276 BOTANICAL GAZETTE [APRIL 
communicating with the preceding through a slender rubber tube, is the 
reservoir cylinder, ungraduated but with index marks 25° apart. Both 
tubes are supported vertically by any convenient laboratory clamps which 
permit the reservoir tube to be slipped up and down. 
For demonstration purposes it is best to select seeds in which the 
oxygen absorbed and the carbon dioxid released are as nearly as possible 
equal in volume, e. g., oats. Ten of these of average size are soaked, or 
better, are selected from a lot which have been started in a germinator until 
the roots are about 5™™ long; they are then placed in the chamber, root 
ends down, just above the bulb, where they will stick if previously wetted. 
These occupy approximately 1°° of volume; and if now 1°¢ of water be 
placed in the bulb, there will be 2°° of these materials and 100% of air, 
the composition of which is of course known, above the zero mark. Where 
greater accuracy is desired, it may be attained by first dropping the ten seeds 
into a proper measuring-glass, then filling this with water to the 2°° mark, 
and finally placing both seeds and water in the chamber. The index 
liquid to be used is now poured through the reservoir tube until it stands 
level at the 100° mark of the graduated tube and at the upper index mark 
of the reservoir (or the lower when mercury is used). The stopper, properly 
lubricated, is inserted with its air opening matching that of the neck, and 
is then twisted, thus sealing the chamber without any compression of air. 
The apparatus is now shielded from light and placed under proper condi- 
tions for growth. 
For the most effective educational demonstration it is desirable, though 
not necessary, to use two of the respirometers. In one is placed a strong 
solution of caustic potash, which rises steadily, as the seeds grow, to near 
the 21 per cent. mark and then stops. This proves that some gas has 
vanished from the tube, and the amount suggests oxygen. In the other 
mercury is used, and the constant level it maintains proves that for any 
gas absorbed by the plants an equal volume of some other has been released. 
Since however caustic postash absorbs only carbon dioxid, it is plain that 
the gas released must be carbon dioxid, while the gas absorbed is presum- 
ably oxygen. If it is thought this latter point still needs proof, it can be 
supplied by a third instrument using pyrogallate of potash. This promptly 
absorbs all oxygen, and the failure of the seeds to grow shows that oxygen 
is the gas absorbed in the other instruments. If but one instrument is 
available, it is best to use it with mercury, and, after the seeds have been 
growing for some three or four days, to make an analysis of the gas by the 
method earlier described? for the photosynthometer. The reservoir and 
? Bot. GazEtreE 41:211. 1906. 
