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 h 30 m 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 h 30 m a. m., being 

 accompanied by a decrease in the water content of the leaves and an increase 

 in that of the twigs. Until ll h 30 m 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 h 30 m 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 9 h 30 m a. m. to 10 h 30 m 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 10 h 30 m 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 



