824 EXPERIMENT STATION RECOED. [Vol.85 



The rate of water loss from a corn plant was found to be affected in a rather 

 similar manner by changing climatic conditions as it was from a physical free 

 water surface. The amount of water transpired from a given area of leaf 

 expanse was determined as approximately one-third as great as the evaporation 

 from a free water surface of the same area. The rates of transpiration and 

 evaporation, following the same general sequence as the diurnal climatic factors, 

 increased gradually from early morning, reached the maximum between 1 and 

 S p. m., and gradually receded until late in the evening. Transpiration was 

 reduced relatively more than evaporation during the night, suggested as due 

 to the almost complete closing at night of the leaf stomata. 



The daily variation in the water requirement was found to be very marked, 

 the maximum observed in two successive days being 600 per cent. The data 

 further showed that under extreme atmospheric conditions as high as 10 lbs. 

 of water may be transpired by a single corn plant during 24 hours. Such days, 

 it is pointed out, are very critical for corn when the soil moisture is insufficient 

 to meet this demand. The weekly amount of water used gradually increased 

 up to the development of the maximum leaf area, and for four or five weeks 

 after this time the transpiration rate remained high, fully one-half of the total 

 water used by the plant being transpired during this period. While a marked 

 variation in the water requirement of different years was observed, there existed 

 a rather consistent relationship in the relative seasonal variations between the 

 transpiration per unit of dry matter, the transpiration per unit of leaf area, 

 and the evaporation from a free water surface. As an average for three green- 

 house tests conducted during two years a difference of 22 per cent in relative 

 humidity and 1.7° F. during the day cau.sed a difference of 42 per cent in the 

 water requirement per pound of dry matter, 38 per cent in the transpiration 

 from a unit of leaf area, and 46 per cent in the evaporation from a free water 

 surface. Corn plants in greenhou.ses transferred from a humid atmosphere to a 

 dry one and vice versa exhibited no difference in transpiration rate under the 

 changed conditions as compared with each other. 



A transpiring leaf was found to be uniformly cooler than a dead, dry one, 

 amounting under extreme conditions to 8.6° in the sun and 4.2" in the shade at 

 2 p. m., when transpiration was maximum. The transpiring leaf as compared 

 with the air at that time was 4.1^ cooler in the sun and 3.2° cooler in the shade. 

 Other data secured showed that vegetation is a great protection against exces- 

 sive transpiration and also against evaporation from the soil surface. 



A reduction in soil moisture content below the optimum during three years 

 reduced the water requirement per pound of ear corn 4.3 per cent and per pound 

 of total dry matter 7.9 per cent, but it also reduced the stalk yield by 37.3 per 

 cent, the yield of ear corn by 28.5 per cent, and the yield of total dry matter 

 by 30.7 per cent. An increase in the soil moisture content above the optimum 

 during three years increased the water requirement per pound of ear corn 13.5 

 per cent and per pound of total dry matter 8.2 per cent, being accompanied by a 

 reduction in stalk yield of 11.3 per cent, yield of ear corn of 21.1 per cent, 

 and yield of total dry matter of 16.7 per cent. 



The water requirement per pound of dry matter was found to be much larger 

 in an infertile than a fertile soil. As an average for two years, equal applica- 

 tions of sheep manure to infertile, intermediate, and fertile soils reduced the 

 water requirement for ear-corn production 42.6, 25.4, and 10.5 per cent, respec- 

 tively ; for total dry matter the reduction \yas 28.9, 17.1, and 8.1 per cent, respec- 

 tively. On the other hand, the total water requirement per plant was increased, 

 respectively, 106.7, 42.6, and 28.7 per cent. The reduction in water requirement 

 per pound of dry matter is regarded as being more a matter of plant nutrition 

 than of transpiration. 



