SECT. VI. ELABORATION OF OXYGENfi. 163 



in this case vegetation is altogether suspended. The 

 plant soon discovers a tendency to putrefaction, 

 furnishing from its own substance, and in conse- 

 quence of its decomposition, the two elements of 

 carbonic acid gas. 



The foregoing results which were obtained from Condi- 

 experiments on Cactus Opuntia are equally appli- SUC eessful 



cable to the leaves of other plants, though not in so 

 perceptible a degree. But the more succulent any the leaves - 

 leaf is the better it is fitted for experiment ; because 

 succulent leaves contain a great quantity of green 

 herbaceous matter within a small volume, while the 

 leaves of most other plants are so thin and fine, and 

 their surface so much extended that the experiment 

 is not only more difficult but the result less striking. 

 According to Saussure, the following conditions are 

 necessary in order to ensure success in experiment: 

 the leaves must be perfectly sound and fresh, and 

 they must displace from about the T V to ~ part of 

 their volume of the air contained in the receiver, for if 

 they displace less, the effect is not sufficiently per- 

 ceptible, and if more, there is too little oxygene 

 left. When the apparatus is placed in the sun, the 

 leaves must not touch the sides of the receiver, 

 which is then so hot as to disorganize the structure 

 of the plant. 



But although the quantity of oxygene extricated 

 in the day is proportional to the quantity inhaled in 

 the night, yet the specific quantity inhaled is very 

 different in the leaves of different plants. The leaves 



M 2 



