LEAF TEMPERATURE. 57 



DISCUSSION. 



From an inspection of the leaf temperature curves in figure 25 it is seen 

 that in spite of small hourly differences the two trees show much the same 

 behavior as compared with the air temperature. In general the tempera- 

 ture curves for the leaves are below the air temperature curves at night 

 and in early morning until about 10 o'clock, when the lines cross the air 

 temperature lines and then remain above until some time between 12 and 

 1 o'clock, when they again cross and go below air temperature. When 

 compared with the water content curves, no actual hourly relationship 

 appears, there being merely the general relationship that the lowered water 

 content during the day is accompanied by a rise in the leaf temperature. 

 Since both humidity and temperature enter into evaporation control and 

 since the amount of heat received from the sun and air is accumulative, 

 too many factors must be considered to make conclusive deductions. The 

 unknown factor is the balance between the heat received and that used in 

 evaporation. The increase of transpiration for the morning does not keep 

 pace with the increasing amount of heat energy received, and hence it is 

 to be expected that less of the heat received by the leaves would be used 

 by the water in evaporation and therefore more would be available for 

 raising the temperature of the tissues. The rise in leaf temperature shows, 

 at least, that transpiration and relative transpiration are not lessened during 

 the middle of the day because the sun's energy is being used in other ways, 

 i. e., in photosynthesis, for the heating of the tissues shows that there is 

 heat energy to spare. 



It will be noted that the leaf temperature begins rising above the air 

 temperature about the time the closure of stomata commences, and begins 

 sinking with the opening of the stomata. While the above temperature 

 curves show no conclusive evidence of a connection with the dip and rise in 

 relative transpiration in the morning, they present no negative evidence for 

 the application of the theory of drying, since the ratio of the energy received 

 to the amount of evaporation can not be determined. It is interesting to 

 note that the lowered evaporation rate during the middle of the day is not 

 great enough to keep the leaf temperature down to air temperature, and 

 that, while there is some evidence of the desiccation theory, the unknown 

 amount of accumulated heat energy makes this evidence inconclusive. 



DAILY COURSE OF TRANSPIRATION UNDER CONDITIONS OF HIGH 

 AND LOW EVAPORATION. 



If the occurrence of the dip is caused by a failure of the ratio of water 

 supply to demand, to equal unity, then on a priori grounds a plant showing 

 the dip under conditions of high evaporation either ought not to show it 

 at all under conditions of low evaporation or to a much less extent. This 

 was tested to some extent in experiment VIII, where a plant which had 

 previously shown a dip was run in a room of the laboratory. Although 

 the plant showed no dip under these circumstances, the absence of sunlight 



