9 6 



PL4NT PHYSIOLOGY. 



them in darkness at a fair growing temperature, and observe and 

 interpret results. 



(The mercury tube in the above experiment is needed partly as a con- 

 trol, and partly to settle a certain point which conies up in tJie inter- 

 pretation of the experiment. Water cannot well be used (though / 





FIG. 21 SIMPLE ARRANGEMENTS FOR DETERMINING THE GAS-EXCHANGE IN 



RESPIRATION. 

 One-fourth the true size. 



formerly recommended it) because it absorbs carbon dioxide and rises 

 in the tube. The V tubes must not be allowed to become wet inside or 

 the potash will rise and destroy the seeds. Practically in the third 

 tube it i.\ better to place the solution of potash in the holder, and the 



mixture in communication with the air outside, the seeds will germinate, though .slowly. 

 Also, peas will germinate somewhat in the closed pyrogallic chamber as they do in 

 the intramolecular respiration experiment (page 99), which they would not do if 

 killed by the influence of the mixture. Removing the oxygen by phosphorus, how- 

 ever, seems to kill the seeds. 



In the other tubes, the seeds over the potash always grow somewhat more vigor- 

 ously than those over the mercury, because, no doubt, of the prompt removal of the 

 carbon dioxide in the former case. 



