INFLUENCE OF THE SHADE ON PLANTS. 51 



pounds in which nitrogen is present, ought to absorb 

 more oxygen than those which do not contain such 

 matters. The correctness of these inferences has 

 been distinctly proved by the observations of De 

 Saussure ; for, whilst the tasteless leaves of the 

 Agave americana absorb only 0*3 of their volume of 

 oxygen in the dark, during 24 hours, the leaves of 

 the Pinus Abies, which contain volatile and resinous 

 oils, absorb 10 times, those of the Quercus Rohur 

 containing tannic acid 14 times, and the balmy leaves 

 of the Populus alba 21 times that quantity. This 

 chemical action is shown very plainly, also, in the 

 leaves of the Cotyledon calycinum, the Cacalia 

 JicoideSj and others ; for they are sour like sorrel in 

 the morning, tasteless at noon, and bitter in the 

 evening. The formation of acids is effected during 

 the night by a true process of oxidation : 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 excess 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 color, by the influence of the atmosphere. 

 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 example, that 

 the leaves of the Ilex aquifolium, distinguished by 

 the durability of their color, 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 9J times their volume ; both the beech and 

 poplar being remarkable for the rapidity and ease 

 with which the color of their leaves changes. 



When the green leaves of the poplar, the beech, 

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



