ORIGIN OF THE CARBON. 293 



disputable fact, like every other great discovery, has been exposed, to such an 

 extent, indeed, that some forty years ago the weighty word of Justus von Liebig 

 was needed, before the fact that the carbon of plants is derived solely and simply 

 from the atmosphere was again completely established. We shall not be concerned 

 more in detail with how plants obtain the carbon from the carbon dioxide of the 

 atmosphere until the following lecture : meanwhile the source only of this substance 

 interests us here, quite apart from the physiological work of the plant. Many 

 so-called investigators were prevented, for more than forty years after the discoveries 

 of Ingenhouss and De Saussure, from believing in the atmosphere as the exclusive 

 source of the carbon, by the fact, known even then, that the relative quantity of 

 carbon dioxide contained in the air is extremely small. Innumerable analyses of air 

 have shown, in fact, that in 10,000 litres (that is in 10 cubic metres) there are 

 contained on an average but 4-6 litres of carbon dioxide ; and since i litre of 

 carbon dioxide weighs about 2 grams, 10 cubic metres of air contain 8-12 

 grams of carbon dioxide. This quantity certainly appears very small, the more 

 so since only j\ of it consist of carbon, and, as we shall see later, the whole 

 of the oxygen of the carbon dioxide (that is y\) are given off by the plant. 

 Nevertheless so much is clear, that 10 cubic metres of air contain somewhat 

 more than 2 grams of carbon, from which, by combination with hydrogen, 

 oxygen, nitrogen, and sulphur, according to circumstances in each case, 4-5 

 grams of vegetable substance may be formed. A plant, however, which in the 

 living state contains 4-5 grams of organic substance, weighs, together with 

 the water contained in it, about 20-25 grams on the average, and this would 

 certainly be a rather small specimen. However, not only a plant growing in the 

 open, but even one in a room, has at its disposal much more than 10 cubic metres 

 of air, for, on account of the continual movement of the atmosphere, new air con- 

 taining carbon dioxide is always being conveyed to the leaves ; and on closer 

 consideration it is seen to depend essentially only upon whether the green leaves 

 are able to snatch these relatively small quantities of carbon dioxide from the 

 atmosphere with sufficient rapidity. That they actually do this is shown by the 

 result— the fact that very considerable quantities of carbon are accumulated in a 

 plant in a relatively short time. I found, for instance, that a very vigorous 

 Tobacco plant in the course of 100 days during its development in the garden 

 absorbed more than 400 grams of carbon from the air, and a Sunflower 

 fixed more than 800 grams in the same time; quantities which were contained 

 in several thousand cubic metres of air. Moreover, the further fact that a Gourd 

 plant, with a leaf-surface measuring one square metre, is able in the course of ten 

 hours of daylight to produce as much as 8 grams of starch, in which neai'ly 

 4 grams of carbon are contained, shows that the carbon dioxide of 20 cubic 

 metres of air has been made use of by this area of leaf-surface in one day ^. 



These reflections on the source of the carbon enable us at the same time to 

 perceive that it would be exceedingly unreasonable to make such experiments on 

 nutrition as have been described, in a completely closed space to which the air has 



1 [See further Sachs, in Arbeiten des hot. Institut zu Würzburg, B. Ill, 18S4, where the 

 quantities of starch produced are shown to be considerably greater.] 



