CH. Il] GAS EVOLVED. 31 



in a beaker of spring water. The cut ends of the plants 

 must be upwards; and must be below the surface, to 

 effect which it may be necessary to tie the specimens to 

 a glass rod (see Pfeffer, Physidogie, I. fig. 17, and Detmer, 

 fig. 12). The beaker is to be placed in sunlight, and 

 evolution of gas from the cut ends of the specimens to be 

 observed. To obtain a convenient series of small bubbles 

 Pfeffer recommends varnishing the cut end of the shoot 

 and pricking a fine hole in the membrane so produced. 

 Select a branch which seems to be yielding a satisfactory 

 amount of gas, and record, with a stop-watch, the time 

 which elapses while 10 or 20 bubbles are given off. The 

 observation must be repeated until the rate of bubbling 

 is fairly constant. [It is important to know that the 

 evolution of bubbles of gas may be produced by other 

 causes than illumination. Thus a plant which is exposed 

 to feeble illumination and is not giving off bubbles 

 may be made to do so by being transferred to a beaker 

 containing soda-water freshly drawn from a "syphon." 

 Devaux^ has shown that this depends on the internal 

 atmosphere rapidly assuming the gas-pressure of the 

 water, by the diffusion of CO2 from outside into the 

 intercellular spaces.] 



(42) Light of different intensities. 



Now move the beaker into the shade, or cover it 



with a sheet of white paper, and take a fresh series of 



readings, and finally replace it in sunshine and record the 



rate once more. In the absence of sunshine, an incan- 



1 Ann. Sc. Nat. 1889. 



