INFLUENCE OF THE SHADE ON PLANTS. 29 



dation : these are deprived of their acid properties 

 during the day and evening, and are changed, by 

 separation of a part of their oxygen, into compounds 

 containing oxygen and hydrogen, either in the 

 same proportions as in water, or even with an ex- 

 cess of hydrogen, which is the composition of all 

 tasteless and bitter substances. 



Indeed, the quantity of oxygen absorbed could 

 be estimated pretty nearly, by the different periods, 

 which the green leaves of plants require to undergo 

 alteration in colour, by the influence of the atmo- 

 sphere. Those which continue longest green, will 

 abstract less oxygen from the air in an equal space 

 of time, than those, the constituent parts of which 

 suffer a more rapid change. It is found, for ex- 

 ample, that the leaves of the Ilex aquifolium, dis- 

 tinguished by the durability of their colour, absorb 

 only 0.86 of their volume of oxygen gas, in the 

 same time that the leaves of the poplar absorb 8, 

 and those of the beech 9^ times their volume ; both 

 the beech and poplar being remarkable for the 

 rapidity and ease with which the colour of their 

 leaves changes. 



When the green leaves of the poplar, the beech, 

 the oak, or the holly, are dried under the air pump, 

 with exclusion of light, then moistened with water, 

 and placed under a glass globe filled with oxygen ; 

 they are found to absorb that gas in proportion as 

 they change in colour. The chemical nature of 

 this process is thus completely established. The 



