R. H. BURRIS 95 



action by enhanced respiration has been derived from observations of 

 intact plants, whereas the idea of herbicidal action by respiratory inhibi- 

 tion has arisen from studies of tissue sHces or cell-free enzyme systems. 

 We must accept Dr. Went's warning that the percentage of water in 

 any given segment of a steamship does not necessarily define the role of 

 water therein. Whenever possible we must devise experimental checks 

 with intact plants to test our observations made with isolated enzyme 

 systems. 



Of the enzyme systems investigated for their relation to herbicidal 

 action the respiratory enzymes have received most attention. This is 

 not surprising, for they are perhaps the most intriguing and widely 

 investigated group of enzymes in other regards, and we all admit their 

 vital importance in supplying the driving force for cellular processes. 

 In addition, work with growth substances has often turned up effects 

 which strongly implicate respiratory processes. 



In this statement of the problem we have chosen to discuss the mode 

 of action of herbicides, but this has merely served as an example. It is 

 open to question whether herbicidal action and other effects of growth 

 substances stem from an influence on a common enzyme system. We are 

 still some distance from a complete explanation in biochemical terms of 

 the mode of action of plant growth substance. It is our hope that the 

 accumulation of additional data and the synthesis of information in 

 common meetings, such as we have here today, may lead to an elucidation 

 of the basic mechanisms of the many faceted activities of plant growth 

 substances. 



