166 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



Method of potted plants. 



The mot hod of determining the transpiration rate, either in the 

 laboratory or under a variety of field conditions, by means of potted 

 plants, i- theoretically the most reliable, since, if the potted plants 

 are kept in ;i healthy condition and the moisture supply in the 



I i- properly regulated, a very near approach to natural growth 

 conditions may be obtained. Unquestionably, the rate of transpira- 

 tion is limited by the moisture supply. Where tests of this char- 

 acter arc made in the field the moisture content of the pots may 

 at all times he similar to that of the native soil at the point studied, 

 allowance being made, of course, for differences in physical prop- 

 erties of the two soils, if any exist. A much simpler method is to 

 maintain the pots constantly at a standard moisture content known 

 to he near the optimum. Thorn and Holtz (168) and Kiosselbach 

 L56) have shown that the maximum transpiration occurs when the 

 <n\\ is about half saturated. This may be the result of aeration, or 

 it may be that "about half saturation" corresponds to the " critical 

 moisture content" of Cameron and Gallagher (117'. in which <•;: 

 it is more likely to be a question of osmosis. 



A method of treating plants that seems especially adapted t<» 

 good-sized trees is that of Briggs and Shantz (153, 154), having 

 been employed to determine the amount of water used by various 

 agricultural crop plants under semiarid conditions at Akron. Colo. 

 These tests were made on a large scale, with all arrangements de- 

 signed for outdoor exposure. The pots were galvanized ash cans, 

 through the covers of which the stems of the plant- were made t«» 

 extend after growth had become well established. The weighing 

 of such pots was laborious, of course, and required the use of a 

 traveling crane arrangement, by means of which the can- were 

 lifted to the scales for each weighing. A very similar arrangement 

 would appear practicable for determining transpiration from tree 

 specimens, until they had attained the height of at least 1 or 5 



feet. 



For the convenient handling of forest tree seedlings up to •"> or 

 •'» years of age, a galvanized iron can. 4 inches in diameter and 10 

 inches deep, has been used by Bates (105). This can is soldered 

 and there are no perforations in the bottom through which water 



l escape. Before the seedlings are potted, a 2-inch clay flower- 

 pot is inverted in the bottom of each can. Through the whole in the 

 base of the flowerpot a glass tube of small bore, bent with two 

 right angles so that the main part of the tube rests against the 

 wall of the galvanized can, is inserted. It extends slightlv above 

 the wall of the can. This glass tube serves to \\hu\ water into the 

 porous pot, whence it is readily diffused through the soil, and at 



