. If, for instance, a few 

 micrograms of 2,4-D are placed on a leaf, the plant can, under some 

 conditions, rapidly absorb and translocate the chemical to the stem 

 where it is moved in both an upward and a downward direction (12). 



The amount of growth-regulating substance translocated from a leaf 

 depends upon several factors. Young rapidly growing leaves may absorb 

 2,4-D but fail to translocate it to other parts of the plant (35). Phenoxy 

 compounds are translocated more readily from leaves of medium age 

 than from either younger or older ones (22,39). Similarly, in stem tissues 

 most marked over-all responses have been observed when the chemical 

 was applied to the young, succulent portion of the stem, least when the 

 chemical was applied to the older, hgnified portion near the soil level 

 (24). Movement of 2,4-D from leaves apparently involves the same 

 mechanism as that used by the plant to translocate the products of 

 photosynthesis or a similar one. Growth regulators of the phenoxy type 

 are not translocated from leaves under conditions unfavorable for the 

 production and translocation of photosynthate (25,35,48,58). Thus 2,4-D 

 was not translocated from leaves kept in C02-free air or in darkness. 



