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. é., 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 
