Occurrence and Role of Growth Hormones 93 



delay in expression of the auxin reduction would suggest that the 

 auxin-producing system had been suppressed by the heat treatment. 



It is known that high temperatures can reverse both the vernaliz- 

 ing effects of low temperatures and also the reproductive stimulus fol- 

 lowing the auxin treatment of seeds (Purvis and Gregory, 1945; Leo- 

 pold and Guernsey, 1953). It is possible that these high temperature 

 effects may act in part through auxin destruction or suppression of 

 auxin formation. Exposure of rice seedlings to warm temperatures 

 (26° C) is known to produce striking reductions of auxin content when 

 they are compared to seedlings in cool temperatures (10° C) (Sircar 

 and Das, 1951). 



