THE EVOLUTION OF OXYGEN BY PLANTS. 



297 



containing much oxygen a considerable quantity of oxygen must have been hberated, 

 since otherwise the organic substance could not be combustible. 



It is seen from this simple consideration that somewhere and in some manner 

 oxygen must be separated in nutrition ; and long before these reflections were estab- 

 lished this evolution of oxygen had been, as a matter of fact, observed. Towards 

 the end of the last century, Priestly, Senebier, Ingenhouss, and later, with classical 

 completeness, De Saussure, established the fact that the green organs of plants evolve 

 oxygen under the influence of light, if they are at the same time in a position to take 

 up carbon dioxide from without. It is of importance to the history of botany to 

 make clear how this important fact was gradually established and explained by the 

 men referred to, and how, for forty years subsequently, almost inconceivable 

 misapprehensions again obscured the clearly established fact, until more recently 

 it came to be counted among the indisputable acquisitions of science. 



As an essential point, it is at the same time to be insisted upon here that the 

 volume of oxygen evolved is equal to the volume of the carbon dioxide taken in, 

 as De Saussure and, later and more exactly, Boussingault, have already established. 

 This signifies, in other words, that the chemical processes in the cells containing 

 chlorophyll run their course as if all the oxygen of the carbon dioxide taken up 

 was evolved. This fact is of the utmost importance, because further consequences 

 respecting the processes in the assimilating cells can be concluded from it. 



Since we are here treating of those chemical processes by means of which the 

 production of organic substance from inorganic materials (carbon dioxide and water) 

 originally takes place in the natural course of events, and since this organic substance 

 affords the constructive material not only for plants but for the whole animal kingdom 

 as well, it must interest the student of vegetable physiology to observe the process 

 for himself. This can, of course, only be effected indirectly, and this most simply by 

 placing water-plants such as Ceratophyllum, Myriophyllum, Udora (Eloded), Potavio- 

 gelon, etc., in water containing carbon dioxide, and taking care that a smooth cross- 

 cut is made at the lower part of the leafy shoot-axis. If the vessel filled with water 

 containing carbon dioxide, in which the shoot is submerged, is placed at a window on 

 which the sun is shining, a stream of small bubbles of gas is at once seen to spring 

 from the cut end of the shoot-axis, and to escape upwards. If a glass tube previously 

 filled with water is inverted over the surface of the water in the vessel, the gas evolved 

 by the water-plant may be collected, and it is then shown that this gas, if the experi- 

 ment has been properly made, is very rich in oxygen, but contains also carbon dioxide 

 and nitrogen. These two latter gases, however, really owe their presence to a falla- 

 cious arrangement of our experiment : for the bubbles of oxygen evolved by the plant 

 ascend into the collecting vessel through water laden with nitrogen and carbon dioxide, 

 and, according to the general laws of diffusion of gases, they must take up carbon 

 dioxide and nitrogen from the water during the ascent. Although this experiment 

 with the above water-plants is particularly suitable for ocular demonstration of 

 the work of plants, a more circumstantial and less obvious mode of procedure 

 is more convenient for rendering clear the true process of the decomposition of 

 carbon dioxide. For this purpose, it is better to employ leaves, or portions of the 

 leaves, of land-plants ; i)ushing them into a glass tube which is cylindrical and gradu- 

 ated below, and dilated above. Into this tube, closed below with mercury, an accurately 



