2 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTUEE. 



It is a matter of common knowledge that the life cycle and struc- 

 tural characteristics of plants are largely determined by the climatic 

 conditions prevailing in the habitat, but the quantitative relations 

 existing between the potent climatic factors and the vegetative activi- 

 ties are not well known. Though the ecologist and plant geographer 

 have shown that a given plant association may have well-defined 

 geographical limits/ which in turn are characterized by rather dis- 

 tinct complexes cf environmental (climatic) conditions, they have 

 not as yet definitely determined which climatic factor,' or set of fac- 

 tors, is most influential in affecting distribution, growth, and physio- 

 logical activities generally.^ This is attributable to several condi- 

 tions. In the first place, the relation of plant development to envi- 

 ronment is exceedingly complicated and can be determined quanti- 

 tatively only when the most influential physical factors are recog- 

 nized, recorded, and properly interpreted. Secondly, the climatic 

 factors of a given habitat, and, indeed, of different habitats, which 

 have to do with the limitation of the life process, are in themselves 

 more or less indefinite; they are highly complicated and variable, 

 and their intensity can not always be measured fully by instruments. 

 In the third place, methods have not been sufficiently advanced to 

 warrant serious investigations. Owing to present lack of knowledge 

 of the response of plant activities to climate, there is wide diversity 

 of opinion as to how best to summarize and integrate climatic data. 

 Temperature studies conducted by Livingston,^ Lehenbauer,* Mer- 

 riam,^ and McLean,'' and researches on soil humidity and " growth 

 water " carried out by Briggs and Shantz,^ Shreve,^ Fuller,'' and 

 others have shown that climatic factors can not to advantage be 

 expressed empirically. Suitable methods of integrating the potent 

 climatic factors, as well as of recording growth and other plant f unc- 



^ Drude, O. Entwurf einer biologischen Eintheilung der Gewachse. (A. Shenk, Hand- 

 buch der Botanik, III, p. 487.) 



^ As a preliminary study It would be desirable to reduce the complexes of the environ- 

 mental factors to their simplest form, which, under controlled conditions, might be ac- 

 complished by maintaining constant all but the factor investigated, and in this way 

 determining the effectiveness of each. This being done, however, the combined influence 

 would have to be integrated in order to approach conditions in nature. Further, since 

 under natural conditions climatic factors vary widely, bo'.h in intensity and in dura- 

 tion, such important variations must necessarily be included in the equation. 



3 Livingston, B. B. and G. J., Temperature coefficients in plant geography and cli- 

 matology. Bot. Gaz. 56: 346-375. 1913. 



* Lehenbauer, P. A., Growth of maize seedlings in relation to temperature. Physiol. 

 Res. 1 : 247-288. 1914. 



^ Merriam, C. Hart, Laws of temperature control of the geographic distribution of 

 plants and animals. Natl. Geog. Mag. G : :^20-238. 1894. 



^ McLean, F. T., A preliminary study of climatic conditions in Maryland, as related 

 to plant growth. Physiol. Res. 2 : 129-207. 1917. 



' Briggs, Lyman J., and Shantz, H. L., The wilting coefficient for different plants and 

 its direct determination. U. S. Dept. Agr. Bui. No. 230 : 7-77. 1912. 



^ Shreve, F., Rainfall as a determinant of soil moisture. Plant World. 17 : 9-20. 

 1914. 



^ Fuller, George D., Evaporation and soil moistui-e as related to the succession of plant 

 associations, Bot. Gaz., 58 : 193-234. 1914. 



