58 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



EFFECT OF EVAPORATION AND TEMPERATURE ON THE GROWTH OF THE PLANT 



AS A WHOLE. 



It has been shown that the efficiency of the leaves, both as users of 

 water and as producers of material, is much influenced by climatic 

 conditions. The true significance of this fact, however, can be fully 

 understood only when the development of the plant as a whole, 

 including stem height, total seasonal and periodic leaf expansion, and 

 similar activities, is correlated with the controlling climatic factors., 



Temperature and evaporation are, as the preceding discussion has 

 shown, undoubtedly the limiting factors in the locality in which this 

 investigation was conducted, and hence it is the chief aim to show 

 physiological activities in relation to these factors. 



EFFECT OF EVAPOEATION AND TEMPERATURE ON THE GROWTH OF WHEAT. 



Since more measurements were taken in the case of wheat than of 

 the other species, wheat is here selected to show correlations between 

 its development and temperature and evaporation. In this connec- 

 tion it should be stated that the correlations obtained between cli- 

 matic factors and growth of wheat hold generally for the other 

 species employed. 



Four sets of measurements of wheat and the daily temperature and 

 evaporation obtained for the period of growth concerned were 

 recorded in each type station simultaneously. The data are summa- 

 rized in Table 17 and platted in figure 32. 



TABLE 17. Growth of wheat as related to evaporation and temperature in type 



stations. 



In figure 32 the direction of slope of the graphs representing total 

 leaf length and average dry weight per plant is similar, a pronounced 

 convexity upward occurring in the aspen-fir association. These meas- 

 urements, then, are in inverse proportion to the evaporation. On the 

 other hand, no apparent correlation exists between the curves repre- 

 senting the average stem length and evaporation. Since the curve of 

 evaporation bears no distinct correlation to the temperature summa- 

 tion curve, it would appear that the height growth, or elongation of 

 the plant, is determined more by the temperature than by the evapo- 

 ration, the factor which apparently determines elongation and ex- 



