264 Auxins in Agriculture 



larly through the phenomenon spoken of as apical dominance. The 

 dualism of the effects of auxin on Inid growth has been clearly pointed 

 out by Thimann (1937) (see figure 46, p. 95). The response of buds 

 to auxins is essentially like the response of stems, but with the opti- 

 mum concentration much lower. A specific case showing the dual 

 action of auxin on bud growth is seen in the data of Skoog (1939). 

 given in figure 108. Small amounts of auxin promoted bud growth, 

 whereas larger amounts inhibited or entirely prevented it. 



At about the same time, Guthrie (1939) discovered that the treat- 

 ment of potato tubers with auxins could prolong dormancy in stored 

 tubers. Sample data showing the quantitative inhibitory effects arr 

 given in figure 109. This experiment was carried out with the methyl 

 ester of naphthaleneacetic acid placed on paper shreds which were 

 scattered among the tubers. It is interesting to note that at lower 

 temperatures (10° C) higher concentrations of the auxin were neces- 

 sary to control sprouting. Also it is of particular interest to notice 

 that even in this first report Guthrie observed that under some cir- 

 cumstances, not then clearly defined, auxin treatment could actually 

 stimulate sprouting of potatoes. This stimulation is seen in the data 

 for tubers held at 22° F in figure 109. In some of the experiments 



O 



Q 



ID 

 GQ 



10"* 10'^ 10"* 



CONG. OF AUXIN 



10" 



10'^ M 



Fig. 108. The effects of various concentrations of auxin (indoleacetic acid) on the 

 growth of pea buds in excised culture (Skoog, 1939) 



