WATER CONTENT OF PLANT PARTS. 45 



the stomata aided in the rise })ut are not necessary to it. So, then, a defi- 

 ciency of water in the evaporating tissues would ])c the first cause of the drop 

 in transpiration rate, while the evaporation rate is still increasing. If now 

 the supply of water from below is unchanged, the smaller evaporation rate 

 Avould allow a balance to be obtained when we should expect the relative 

 rate to become a straight line for a time at least. It is hard to see how the 

 gain could be sufficient to send the relative rate up again unless some other 

 factor entered, either to check the rate of outgo or to increase the rate of 

 intake from the lower parts. If Dixon's hypothesis* of the ascent of sap be 

 accepted, it might be supposed that the deficiency of water in the terminal 

 tissues causes a "pull" on the water columns, which enter the tensile state, 

 and thus the intake is increased. As soon as the supply begins to catch up 

 with the demand the tensile pull is lessened and the relative rate begins to 

 go down again. 



If the explanation is the latter one, based on Dixon's hypothesis, then 

 there might be a relation between the moisture content of different parts of 

 the tree which would throw light on the matter. Consequently a series of 

 moisture content determinations was carried out as, follows . 



DAILY COURSE OF WATER CONTENT OF LEAVES. TWIGS, AND STEMS. 



EXPERIMENTATION. 

 Experiment XVIII. 



Three adult trees, designated I, II, III, were selected in open sunny 

 places, and at two-hour intervals small limbs were sawed off and samples for 

 the determination of water-content were taken as follows: Ii, IIi, and IIIi 

 were end twigs about 3 to 5 cm. long, with diameters from 0.1 to 0.2 cm.; 

 I2. II2, and IIIo were taken about a meter from the end twigs and had diam- 

 eters about 1.2 to 1 .6 cm. ; I3, II3, III3, were taken 2 meters from the end twig 

 and were from 3.5 to 3.9 cm. in diameter. The work of sawing and cutting 

 was done very quickl}^, with the aid of an assistant, and the pieces were 

 weighed immediately after the cuts were made. The samples were allowed 

 to dry in the sun for about a week and then dried to constant weight in an 

 oven at 95 to 100 C. Accidents which happened to the end twigs of II 

 left too few good readings for the results to be plotted. For the sake of a 

 comparison with the Avater-content of a tree which had lost no branches, 

 samples were taken from a fourth tree at 2'' 30'" p. m. The two readings 

 marked III4 and IV4 were taken about 4 meters from the tips, where tlie bark 

 is brown and not green as in the other samples. In the sawing great cafe 

 was taken to get branches widely separated on the tree, in order that the 

 wounds might cause as little disturbance as possible. The evaporation rate 

 was measured by exposing atmometer No. 11-1 in an open sunny place. 



* Dixon and Joly, On the ascent of sap, Proc. Roy. Soc. Lond., vol. 57 B. 



