THE CLIMATIC CONDITIONS OF THE UNITED STATES. 



I. INTRODUCTORY. 



From the last paragraph of the preceding chapter it is clear that no 

 adequate description of the environmental conditions that obtain in 

 any area is even to be attempted for a long time. In the present 

 chapter will be brought together merely the results of certain studies 

 which we have been able to carry out upon a very few conditions, and 

 upon large areas. Some of the conditions studied do not directly 

 affect plant Hfe at all, it being usually impossible as yet to obtain 

 quantitative information upon the subjects most pertinent to our 

 general line of inquiry. Only in a single case have we attempted 

 actually to obtain measurements of an environmental factor de novo; 

 for the rest we have simply made use of data already collected. As is 

 clear from the preceding analyses, to obtain the kinds of information 

 most needed for such a study as the present methods, will have to be 

 employed which are as yet quite unknown; adequate procedures re- 

 main to be devised. Nevertheless, so great is the inertia of routine 

 that there is little hope that the trend of observational work will alter 

 very profoundly in the near future, and, as has been stated, we have 

 deemed it advisable to make what use is now possible of the informa- 

 tion at hand, with the hope that the very inadequacy of our whole 

 presentation may itself be a potent stimulus toward the acquu-ement, 

 in the future, of the kind of climatic observations upon which alone 

 anything like a scientific ecology or agriculture may eventually be 

 founded. 



Of the five main groups of external conditions which influence plant 

 activities, discussed in outline in Chapter II, we shall consider here, 

 and very inadequately, only the first three — moisture, temperature, 

 and hght. For the subjects of chemical and mechanical conditions 

 no information that is at present available can be brought to bear 

 upon the problem of plant distribution in a broad way. It seems 

 probable, indeed, that the distribution of vegetation types is only 

 rarely determined by any of the last-named conditions, though the 

 detailed distribution of many species in any relatively small area is 

 probably often related to chemical and mechanical influences. 



The information so far accumulated upon environmental conditions 

 has not been obtained primarily with reference to plant activities; it 

 has been brought together mainly in the interest of meteorology, clima- 

 tology, and weather prediction. Therefore it is impossible at present 

 generally to select for study those conditions that directly affect the 

 plant. We have been forced, in the main, to study conditions or 

 factors that are more or less remote causes of the immediate conditions 



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