CLIMATIC CONDITIONS OF THE UNITED STATES. 371 



temperature index are presented below, and these are followed by those 

 obtained by Livingston. 



It is perhaps not out of place here to remark that these moisture- 

 temperature indices represent no more than a first rough approxima- 

 tion toward an environmental index, which might state the efficiency 

 of the environment as a whole to produce plant growth. It is quite 

 obvious that such an environmental index will not really be attainable 

 for a very long time; it must embrace many other terms besides those 

 representing climatic conditions, and also terms for all of the influential 

 climatic ones, and, as has been emphasized, methods for the measure- 

 ment and weighting of most of the environmental conditions are yet 

 to be devised. Nevertheless, progress can best be favored by employ- 

 ing the two climatic indices that seem most promising, with the hope 

 that the shortcomings of the resulting interpretations may suggest 

 closer approximations to the form of index required. 



It may also be remarked that but little real progress can be hoped 

 for in this direction until laboratory facilities become available, by 

 which the relations between plant growth and environmental condi- 

 tions may be experimentally studied. As has been emphasized, this 

 sort of experimentation will require well-planned physical equipment 

 for the control of environmental conditions. It will also require a 

 group of workers who can bend their energies toward gaining a com- 

 mon end, for a single individual, no matter how well equipped with 

 apparatus, can not hope to find it in his power to enter very deeply 

 into these complex relations. Nevertheless, expensive and difficult 

 as the project may seem at present, there can be no doubt that it will 

 be eventually undertaken, nor can it be doubted that the benefits to be 

 derived from properly planned and conducted experimental studies 

 on plant environmental relations will prove fully as great and as 

 valuable to the human race as have been those derived from experi- 

 mental physics and astronomy. It is in the laboratories and observa- 

 tories of these sciences that the nearest approach to the sort of work 

 here contemplated is now being carried on. On the practical, bread- 

 winning side, it needs only to be suggested that the greatest and most 

 important of all human industries, agriculture, rests entirely upon 

 what little knowledge we already happen to possess in regard to the 

 relations between plant growth and environmental conditions. When- 

 ever a workable environmental index for plant growth may be 

 approached, it is certain that the arts of agriculture and forestry will 

 be markedly improved. 



B. MOISTURE-TEMPERATURE INDICES BASED ON TEMPERATURE SUMMATION- 

 INDICES OBTAINED BY THE REMAINDER METHOD (ABOVE 39° F.). FOR 

 THE PERIOD OF THE AVERAGE FROSTLESS SEASON. (TABLE 22, PLATE 70.) 



These values were derived by multiplying the suromation index 

 for the period of the average frostless season, for each station (table 



