Application and Persistence 193 



quent stimulative aitereffect after the roots were removed from the 

 solutions. 



Various other methods of application of auxins have been de- 

 scribed for special purposes, including the overall application of 

 auxins to greenhouse crops by volatilization of esters (Zimmerman 

 et al, 1939), and the incorporation of auxins into sprays such as fungi- 

 cides, insecticides, and fertilizers. Auxins appear to be compatible with 

 most ordinary spray materials provided they do not contain lime or 

 other cations which would form insoluble salts with the auxin. 



The use of a lanolin paste of auxin has advantages for some types 

 of treatments. Unlike the water sprays, lanolin provides a fairly con- 

 tinuous supply of auxin to the treated plant part. Where an effect of 

 long duration is desired, lanolin pastes are very good. 



AUXIN PERSISTENCE AND DESTRUCTION 



Indoleacetic acid is spontaneously inactivated to a certain extent 

 in solution and, to a lesser extent, even in the dry crystalline state. 

 Crystalline indoleacetic acid in time breaks down and turns brown as 

 it does so. For this reason, storage of the dry crystals for more than 

 two or three years tends to lower purity and activity. In solution the 

 breakdown is much more rapid, and noticeable changes are observed 

 in ten days' time even under refrigeration. For this reason it is worth- 

 while to discard stock solutions of indoleacetic acid after one week. 

 Breakdown of auxins other than members of the indole series is 

 generally much slower and storage precautions are less important. 



Persistence in Plants 



The persistence of auxins applied to the plant varies widely with 

 the auxin which is applied. The currently recognized enzyme systems 

 of the plant which destroy auxin are more or less specific to indole- 

 acetic acid as described in chapter III, and for this reason such auxins 

 as naphthaleneacetic acid and 2,4-D are often more effective and per- 

 sistent than indoleacetic acid over periods of time. For example, it 

 is known that sprays of many auxins will force flower initiation in the 

 pineapple plant, but indoleacetic acid sprays are notably ineffective, 

 even though the bulk of the growth hormone in the pineapple plant 

 is indoleacetic acid. Consequently pineapple growers employ sprays of 

 naphthaleneacetic acid or sometimes 2,4-D. Another example of the 

 poor efficiency of indoleacetic acid is in the case of induction of par- 

 thenocarpic fruit-set in the tomato. Spraying with indoleacetic acid is 

 very much less effective than applying many other auxins to stimulate 

 fruit-set. Some auxins which are highly effective in setting fruit in the 



