298 LECTURE XVIII. 



measured quantity of carbon dioxide gas is placed, together with atmospheric air, and 

 the whole apparatus is then placed in the sunlight. After some time, perhaps an 

 hour, the leaf is withdrawn, and the carbon dioxide and oxygen contents of the air in 

 the glass tube examined, according to known gasometric methods. It now turns out 

 that the quantity of carbon dioxide has diminished, while the proportion of oxygen has 

 increased ; and, moreover, it is shown that for each cubic centimetre of carbon dioxide 

 which has disappeared, a cubic centimetre of oxygen has become free. Such an 

 experiment, it is true, is little suitable for demonstration in the lecture room, since it 

 demands at least three or four hours of work, besides corrections and calculations : it 

 supplies, however, incontestable proof that carbon dioxide is decomposed by means 

 of a green leaf, under the influence of light, and that an equal volume of oxygen is 

 evolved in its place. What we here observe, however, in the case of a single small 

 leaf, or portion of a leaf, is proceeding more or less energetically during the whole 

 day in all green leaves of plants in the open, even when the sun is overshadowed 

 by clouds ; and what appears in our experiment as the decomposition of a few cubic 

 centimetres of carbon dioxide, is represented in the case of a meadow, a corn-field, or 

 a forest, by hundreds or thousands of litres of this gas. 



That it is by means of the chlorophyll of the plant that the absorbed carbon 

 dioxide is decomposed and its oxygen evolved, follows wth certainty from the fact 

 that it is only and exclusively those parts of plants which contain chlorophyll that 

 are able to eifect this decomposition \ No organs devoid of chlorophyll, such as 

 roots, subterranean tubers, floral leaves and stamens ; no chlorotic white leaves, or 

 yellow etiolated leaves grown in the dark; and again, no non-green parasites or 

 Fungi, yield oxygen in the experiment just described: on the contrary, they 

 continually evolve carbon dioxide from their interior, though in relatively small 

 quantities. In such organs devoid of chlorophyll, therefore, no reconstruction of 

 combustible organic substance poor in oxygen can take place; and thence follows 

 at once that in all plants containing chlorophyll, the organs which are not green 

 consist of organic compounds which have been produced in the organs containing 

 chlorophyll, and in like manner that plants generally which contain no chlorophyll, 

 such as true root-parasites and all Fungi, are necessitated to absorb organic sub- 

 stance poor in oxygen from without, because they lack the organs for decomposing 

 carbon dioxide and producing organic substance. Since, however, all animals are 

 likewise devoid of these organs, and are thus unable to form organic substance 

 from carbon dioxide and water, although they build up their bodies from such 

 substance, it follows obviously that the substance of the bodies of all animals is 

 originally produced in the chlorophyll-cells of plants. The few lower animals which 

 apparently contain chlorophyll — certain Infusoria, Sponges, and Plaiiarice — contain 



' That the green-coloured protoplasm, the chlorophyll, is the organ of assimilation, was first 

 expressed definitely by me in my ' Experimental-Pkysiologie' (1865, p. 319), in opposition to the 

 utterly indefinite statements of earlier physiologists, according to which green plants possess a 

 daily respiration with the evolution of oxygen, and a nocturnal respiration with the formation of 

 carbon dioxide, and after the doubts which even the distinguished Theodore de Saussure entertained 

 as to the signification of chlorophyll. I expressly insisted upon the point that ' it is a striking 

 phenomenon in the history of vegetable physiology that the chlorophyll has not long ago been 

 distinguished definitely as the instrument for the separation of oxygen.' 



