STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



25 



the petioles of the leaves; one leaf in position is shown in the drawing. 

 Fresh sterilized solutions are of course used for each experiment, 

 the entire chamber and dish being sterilized each time. 



The air-stream enters the chamber through the upper tube and 

 leaves through the lower one. To the latter a trap is attached to 

 catch the water-vapor which is carried out and condenses in the 

 tubes. By means of glass tubing the air-stream is distributed 

 through four glass stopcocks to the four 

 mercury bell-valves. These valves are 

 electrically controlled by means of a 

 clock and require detailed description. 



Our preliminary studies on the res- 

 piration of leaves soon revealed the 

 necessity of making carbon-dioxid deter- 

 minations over a longer period of time 

 than had usually been done. From ob- 

 servations of a few hours or a whole day 

 only limited information can be gained 

 as to the true nature of this process, 

 especially if the leaves are to be fed 

 various substances. Moreover, ^5 too 

 long periods, as during an entire night, 

 are for obvious reasons to be avoided. 

 These requirements necessitated atten- 

 tion being given to the experiments 

 during the entire 24 hours of the day. 

 Therefore, in order to obviate the night 

 work, an apparatus was devised which 

 automatically changed the course of the 

 air-stream from one absorbing tube to 

 another at definite times. 



As was stated, the air-stream passes 

 from the distributing tubes to the mer- 

 cury valves. The construction of these 

 valves, together with the electric coils 

 which control them, is pictured in figure 

 2; a detail of the mercury valve is shown in figure 3. In figure 2 

 the valve is show^n open, giving free passage of the air-stream from 

 the distributing tube through the valve to the absorbing tubes. The 

 valve is connected by means of a heavy wire to a piece of laminated 

 iron which forms the core of an electric magnet. When the current 

 passes through the wire coil, the core is drawn up and the valve 

 remains open as long as the current passes through this coil. As soon 

 as the current stops passing through the coil, the iron core drops 

 and the valve is closed by means of the glass tube which is immersed 



Figure 3. 

 Detail of bell- 

 valve, made of glass 

 and sealed with mer- 

 cury. The upper 

 portion of the glass 

 tube is attached to 

 the core of the elec- 

 tromagnet; the lower 

 end is immersed in 

 mercury when the 

 valve is closed and 

 drawn out of the 

 mercury by a magnet 

 to permit the air- 

 stream to pass into 

 the absorbing tubes. 



