Biological Preparation of Oxygen

Under the influence of light the green parts of plants steadily assimilate carbon dioxide and water, converting them into starch and evolving oxygen as a by-product. The volume of oxygen set free is approximately equal to that of the carbon dioxide absorbed, so that the initial and final stages of the very complex series of reactions involved may be represented by the equation

6CO₂ + 5H₂O = C₆H₁₀O₅ (starch) + 6O₂.

The energy necessary for this reaction, which is endothermic, is obtained from the light, the most active rays being, curiously enough, those of the red, orange, and yellow portions of the spectrum, and not the chemically reactive rays of the blue and violet end.

The evolution of oxygen from plants is readily demonstrated by placing fresh green leaves, such as those of mint or parsley, in a jar of water more or less saturated with carbon dioxide and exposed to sunlight. If the mouth of the jar is closed with ah inverted funnel fitted with a tap, sufficient oxygen will collect in an hour or two to admit of being tested with a glowing splinter. The experiment may be carried out in a glass cell in a projecting lantern, an image of the whole being thrown on to the screen by means of electric light. Bubbles of gas will be seen to collect rapidly on the leaves under the influence of the light.

This reaction is of particular interest inasmuch as it constitutes nature's method of replenishing the free oxygen content of the atmosphere. The efficiency of the process is evident when, to quote an example of medium assimilatory activity, it is remembered that one square metre of sunflower leaf can effect the decomposition of some 40 grams of carbon dioxide, and the simultaneous evolution of 30 grams of oxygen in one summer day of 15 hours duration.