WATER CONTENT OF PLANT PARTS. 49 
of the branches 1 meter down. Then 2 meters down the stem the effects 
might either lag behind those above or, being influenced by the same extra 
pull, might draw enough water from the portions still lower to keep up with 
the water content of the higher portions. Indeed, it would be impossible 
to predict the condition of any portion below the end or those very near the 
end. To return to the relative transpiration curve at 10° 30™ a.m., it will 
be seen that the rate is now lessened as a result of the drying out of the 
tissues. This decreased transpiration gives the water supply a chance to 
catch up with the demand and a straight line of relative transpiration might 
be expected for a short time at least; but here a marked increase in the 
water content of the twigs is observed, accompanied by a decrease 1 meter 
below. This increase is accompanied also by an increase in the relative 
transpiration above, a natural result of the water films coming nearer the 
surfaces of the pores. 
Experiment XIX differs from experiment XVIII in that the tree was in 
leaf and that the transpiration rates are more surely comparable with the 
water content readings. An inspection of the curves shows the following 
facts. The relative transpiration increases until about 9" 30™ a. m., being 
accompanied by a decrease in the water content of the leaves and anincrease 
in that of the twigs. Until 11" 30™ a. m. the water content curves of leaves 
and twigs slant in opposite directions, after which time they run parallel. 
Bearing in mind the theory mentioned in the preceding paragraph, the 
following points are evident: The increase in relative transpiration up to 
9 30" a. m. brings demand greater than supply, causing a decrease in the 
water content of the leaves; this in turn throws the water in the tissues into 
a tensile state, causing a rise of water content in the twigs by the water 
. | brought up from below. From 930" a. m. to 10530 a. m. the transpiration 
‘rate is decreased, caused by the lowered amount of water present, but at 
the same time the water content of the leaves is slightly increased by a 
“pull” which lowers the water content of the twigs. By 105 30™ a. m. the 
transpiration and relative transpiration have been able to increase because 
of the rising water content of the leaves, but this water is gained at the 
expense of the twigs and it is not until after 12 noon that the lower portions 
can supply water fast enough for the water content of twigs and leaves to 
increase together. During the afternoon the relative rate fluctuates slightly 
from a straight line. From 4 p.m. to 5 p.m. the two transpiration curves 
go in opposite directions, so no conclusions from their comparison with the 
water content can be made. 
In cases of this kind, where the tree is in leaf, the drying out is probably 
accompanied by a closure and subsequent opening of the stomata, as was 
mentioned in the section on stomatal behavior. The fall and rise in relative 
transpiration may occur more frequently than the readings at hourly inter- 
vals show. In fact, it is possible that here is an explanation for the well- 
known small fluctuations in relative transpiration observed when readings 
are taken at very short intervals. On page 6 is a brief description of the 
