. Procès-verbal de la Société américaine d'agronomie . v. 12 plantes), grammes. Eau évaporée de la surface FreeWater (Av. 6 Jars), grammes. Juillet 27, 1910 4.550 454 juillet 28 2,333 372 juillet 29 1.579 173 juillet 30 2,802 232 juillet 31 3.561 314 août I 3.982 374 août 2 3.419 311 août 3 2.143 204 certains jours, trois fois plus d'eau était nécessaire que d'autres, et la même variation a été observée dans l'évaporation de la surface de l'eau libre. Figure 49 montre la variation quotidienne de la perte d'eau des plantes ainsi que la variation de l'eau perdue d'une surface d'eau libre pendant une période de 30 jours, de juillet 20 à Augus

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. Proceedings of the American Society of Agronomy . v. 12 Plants), Grams. Water Evaporated from FreeWater Surface (Av. 6 Jars), Grams. July 27, 1910 4.550 454 July 28 2, 333 372 July 29 1.579 173 July 30 2, 802 232 July 31 3.561 314 Aug. I 3.982 374 Aug. 2 3.419 311 Aug. 3 2.143 204 On some days three times as much water was required as on others, and about the same variation was shown in the evaporation from afree water surface. Fig. 49 shows the daily variation in water lossfrom the plants as well as variation in water lost from a free watersurface for a period of 30 days, from July 20 to August 20. We seehere a very close correlation between water lost from a free water sur-face and that lost from the plants. This same principle is illustrated in a more marked way by Fig. 50, which gives the average water loss together with evaporation andcertain weather factors plotted by the hour for the 24 hours of day.^ ^ Montgomery and Kiesselbach. 24th Ann. Rept. Nebraska Exp. Sta., p. 100, 1910. 28o 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

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