Little is known concerning the effect of humidity on the absorption of foliarly applied materials or their subsequent translocation. It is generally believed that the higher the humidity at any one temperature, the larger the amount of a foliarly applied material that penetrates the leaf per unit time. Most of the evidence for such an effect has been obtained indirectly. Koontz and Biddulph (16) found that the amount of phosphorus translocated from a given compound seemed to be related to the drying time of the solution on the leaf. By adding glycerine to the treatment solution, the translocation of P32 from an application of KH,PO4 solution was increased. Other workers have reported increased penetration of foliarly applied materials by additives which increase moisture retention. The recent review by Currier and Dybing (7) covers this work. It has been suggested that thin aqueous films on the leaf surface are important in promoting the absorption of nutrient sprays, the existence of the film depends upon the vapor pressure gradient at the leaf surface (4).In general, increasing temperature, within physiological limits, has been found to result in increased penetration. Rice (19) using red kidney bean plants studied the absorption of the ammonium salt of 2,4-D. He found that the amount absorbed was positively correlated with temperature over a three-level range of 46 to 580 F, 79 to 820 F and 86 to 920 F. Other workers using soybeans as the test plant found that with increasing temperature, there was an increase in the absorption of the sodium salt of 2,4-D (5,10). Barrier and Loomis (2) reported a temperature effect on the absorption of 2,4-D by soybean seedlings, but no effect upon the absorption of P32. Increased rates of absorption with increasing temperature have been reported for Co60 (9) temperature or humidity gave an increase in absorption of the maleic hydrazide. The preponderance of evidence to date indicates a maximum in translocation over the range of 20 to 300 C. At temperatures below and above this range translocation is reduced (2, 12, 22). MATERIALS AND METHODSSelected seed of Phlaseolus vulgaris L. var. Red Kidney was used. Seeds were germinated at 250 C in Vermiculite saturated with distilled water. All temperatures reported are accurate within ± 10 C and relative humidities are accurate to + 3 % as measured by a hair hygrometer calibrated periodically with a hand phychrometer. Forty to 42 hours after sowing, 90 germinated seeds with radicles between 1.5 cm and 1 cm long were selected and planted in 4 inch pots containing Yolo clay loam soil fertilized with 16 ppm N and 6 ppm P on an oven dry weight basis. After subirrigation the pots were transferred to a growth chamber where they remained at 250 C until the 5th day after sowing, when the lights were turned on at 5:30 A.M. At this time the beans were in the crook stage. Subsequent growth conditions were: 1 5Y2 hours light, temperature 250 C, relative humidity 60 to 76 % and 8/2 hours dark, temperature 15°C, relative humidity 85 to 95 %. Lig...

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Conductance to gaseous transfer is normally considered to be greater from the abaxial than from the adaxial side of a leaf. Measurements of the conductance to water vapor of peanut leaves (Arachis hypogaea L.) under well watered and stress conditions in a controlled environment, however, indicated a 2-fold higher conductance from the adaxial side of the leaf than from the abaxial. Studies of conductance as light level was varied showed an increase in conductance from either surface with increasing light level, but conductance was always greater from the adaxial surface at any given light level. Soil water was monitored in each container with electrical resistance blocks. The soil-water potential was maintained above -I bar throughout the growth period by rewatering to -0.05 bar as determined by soil-water desorption curves. Plant-water potential was measured as described in ancillary experiments (16). The plants were grown in water-cooled chambers programmed for 25 C, 60%o RH, and 350 pl CO2 I`air for 14-h photoperiods, and 20 C, 90%o RH and 400 pl CO2 I ' for 10-h nyctoperiods. The primnary light source for growth and experimentation consisted of a bank of VHO cool-white fluorescents supplemented with incandescents with a total light intensity of 1.17 joules cm-2 min-' of total radiation at the soil surface of which 49% was IR (340 ,uE m-2s-PAR). During experimental periods a constant day-night temperature of 25 C was maintained with 60%o daytime RH and a 90%1o nighttime RH.Assimilation and Conductance Measurements. Most plants studied ranged from 1 to 2 months in age. Some diffusive resistance measurements were made simultaneously with the leafand stem-water potential measurements already reported (16) and others were made simultaneously with Pn2. All measurements were made on recently fully expanded unshaded leaves. Pn measurements were made in a semiclosed compensating system where air was circulated through a Plexiglas water-cooled leaf chamber at 42 cm s-'. The chamber air temperature and VPD were controlled by bringing humidified air to the desired dewpoint temperature and reheating it before it entered the leaf chamber. The CO2 content of the chamber was monitored by a model 315 NDIR Beckman IR CO2 analyzer and held at steady-state 300 + 2 PI CO2/l by the compensating system. Air VPD was monitored continuously by a model 880 thermoelectric dewpoint hygrometer and leaf and air temperatures were measured by small thermocouples. Pn was first measured for 1 standard day (conditions similar to those during plant growth) and then Pn and leaf resistance were measured for several days at differing light intensities. The effect of varying light intensity on Pn and leaf resistance was measured only during those hours of the day when the rates of Pn showed the least fluctuation due to endogenous control, i.e. 0900-1500 h.At any light level other than 340 ,uE m-2 s-' PAR the radiant energy was applied for 15-min intervals. Light was normally increased in at least four steps (i.e., 340-570, 570-920, 920-

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