212 BOTANICAL GAZETTE [MARCH 
removed will give directly the percentage originally present. Some slender 
vessel may then be slipped under the tube which is removed and supported 
as shown by the figure. 
Thus may the gas exchange in photosynthesis be demonstrated accu- 
rately, completely, logically, and conveniently. 
n studying the process with beginners, the demonstration is the more 
striking and conclusive to them if a second instrument is set up like (and 
beside) the first, but covered completely from light; while even a third, like 
these two except that it has no plant, may advantageously be added. The 
comparative analyses of the gases at the close of the experiment give results 
leaving nothing to be desired in logical demonstration. 
For all ordinary purposes, water may be used in the pneumatic trough, 
its slight absorption of carbon dioxid being negligible; but for great accu- 
racy mercury may be employed. Similarly the corrections for capillarity, 
vapor tension, etc., may for elementary demonstration be ignored, though in 
precise work they would be taken into account. Temperature and baromet- 
ric pressure are compensated, obviously, by the method of use of the instru- 
ment. 
A much larger instrument, capa- 
ble of taking large leaves, branches, 
or even entire potted plants, but 
operated upon substantially the same 
principle, is now in advanced prep- 
aration and will be described later. 
VII. ALUMINUM SHELLS FOR TRANS- 
PIRATION EXPERIMENTS, ETC. 
In transpiration studies with 
potted plants it is of course neces- 
sary to eliminate evaporation from 
pot and soil. There are many ways 
of effecting this, of which the best, 
perhaps, is the use of a tin cup or 
glass jar to cover the pot, and a roof 
of rubber sheeting. The advantage 
of this method over those in which 
the plant is wrapped in rubber, 
sealed in melted wax, etc., is this,— 
that the rubber roof may be readily detached from the can or jar and 
lifted, permitting a complete change of air to the roots when the plant is 
* watered, thus contributing greatly to the health of the underground parts. 
Pies Ss. 
eT ee ee ae ee 
