ELIMINATION 175 



demonstration use; but sliding weights afford a fair substitute. Following 

 the general form of Pfeffer's balance, but using a sliding weight for the 

 coarse adjustment, I have designed a new transpiration balance which is 

 among my normal apparatus (page 46) and is figured in the catalogue 

 descriptive thereof. 



Demonstration Methods. With large classes I have found it most 

 effective to use a large leafy potted plant (see Materials earlier), having pot 

 and soil enwrapped with aluminum shell and rubber roof; it is weighed on a 

 torsion balance sensitive to a gram. After being prepared tefore the class, 

 the plant is then watered at regular intervals until the class meets again, 

 or for an entire week. If the quantities found to be lost are then exhibited 

 in measuring-glasses, it adds much to the effectiveness. If, however, only 

 the fact, and not the amount, of transpiration is to be shown, then this is 

 very easily effected by simply covering the plant with a bell jar, upon which 

 in a few minutes water abundantly collects. Of course evaporation from 

 pot and soil must be prevented either by enwrapping them or by keeping 

 them outside the bell jar, the stem being passed through a split glass plate; 

 this is effected very conveniently and perfectly by the supported bell jar 

 later described (page 187), for which a substitute may be adapted from 

 any waterproof stiff material (e.g., an indurated fiber saucer) arranged to 

 rest upon the pot. For exhibition to an audience it is possible to place in 

 the bell jar a large disc of paper infiltrated with cobalt chloride, of which 

 the change of color, with accumulating moisture (described later on page 

 190), can be seen from a distance. The simplest arrangement of all, one 

 very effective for elementary work, is a shoot or a single leaf thrust through 

 a small hole in cardboard resting upon a nearly filled tumbler of water, and 

 covered by another inverted tumbler. 



Other Methods of Measuring Transpiration. Weighing is by far 

 the most accurate and satisfactory method of measuring transpiration yet 

 known, but it has one obvious drawback, viz., it is applicable in general 

 only to potted plants. Other methods which overcome this difficulty are 

 the following. First, the vapor released by the plant in a closed space is 

 absorbed by a chemical whose increase of weight gives the transpiration; 

 the method is described by Detmer, 214. But it has an inseparable error, 

 — the vapor conditions are not normal in the chamber. Second, the 

 relative humidity of a plant chamber of known capacity, in which is a tran 

 spiring plant, is determined by a suitable instrument, whence the abso- 

 lute humidity and transpiration can be calculated. This is recommended, 

 with use of a polymeter, by Cannon, for plants in place in the ground, in 

 Bulletin of the Torrey Botanical Club, 32, 1905, 515. Related in general 

 principle is the method of drawing a known amount of air through a plant- 

 containing chamber over metal tubes arranged to show the dew-point, 

 as described by Leavitt in the American Journal of Science, 5, 1898, 440. 

 Both methods are somewhat elaborate, and moreover do not allow the plant 

 wholly natural transpiration conditions. Third, the water absorbed by 

 the plant is measured or weighed, and is assumed to equal the transpira- 

 tion, as in the long run it must do. This is accomplished for potted plants 



