THE DIFFERENTIATED PLANT 197 



space thus left. Some of the brighter pupils, observing the 

 pyrogallic and the potash tubes alone, would say that all the 

 potash tube proves is that something is absorbed, doubtless 

 oxygen, from the air by the plants, and that nothing is proved 

 to be given off since removal of the oxygen alone necessitates 

 the rise of the liquid. In answer to this is the water tube, 

 the failure of the liquid in which to rise proves that some- 

 thing is given off as well as absorbed, and since the only 

 gas absorbed by potash is carbon dioxide, that gas must be 

 given off in volume equal to the oxygen absorbed. This 

 experiment proves that oxygen is necessary to growth, that 

 carbon dioxide is given off, and that the volumes of gas thus 

 exchanged are equal. This exchange is, of course, respiration, 

 necessary to supply energy for growth. 



This experiment always works very well with me ; if it fails, 

 it will probably be found that the pyrogallic-potash mixture is 

 not concentrated enough. Practically, it is best to place the 

 pyrogallic acid in the arm of the U-tube, and the potash in the 

 test-tube, when the mixing occurs where it will do most good. 



There are other very simple experiments to substantiate 

 these results. Thus the giving off of carbon dioxide may be 

 proved by placing in a closed bottle a number of soaked seeds 

 with a small dish of clear lime water, the milkiness of which, 

 after two or three days, will prove the presence of the gas ; 

 while in a similar bottle without the seeds it will remain clear. 



The students will themselves suggest the identity of this 

 process with respiration in animals ; it is also called respiration 

 in plants. The teacher, by a lecture or otherwise, can make 

 it plain that it is by oxidation of food that both plants and 

 animals obtain the energy needed in the work of growth. Of 

 course, the results of this experiment, with a drawing of the 

 apparatus, should be carefully worked out by the students. 



