142 PLANT GROWTH SUBSTANCES 



results have been obtained regarding the absorption of plant growth 

 regulators. 



Such substances as indoleacetic acid and the phenoxy compounds are 

 apparently absorbed by most leaf-surface cells of plants. Their absorption 

 by leaves does not seem to be related to the presence of stomatal openings 



(59). 



Because of the Inadequacy of methods at hand we have not yet 



been able to measure directly the effect of age or stage of development 



on the rate of absorption of growth regulators by plants. It Is known that 



when older, more mature leaves or stems are treated the plants do not 



respond as readily to growth-regulating substances as when the chemicals 



are applied to young vigorously growing parts, but this may be due to 



factors other than absorption of the chemical (59). 



The evidence, so far, Indicates that leaves of certain dicotyledonous 

 plants may absorb certain growth-regulating substances somewhat more 

 readily than do leaves of monocots such as oats and barley (62). Thus 

 when 10 micrograms of radioactive 2-iodo-3-nitrobenzoIc acid was ap- 

 plied the young leaves of bean plants absorbed roughly twice the amount 

 absorbed by the leaves of barley plants. Thus far, most of the work 

 concerning the absorption of growth regulators by plants has not dealt 

 with the direct measurement of the rate of absorption of the chemical. 

 However, Rice (48), using a direct method of measurement, reports that 

 a large part of the 2,4-dIchlorophenoxyacetic acid that he placed on 

 bean leaves was absorbed during the first 4 hours following treatment. 

 He measured the amount of the chemical that was left on the surface 

 of the leaf then subtracted this amount from that which had been 

 Initially added thus obtaining the rate of absorption. 



Bean plants have absorbed a sufficient amount of 2,4-D within 4 

 to 6 hours to bring about a maximum response (60). Absorption of 

 2,4-D apparently begins as soon as the chemical comes in contact with 

 the plant. In order for activated charcoal to absorb effectively 2,4-D 

 that had been added to the surface of a plant, Weaver (57) had to apply 

 the charcoal to the surface within 15 minutes after the chemical came 

 into contact with the plant. It Is a matter of common observation that 

 young and succulent plants often show marked growth responses within 

 30 to 60 minutes following the application of a growth regulator of 

 the phenoxy type, and it has long been known that indoleacetic acid is 

 quickly absorbed by succulent tissues of such plants as tomato, bean, 



