. Proceedings of the American Society of Agronomy . MONTGOMERY: WATER REQUIREMENTS OF CROPS. 281 These figures are the result of hourly weighings for a period of twoweeks. We find that during the night the water loss from the plantsis very low, but it begins to increase about 7:00 oclock in the morn-ing, reaching the maximum loss per hour at 2:00 oclock and againdeclines till about 8:00 in the evening. It is remarkable that theevaporation from a free water surface varies in exactly the same wayand in practically the same degree. It might be expected that duringthe heat of the day, when maximum
. Proceedings of the American Society of Agronomy . MONTGOMERY: WATER REQUIREMENTS OF CROPS. 281 These figures are the result of hourly weighings for a period of twoweeks. We find that during the night the water loss from the plantsis very low, but it begins to increase about 7:00 oclock in the morn-ing, reaching the maximum loss per hour at 2:00 oclock and againdeclines till about 8:00 in the evening. It is remarkable that theevaporation from a free water surface varies in exactly the same wayand in practically the same degree. It might be expected that duringthe heat of the day, when maximum evaporation was taking place, theplants would .exercise some physiological control of their water andtend to lag behind evaporation from a free water surface. It alsowould be expected that during the night, when evaporation from afree water surface was very low, the plants would tend to increasetheir loss in proportion to that from a free water surface. The cor-relation, however, between the two is very close. It is also noticeable. Fig. 51.—Chart showing the evaporation in inches during the six summermonths from April to September, inclusive, for various points in the UnitedStates. The circles show stations where evaporation measurements are beingmade in connection with the field experiments of the Bureau of Plant evaporation at these stations is measured in the same way and can bedirectly compared. (From U. S. Dept. Agric, Bur. Plant Industry, Bui. 188.) that the curves for humidity, temperature, and wind velocity reachedtheir maximum at 2:00 oclock in the afternoon or at practically thesame point where evaporation and transpiration are at would indicate that plants respond to climatic conditions in ex- 282 PROCEEDINGS OF THE AMERICAN SOCIETY OF AGRONOMY. actly the same way that a free water surface responds, and apparentlyexercise little or no physiological control of their water at periods ofgreatest water requirement. We should therefore expect
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