60 BULLETIN 700, U. S. DEPARTMENT OE AGRICULTURE. 



panslon of the leaf. The curve representing the water used per plant, 

 on the other hand, is more or less intermediate in slope between the 

 temperature and evaporation summation curves, and it is indicated 

 that the water used per plant is determined both by temperature and 

 evaporation. I 



The above figures are based on average measurements of 20 speci- 

 mens in each type, but the data may not be adequate to justify the 

 statement that leaf elongation and expansion of plants in general 

 are locally, and under similar conditions, controlled more by evapo- 

 ration than by temperature. Where the evaporation is especially 

 high owing chiefly to factors other than high- wind movement, how- 

 ever, as in the oak-brush type, the data appear to warrant the con- 

 clusion that evaporation is the limiting factor in leaf expansion and 

 consequently in the production of dry matter and other physio- 

 logical activities of economic importance. This conclusion is fur- 

 ther substantiated by the data presented in figures 30 and 31, show- 

 ing on the one hand relatively high water requirement and on the 

 other a correspondingly low production of dry matter in a unit of 

 leaf area in the oak-brush type. The correlation between high evapo- 

 ration and low production of dry matter may be explained either by 

 the lack of proper turgor in the leaf cells during the long diurnal 

 periods of high transpiration, or by the fact, that egression of water 

 molecules from the stomata and cells adjacent thereto is so great as 

 to prevent free ingression of carbon dioxide essential to photosyn- 

 thesis. 



From the lower border of the aspen-fir type (about 8,000 feet eleva- 

 tion) throughout this 'association and in the less exposed sites of the 

 spruce-fir type temperature and evaporation may exert equal effect 

 on the plant. 



EFFECT OF EVAPORATION AND TEMPERATURE ON SEASONAL MARCH OF GROWTH RATES 

 OF WHEAT AND BROME GRASS. 



While a measure of the relation of climate to the development of 

 vegetation may be integrated by summarizing the climatic data 

 and recording the dry matter produced by comparable plants during 

 the entire growing season, the relation may best be known through 

 concrete comparisons made at more or less regular intervals through- 

 out the season. This is especially true of the more elevated regions, 

 where weather within a season is subject to wide variation. Even if 

 the relations between plant growth and weather were known, how- 

 ever, the factors affecting growth vary in a more or less unpredictable 

 manner, so that the yield of a given crop could not be correctly 

 judged much in advance of actual harvest. 



If the assumption that leaf-expansion rate is retarded by evapora- 

 tion is correct, the graph of evaporation platted period by period 



