Developmental Effects of Auxins 101 



effectively preventing lateral bud growth. If the plant apex were the 

 only source of auxin in the plant one would expect that it alone would 

 control bud inhibition. But of course young expanding leaves are also 

 rich sources of auxin, and leaves are known to exert an inhibitory 

 effect on the development of the bud in their axil (Snow, 1929; Good- 

 win, 1937). The production of relatively large amounts of auxin in 

 the apex has been associated with the development of a relatively 

 unbranched plant form and conversely, plants which contain relatively 

 low auxin levels develop a branching habit (Delisle, 1937). Perhaps the 

 most dramatic demonstration of auxin levels as determining factors 

 in plant form was that made by van Overbeek (1935), in which he 

 showed that a dwarf variety of corn (variety Nana) contained a much 

 lower auxin level than non-dwarfed varieties. Associated with this low 

 auxin level was a less rapid rate of elongation, a branching habit, and 

 even a tendency to lose the upright position and to become prostrate. 

 This effect of low auxin levels was demonstrated to be attributable 

 not to a lower auxin production rate, but instead to a high rate of 

 auxin destruction. 



Since the auxin level in a plant or a branch of a plant is a 

 primary factor in the inhibition of lateral buds, then treatments which 

 would reduce the auxin level should stimulate branching. Various 

 treatments which might destroy auxin in a plant, such as x-irradiation, 

 the injection of dyes in the light, the addition of auxin antagonists, 

 and changes in photoperiod, have all been found to increase branch- 

 ing of dicots or tillering of grasses (Skoog, 1935; Johnson, 1936; Caj- 

 lachjan and Zdanova, 1938; Leopold, 1949). 



The control of branching in roots appears to involve an auxin 

 mechanism similar to that operative in shoots, for the decapitation 

 of roots can cause branching. However, an additional feature appears 

 to be involved in the control of root branching, presumably a factor 

 originating elsewhere than in the tip (Torrey, 1950). 



The inhibition of the growth of buds by auxins is not an entirely 

 simple phenomenon and it has been shown that bud inhibition may 

 not be strictly a function of absolute auxin level. Le Fanu (1936) re- 

 ported that auxin applied to the base of pea stem cuttings is con- 

 siderably more inhibitory than auxins applied to the apex. The evi- 

 dence that some auxin can move in a non-polar manner would seem 

 to suggest that the direction of auxin movement may have a consider- 

 able bearing upon its inhibitory effectiveness. 



The development of flowers at the apex of a stem is often associ- 

 ated with the development of lateral branches and a loss of apical 

 dominance, a good case in point being the zinnia. Since it has been 



