488 PLANT GROWTH AND PLANT COMMUNITIES 



active site of action, but also with the way that highly active but ap- 

 parently somewhat simple chemical entities must exercise and mediate 

 control over the operation of such highly integrated systems as single 

 cells or groups of cells. This seems to be the case in the photoperiodic 

 response that turns upon the redistribution of growth in the shoot apex, 

 and in this redistribution a relatively small group of otherwise quies- 

 cent cells in the central part of the apex is implicated ( W^etmore et al., 

 1959). 



Let us have all the enzymology, descriptive biochemistry, and 

 "molecular biology" we can get, but none of these is of much avail until 

 one has actually shown how the reactions work in, and the causative 

 agents modify, the system that can grow. Thus many problems of plant 

 physiology are now essentially limited by our lack of knowledge at the 

 supramolecular level— i.e., lack of more exact knowledge of what cells 

 are, how they behave, and where and what are the active sites meta- 

 bolic events occur and morphogenetic responses are determined. 



These problems of organization and integration in the living sys- 

 tem thus present a major objective for the future. It is no longer enough 

 to study each physiological function separately. Understanding of the 

 living system requires a synthesis of knowledge, for all these attributes 

 of the organism work together and are coordinated to achieve that 

 "built-in goal of growth" which is the complexity and the challenge of 

 every cell and every organism. 



References 



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Plant Physiology — A Treatise, Vol. II (N. Y., Academic Press). 

 Blackman, F. F., 1905. "Optima and Limiting Factors," Ann. Bot. 19, 281. 

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