Translocation of Growth- Regulating Substances and 

 Their Effect on Tissue Composition 



JOHN W. MITCHELL 



PLANT growth regulating substances are readily absorbed by plants. 

 These chemicals penetrate the living surface cells of most plant parts 

 and seem to move as readily through intact epidermal cells as through 

 injured ones. Synthetic growth regulators readily gain entrance to the 

 plant when applied to roots, stems, leaves, flowers, or fruits. They are 

 absorbed by these organs even though the surface cells of some may be 

 protected by thick walls and a layer of cutin. 



Three methods have been used in studying the rate of absorption 

 of growth-regulating substances: i) the detection of morphological or 

 histological responses that occur some distance from the treated area 

 has been used to indicate that the plant has absorbed the growth- 

 regulating substance; 2) measurement of the amount of residual growth 

 regulator left on the surface of a treated area has been used to indicate 

 the amount of the chemical absorbed; 3) radioactive tracers have been 

 used to some extent in studying the absorption of these chemicals. As 

 yet, however, we do not have a direct and completely reliable method of 

 measuring the absorption of growth regulators by plants. All three 

 methods have drawbacks. If we attempt to measure absorption by 

 means of radioactivity, or by evaluating a morphological or histological 

 response that occurs some distance from the treated area, then transloca- 

 tion of the chemical is involved as well as its absorption. If we attempt 

 to measure absorption by detecting the amount of the chemical remain- 

 ing on the treated surface of the plant, then we are confronted with the 

 problem of how to remove this surface residue quantitatively so that it 

 can be measured accurately. In spite of these difficulties some reliable 



