INFLUENCE OF THE SHADE ON PLANTS. 23 



matters. The correctness of these inferences has been distinctly 

 proved by the observations of De Saussure ; for whilst the taste- 

 less and inodorous fleshy leaves of the Agave Americana absorb 

 only 0.3 of their volume of oxygen in the dark, during twenty- 

 four hours, the leaves of the Pinus Abies, containing volatile and 

 resinous oils, absorb ten times ; those of the Quercus Robur con- 

 taining tannic acid 14 times ; and the balmy leaves of the Popu- 

 lus alba 21 times that quantity. This chemical action is shown 

 very plainly also in the leaves of the Cotyledon calycinum, the 

 Cacalia ficoides, and others ; for they are sour like sorrel in the 

 morning, tasteless at noon, and bitter in the evening. The forma- 

 tion of acids is effected during the night by a true process of oxi- 

 dation ; they 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 ; for such 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 continuing 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, distin- 

 guished 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 9£ times their volume : 

 both the beech and poplar being remarkable for the rapidity and 

 ease with which the color of their leaves changes. (De 

 Saussure.) 



When the green leaves of 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 

 color. The chemical nature of this process is thus completely 

 established. The diminution of the gas which occurs can only 

 be owing to the union of a large proportion of oxygen with 



