18 TORREYA 



the author finds himself in the awkward position of, on the one hand, agreeing 

 with Mason and not refuting Clements, and on the other hand agreeing with 

 Clements and not refuting Mason. The extreme is the maximum or minimum 

 of an environmental factor beyond which functioning ceases. The mean is an 

 abstraction pure and simple and cannot directly enter any equation of stimulus 

 and response in the physiology of the plant. Under no circumstances is it an 

 environmental factor. The author maintains that Clements' view is to be used 

 "with a long time point of view and with whole associations being considered," 

 while Mason's view is useful when individual organisms in the margin of their 

 range are being considered. He points to the migration of the redwood forest 

 through time as being under the influence of the migration of the mean. He ad- 

 mits, however, that this migration is initiated as a function of the extremes act- 

 ing upon individuals. The species of the flora moved forward with permission 

 of a change in position of the extremes and are eliminated behind under com- 

 pulsion of a shift in position of the extremes. It must be pointed out that an 

 association owes its existence to the complete or partial coincidence of the toler- 

 ance ranges of each of the component species. Hence there is no reason to 

 assume that the behavior of an association of species will be controlled by other 

 factors than those controlling the individuals making up the association. The 

 author further states that the mean "characterizes associations." This is danger- 

 ously close to saying that the mean may serve as an indicator of the association. 

 The mean of enrivonmental factors is often capable of being expressed in very 

 definite figures, but no one has as yet discovered how these figures can be ap- 

 plied directly with significance to the organism. Its significance is solely as a 

 tool of statistical computation. The problem of extremes in their action on 

 plants is not a statistical problem. 



Another point raised by the author concerns the complex problems of the 

 interaction of factors. He points to the environment as being "holocoenotic," 

 meaning that the factors of the environment act collectively and simultaneously. 

 This having been stated, he proceeds to say, "It is erroneous, then, to speak 

 of a single factor as being limiting, quite definitely, the environment is holo- 

 coenotic." I will grant that the interaction of factors complicates enormously 

 our analysis of their operation, but I challenge the conclusion that single factors 

 may not be limiting. The farmer in our western states goes to great expense to 

 build an irrigation system. He knows by experience that water is a limiting 

 factor to the plants he is growing. To be sure, water enters into many of the 

 reactions that go on within the plant and is an agent of transport of a complex 

 series of substances which in themselves may at times be limiting, but it is 

 in these very functions that water may be limiting. This does not imply that 

 water works alone nor that the processes are simple nor that any one factor 

 is more important than any other. The fact remains that water, depending on 



