146 PLANT GROWTH SUBSTANCES 



Rice (48) has reported, however, that absorption of 2,4-D by leaves of 

 bean plants was not afTected by increasing the light intensity from 100 

 to 900 foot-candles. 



The chemical nature of the growth regulator may in itself influence 

 the rate of its absorption and translocation. The morphoHne salt, the 

 butyl ester, and the acid forms of a phenoxy compound (parent com- 

 pound 2,4-dichloro-5-iodophenoxyacetic acid) have been applied in 

 equal molecular amounts to bean leaves and their absorption and trans- 

 location followed by means of radioactive tracers (39). Of the three 

 forms, the acid was translocated in the greatest amount, the salt in the 

 least amount. 



Under constant conditions bean leaves apparently absorb and trans- 

 locate 2,4-D at a relatively uniform rate for a period immediately 

 following treatment. In recent experiments (39) 2,4-D acid was apphed 

 in an aqueous mixture containing Tween 20 to bean leaves, 10 micro- 

 grams per leaf. On the basis of tracer measurements the amount ab- 

 sorbed and translocated to the stems was essentially linear with time 

 during a period of four days following treatment. 



Not all growth-regulating substances are translocated from leaves even 

 under the most favorable conditions. Certain nicotinium compounds, 

 such as 2,4-dichlorobenzylnicotinium chloride, have a systemic regulat- 

 ing effect when apphed to succulent stem tissues of bean plants (44). 

 These same compounds were not effective when applied in amounts of 

 even a milligram or more to leaves or to the cotyledons of the plants. 

 There is reason to believe that the method by which plants absorb and 

 translocate different growth-regulating substances may vary widely in 

 some respects, but so far these differences are not understood. 



Summarizing the data so far, it is evident that the rate of translocation 

 of growth-regulating substances from a leaf is not related to the rate at 

 which the chemical may be absorbed by the leaf. Translocation of such 

 regulators as the phenoxy compounds is associated in some way with 

 the translocation system involved in the movement of the products of 

 photosynthesis. If activated diffusion plays a part in the translocation 

 of growth regulators, as has been suggested by Clark (10), then such a 

 phenomenon must be governed by external factors, including light 

 and carbon dioxide supply; for without adequate illumination and 

 carbon dioxide, growth-regulating substances are apparently not trans- 

 located from leaves of plants. 



