K. V. THIMANN 



In particular, the time of year and the age of the plant from which 

 cuttings are taken remain as important variables. A few species also 

 do not respond at all to auxin, their rooting being limited by other 

 factors which are not yet understood. 



A second major application depends on the fact that auxin not 

 only inhibits lateral bud development, as described above, but also 

 inhibits the formation of the abscission layer, a special layer of cells at 

 the base of a petiole or fruit stalk, whose walls separate very readily, 

 resulting in the falling to the ground of the leaf or fruit. When the 

 fruit is sprayed with auxin solution, therefore, the formatipn of this 

 layer is delayed and the fruit remains on the tree for longer. Such 

 spraying has been very successful with apples, since the delay of a 

 week or two helps greatly to solve the long-standing problem of early 

 fruit fall. The most effective auxin for this purpose appears to be 

 a-naphthaleneacetic acid. 



The effect of auxin in promoting the swelling of the ovary, men- 

 tioned above, has found interesting application in the production of 

 seedless fruit. The auxin is applied to the style of the mature flower, 

 either individually by hand, or by using a liquid spray or even vapor. 

 The ovary begins to swell in a normal-looking way and the process 

 continues, to produce seedless fruit. Tomatoes and holly are the out- 

 standing successes obtained by this procedure, but seedless pears and 

 even watermelons have been produced, and the method may have an 

 important future [see the review by Gustafson (2a)]. 



Lastly, the treatment of seeds with auxin stimulates root de- 

 velopment and may result in a promotion of growth which lasts for 

 many weeks or months. The conditions necessary for this stimulation 

 are, however, not fully understood and the results reported are so con- 

 flicting as to cast doubt on the legitimacy of this application. The 

 toxicity of some auxins in high concentrations has recently led to an 

 application as weed killer. An excellent summary of all of this ap- 

 plied work is given by van Overbeek (8). 



All these applications have led to a scramble for trademarks, 

 and even patents, and the emergence of special methods of advertising 

 and marketing. Indoleacetic, indolebutyric and naphthaleneacetic 

 acids are now on the market in the form of solutions, pastes, powders, 

 and pills. 



(b) Auxin Reserves. The auxin content of plant tissues is of 



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