WATER UTILIZATION BY TREES 



19 



of this paper. This is well illustrated by the results of Eberdt (S3), who 

 reports the following differences between the intake and output of the 

 sunflower (table 5). 



Table 5. — Water intake and output of sunflower leaves in 12 hours 



Observation period 



Water 

 intake 



Water 

 output 



Difference 



7:15-8:45 



Grams 

 16.67 

 4.95 

 5.50 

 6.45 



Grams 

 15.55 

 5.53 

 7.40 

 5.50 



Percent 



+7 

 -11 



8:45-11:45 



11:45-3:00 



-26 



3:00-7:15 



+ 15 









33.57 



33.98 



-1 







Potometers consist essentially of a vessel of water arranged in such 

 a fashion that it is possible to measure amounts removed by a plant 

 inserted in the vessel. They are extremely easy to manipulate, and 

 various types are on the market. Some of the more recent devices 

 register automatically the water losses from the vessel. 



Lysimeters differ from potometers in that they contain soil and thus 

 provide a more natural environment than do potometers, but experi- 

 ments with lysimeters, like those with potometers, do not permit one 

 to distinguish between the water used within the plant and that lost 

 or wasted. Furthermore, in such experiments it is very difficult to 

 control and limit the water supply without interfering with some 

 other function or phase of metabolism such as aeration (respiration), 

 root expansion, or mineral nutrition. Therefore work with lysimeters 

 requires a rather elaborate system of controls, if it is to be helpful in 

 general silvicultural practices. 



Collecting water vapor with a hygroscopic substance, such as P 2 5 

 or H 2 S0 4 (method III, a), is a fairly easy procedure. The substance is 

 weighed carefully before and after, the difference in weight being the 

 amount of water given off by the plant organs. This method, how- 

 ever, has a very serious weakness, inasmuch as the transpiration nec- 

 essarily occurs in a space with an abnormally low humidity. It is 

 thus quite unsuitable for measuring transpiration under normal 

 conditions. 



Minckler (153) has attempted to overcome some of the weaknesses 

 in this method so that it can be used more successfully for field work 

 with trees. By using more " active" drying agents, such as anhydrous 

 magnesium perchlorate and barium perchlorate, distinct results can 

 be obtained during 6 minutes. While it is true that the leaves during 

 this time are probably not influenced much by the enclosure, provided 

 the temperature and quality of irradiation are not appreciably changed, 

 they probably do transpire differently from what they would in an open 

 environment with a natural (and not an experimentally low) humidity. 

 Also it must be admitted that, since the age, exposure, etc., all influence 

 the transpiration of leaves on any given twig, as will be shown in the 

 following pages, calculations on the transpiration of a tree based on 

 any one of its twigs under such an environment will not be very trust- 

 worthy. 



Measuring the changes in humidity with a hygrometer (method III, 

 b) is much more satisfactory, but the objection to this is that it also is 



