THE DAILY PERIODICITY OF TRANSPIRATION 209 



ard day" conditions. Thus the effect of a reduction in the vapor pressure 

 gradient through the stomates in decreasing the rate of transpiration, which 

 first becomes apparent during the mid-day period, continues with augmented 

 effect as the hours of darkness approach. 



Continued diminution in the leaf water content due to the excess of 

 transpiration over absorption eventually results also in a diminution of 

 the turgor of the guard cells due to a decrease in the water content of the 

 leaf. This results in a gradual closure of the stomates. Some of the sto- 

 mates on a plant probably begin to close even before the time at which 

 the peak of the transpiration rate is attained. In all likelihood stomates 

 near the margin or tip of a leaf begin to close before those in the middle, 

 since the effects of deficiency of water usually appear first in these regions 

 of a leaf. With an increasing leaf water deficit more and more of the sto- 

 mates on the plant close. This soon results in a reduction in the aggregate 

 diffusive capacity of the stomatal population of the plant. As the afternoon 

 advances more and more of the stomates become completely closed, resulting 

 in a progressive diminution in diffusive capacity. This gradual closing of the 

 stomates, resulting in an increasingly larger proportion of them being closed 

 as the day advances, is the principal factor reducing the rate of transpiration 

 during the latter part of the day. The effect of this factor and that of a 

 decreased steepness of the vapor pressure gradient overlap during a large part 

 of the afternoon. 



Finally, by late afternoon, complete closure of essentially all of the sto- 

 mates has occurred. Stomatal transpiration is terminated at that time and 

 during the hours of darkness the rate of water loss from the plant is con- 

 trolled by factors influencing the rate of cuticular transpiration. 



Under environmental conditions deviating very greatly from those which 

 were postulated in the preceding discussion, transpiration periodicity curves 

 may be entirely different from those shown in Fig. 50. Variations in tempera- 

 ture, light intensity, humidity, and soil water supply may all markedly in- 

 fluence both the trend of transpiration periodicity, and the magnitude of the 

 daily water loss. 



Low temperatures may result in a complete elimination of stomatal trans- 

 piration by inducing stomatal closure. 



Low light intensities, such as those existing on cloudy days, are unfavorable 

 to stomatal opening in most species. The stomates seldom open completely 

 under such conditions and the period during which they are open is usually 

 of shorter duration than on clear da^s. Furthermore, the vapor pressure 

 gradient through the stomates is seldom as steep on such days as on clear, 

 bright days, since leaf temperatures never appreciably exceed atmospheric 



