396 THIMANN 



fruits, especially apples, when nearly ripe they can be made to stay on the 

 trees through autumn winds, or until it is convenient to pick them. 



Curiously enough, fruit growers now also spray their trees with auxin very 

 early in the season to promote the fall of fruit, a practice widely used as a 

 standard method for thinning apples. The reason for this opposite effect 

 is believed to be as follows: when the ovule is fertilized there is an immediate 

 growth of the ovary, due partly to auxin brought in by the pollen and partly 

 to production of auxin by the ovary tissue itself. Shortly afterward the endo- 

 sperm begins to produce auxin, and shortly after that the growing embryo 

 itself takes over the task of auxin production. As a result a stream of auxin 

 flows from the young fruit into its pedicel, and this is considered to prevent 

 abscission in the same way as above. The stream is interrupted temporarily, 

 when one tissue which forms auxin takes over from another, and to such 

 interruptions the natural drop of the fruit (e.g., "June drop") is ascribed. 

 But the auxin is applied as spray in a concentration high enough to kill 

 many of the young embryos, and perhaps the ovaries too; their death means 

 the cessation of their secretion of auxin, and therefore the falling of the 

 young fruit. The full explanation may be still more complicated, but the 

 working out of these events has in any case brought out one rather general 

 principle, namely, that the production of auxin is usually a transient activity 

 which is soon given up and taken over by other tissues. 



The inhibition of growth processes, such as bud development or abscission, 

 by a substance which primarily causes growth promotion, is hard to under- 

 stand. A great many theories have been proposed, most of which are open 

 to serious objection, and some workers, finding the phenomenon puzzling, 

 have even attempted to deny the facts. It seems probable that some kind of 

 balance is involved between auxin and other factors and that buds or fruits 

 continue to develop only when this balance is maintained, while excess of 

 either one inhibits. Some of these factors are beginning to be known, but the 

 working out of the process in detail will have to be described in the report 

 of the next fifty years. 



The above are not the only inhibiting actions exerted by auxin. For the 

 elongation of roots, in opposition to that of shoots, is strongly inhibited by 

 applied auxin. Only at excessively low concentrations, far below those 

 promoting growth of shoots, does auxin promote root elongation, and then 

 the effect is of small magnitude; isolated roots grown in culture and detached 

 short segments of roots usually show a small promotion, but some plants show 

 none at all, only inhibition. The root tip has been shown, by standard test 

 methods, to produce small amounts of auxin, and this is probably in the 

 inhibiting range, since, at least in some cases, careful decapitation without 

 removal of the elongating zone may slightly promote the growth of the root. 

 One characteristic difference between roots and shoots is their opposite tro- 

 pisms, for roots react positively to gravity and negatively (in many cases) to 



