85 

 This concept is the essence of current maintenance control concepts for 

 waterhyaci nth control on large riverine systems (Joyce, 1977). 



As a final summary to this discussion, the following comments are 

 offered. Despite years of intensive research, the role of growth 

 substances in the life of the intact growing plant appears far from 

 clear. In fact, a new concept appears to be emerging in plant growth 

 substance theory. Trewavas (1981) stated, "Those who work in the area 

 will be only too familiar with the often-confusing contradictions, the 

 apparently endless and puzzling interactions and the plain uncertainties 

 of supposedly established facts. Even the outline of a simple physiolo- 

 gical mechanism of control for any growth substance in the intact plant 

 cannot be deduced with any certainty" (Trewavas, 1981, page 203). The 

 entire concept of plant "hormones" as substances which have localized 

 biosynthesis, control physiological and biochemical events by changing 

 their concentration, and cause actions at a distance from their synthe- 

 sis stems from mammalian hormore theory and was challenged by Trewavas 

 (1981). Much of the confusion surrounding plant growth substances was 

 blamed on the prevailing trend of explaining the actions of these 

 substances in terms of mammalian hormonal theory. Trewavas (1981) 

 suggested that growth substances might represent, instead, a form of 

 cell-to-cell interaction or communication and that the varying responses 

 to substances such as gibberellins by different species and tissues can 

 be explained by differing sensitivity of plants to gibberellins. This 

 concept may explain the variation in responses of waterhyaci nths to 

 GA 3 and 2,4-D under environmental and physiological conditions 

 referenced and investigated in this study. 



