Factors Altering Effectiveness 141 



The rapidity of absorption of aqueous auxin sprays indicated by 

 tlie observations of Overholser et nl (1943) and subsequent workers is 

 not at all unique to auxins. The effectiveness of sprays of inorganic 

 salts such as copper sulphate is practically unaltered by washing off 

 as soon as three hours after application (Blackman et al, 1949). 



The acidity of the auxin solution may greatly change the rate 

 of absorption (Crafts, 1949), and the level of acidity most effective for 

 rapid absorption will often be closely dependent upon the age and 

 constitution of the foliage. Crafts has pointed out that young foliage 

 is generally more responsive to polar solutions, whereas the surface of 

 the more heavily cuticled, older foliage is itself more lipoidal and 

 consequently more responsive to non-polar solutions such as esters. For 

 this reason free acids or salt formulations are most effective on young 

 foliage, while esters are most effective on old foliage. Similarly the 

 surfaces of roots are generally polar in nature. Hence acids and salts 

 are generally more effective than esters when applied to soil solutions. 



The use of carriers such as polyalkylene glycols or detergents 

 apparently permits the entry of polar acids and salts by normally non- 

 polar pathways. This may account for the somewhat slower rate of 

 entry apparent in figure 63. It is pertinent to note that such carriers 

 have no beneficial effect upon absorption of non-polar formulations 

 such as the esters of auxins (Staniforth and Loomis, 1949). 



The absorption of auxins by plant tissues in solutions should be 

 mentioned here. Albaum et al (1937) followed the entry of indoleacetic 

 acid into Nitella cells by means of the ferric chloride color test. They 

 observed that the auxin appeared in the cells most rapidly at pH's 

 below the pK value (4.75). They conclusively demonstrated that auxin 

 entered the cells as the undissociated acid, and that such entry was 

 a simple diffusion process. This problem has been reopened by Rein- 

 hold (1954), who followed uptake by the disappearance of auxin 

 from the external solution surrounding pea and carrot sections. She 

 obtained evidence for two uptake mechanisms — a physical difEusion 

 mechanism, and a metabolic mechanism as well. The former appears 

 to be operative upon undissociated auxin molecules as Albaum et nl 

 had reported, but the metabolic mechanism was independent of dis- 

 sociation of the auxin. How much of the disappearance of auxin from 

 the medium by the metabolic process is in fact destruction of the 

 auxin was not established. 



TRANSLOCATION 



Once the auxin is inside the plant, a major factor determining its 

 ultimate effectiveness will be the ease with which it is translocated 



