06 A TEXTBOOK OF GENERAL BOTANY 
attached, by means of a rubber tube, to a burette containing 
water. The branch is then placed inside a bell jar, when some 
of the water that is transpired will condense on the sides of the 
jar. A potted plant may be used in place of the branch. This 
experiment can be checked by setting up a similar preparation 
with a dead, dry branch substituted for the living one. In this 
case no water will collect on the sides 
of the bell jar. 
The amount of transpiration can be 
roughly determined in the manner shown 
in Fig. 42. A branch is attached, by 
means of a rubber tube, to a burette 
containing water. As transpiration con- 
tinues water is absorbed from the burette, 
the amount being shown by the change 
in the height of the water in the burette. 
This method is inaccurate, ‘as it measures 
the amount of water absorbed rather than 
that given off, and, moreover, the tran- 
spiration of a severed branch is frequently 
very different from that of a branch on 
the plant. A more accurate method of 
Pere tia acorns: eee transpiration is by weighing. 
Wee haorbed by ure potted plant can be used for this pur- 
spiring branch * POSE. The pot and soil should be sealed 
in a waterproof covering so that water 
can evaporate only from the plant itself. The rate of transpira- 
tion can then be determined by weighing the preparation at 
definite intervals of time. 
Harmful effects of transpiration. The most conspicuous effect 
of transpiration is the damage that results from excessive loss 
of water. If water is lost in transpiration faster than it is sup- 
plied from the roots, the leaves wilt ; and if the excess of loss 
over supply continues, the plant will finally die. Even without 
actual wilting, the growth of plants may be greatly retarded 
by high rates of transpiration accompanied by a low moisture 

Fig. 42. Apparatus for 
