126 RESPIRATION [CH. 



When living green leaves are placed in the dark, they 

 absorb oxygen from the air and give out an equal volume 

 of carbon-dioxide, provided the temperature and other 

 conditions of life are not interfered with; and the amounts 

 of oxygen consumed and of carbon-dioxide evolved per 

 hour can be readily estimated by any of the ordinary 

 methods of gas-analysis. If normal green leaves are 

 exposed in a closed atmosphere to a sufficiently bright 

 light, however, the proof of their respiration is less easy, 

 and depends on a thorough understanding of the process 

 of assimilation ; because the carbon-dioxide set free is at 

 once reabsorbed by the chlorophyll-corpuscles and again 

 decomposed into carbon and oxygen, and consequently the 

 gaseous composition of the atmosphere does not alter 

 essentially, a fact already known to De Saussure at the 

 beginning of the last century. 



If the last experiment is conducted under such con- 

 ditions, however, that the light is too feeble to produce 

 carbon-assimilation, the accumulation of carbon-dioxide 

 in the closed atmosphere indicates that oxygen is being 

 respired ; and a similar result follows if etiolated leaves 

 are allowed to remain in such an atmosphere, even in 

 the light. 



The fact is more directly established, however, b}^ 

 employing a closed atmosphere and shallow vessels filled 

 with potassium-hydrate or some other body which absorbs 

 and retains the carbon-dioxide as fast as it is formed : in 

 such a system even green leaves in sunlight can be shown 

 to enrich the environment with carbon-dioxide, because 

 the potassium-hydrate absorbs this gas with such energy 

 that it does not reach the chlorophyll, and is therefore 

 not again decomposed. This experiment yields, in equal 

 times, better results in diffuse light, and still better in 

 darkness, because, as we shall see, the avidity with which 



