THE METABOLISM OF PLANTS. 



these gases in any particular case. We may, in fact, say that 

 the evolution of carbon dioxide is not directly connected with 

 the absorption of oxygen, for it has been found that, in the 

 absence of free oxygen, Yeast can decompose sugar with evo- 

 lution of carbon dioxide, that Bacteria can do the same with 

 proteids, and that fruits, and seeds, and opening flowers con- 

 tinue to exhale carbon dioxide for a time : and again, it has 

 been found by de Saussure, by Mayer, and by Deherain, that 

 plants may absorb oxygen without exhaling any perceptible 

 quantity of carbon dioxide. 



The organs which have been found to be capable of absorbing oxygen 

 without an accompanying evolution of carbon dioxide are succulent 

 leaves, such as those of Agave (Dehe'ram), of Saxifragaceae and Crassu- 

 lacese (Mayer), and stems, such as those of Cacti, and fruits (de Saus- 

 sure). The following table refers to an experiment of de Saussure's with 

 Opuntia. A segment of the stem was placed in a closed receiver con- 

 taining air, and was left all night ; on the following morning the volume 

 of the air was found to be diminished by 79 c.c. 



The air contained at the commencement of the experiment 198 c.c. O 

 conclusion 119 O 



Amount of O absorbed 79 



Hence the amount of O absorbed exactly corresponded to the diminu- 

 tion in volume which the air in the receiver had undergone. No CO 2 

 could be detected in the receiver. These results will be considered 

 subsequently. 



It has been found, however, in most instances that, at 

 ordinary temperatures, the volume of carbon dioxide exhaled 

 is approximately equal to the volume of oxygen absorbed. 



In illustration we may cite the following experiments : 



i. With Fungi (Marcet). 



Three Lycoperdons, weighing 72 grains, were left in a closed receiver 

 containing 100 c.c. of air for six days. 



Composition of the air. At the commencement. At the end of the experiment. 



N 79-0 c.c. 79-0 c.c. 



O 2ro 18-0 



CO a oo-o 3-0 , 



1000 lOO'O 



