J CO BULLETIN 1050, U. S. DEPARTMENT OE AGRICULTURE. 
Method of potted plants. 
The method of determining the transpiration rate, either in the 
laboratory or under a variety of field conditions, by means of potte 1 
plants, is theoretically the mest reliable, since, if the potted plants 
are kept in a healthy condition and the moisture supply in the 
soil is 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 are made in the field the moisture content of the pots may 
at all times be 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 be near the optimum. Thorn and Holtz (168) and Kiesselbach 
(156) have shown that the maximum transpiration occurs when the 
soil 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 case 
it is more likely to be a question of osmosis. 
A method of treating plants that seems especially adapted to 
good-sized trees is that of Briggs and Shantz (153, 151). 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, w T ith all arrangements de- 
signed for outdoor exposure. The pots were galvanized ash cans, 
through the covers of which the stems of the plants were made to 
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 cans 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 5 or 
6 years of age. a galvanized iron can, -1 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 
can escape. Before the seedlings are potted, a 2-inch clay flower- 
pot is inverted in the bottom of each can. Through the hole 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 slightly above 
the wall of the can. This glass tube serves to feed water into the 
porous pot, whence it is readily diffused through the soil, and at 
