658 REPORT—1899. 
Although positive experimental proof was still wanting that aerial plants also 
derive their carbon from carbon dioxide, Senebier regarded this as extremely pro- 
bable; but, taking into consideration the small amount of this gas present in the 
atmosphere, he concluded that it must reach the plant by the roots and leaves 
entirely in a state of solution in water. 
The work of Priestley, Senebier, and Ingen-Housz fortunately attracted the 
attention of a young chemist of high attainments, who, within a period of less than 
ten years, did more for the advancement of vegetable physiology than any single 
observer before or since his time. Théodore de Saussure, the second of that 
illustrious name, and the son of the famous explorer and natural philosopher, 
commenced his researches about the year 1796, and in 1804 published his 
‘ Recherches Chimiques sur la Végétation,’ a modest little octavo volume of some 
500 pages which must certainly take rank as one of the great classics of scientific 
literature, and one of the most remarkable books of the century. 
De Saussure was a past master in the art of experiment, and the methods 
which he devised for demonstrating the influence of water, air, and soil on vegetation 
have been the models on which all such investigations have been conducted ever 
since. It is indeed very difficult, when reading this masterly essay, to bear in 
mind that it was not written fifty or sixty years later than the date on its title- 
page, so essentially modern are its modes of expression and reasoning, and so far is 
the author in advance of his contemporaries. It is to this work we must 
refer for the first experimental proof that plants derive at any rate the greater 
part of their carbon from the surrounding atmosphere. This was shown by De 
Saussure by a variety of quantitative experiments of a sufficient degree of accuracy 
to bring out the great leading facts. By making known mixtures of carbon 
dioxide and air, and submitting them to the action of plants in sunlight, he was 
able to show not only that the gaseous carbon dioxide was decomposed and the 
carbon assimilated, but also that the volume of oxygen disengaged was approxi- 
mately equal to that of the carbon dioxide decomposed.! He also showed that 
plants growing in the open in moist sand, or in distilled water, and therefore under 
conditions in which they could not derive any carbon from other than atmospheric 
sources, not only materially increased in dry weight, but contained much more 
carbon at the close of the experiment than at the beginning, and had also fixed an 
appreciable amount of water in the process. That atmospheric carbon dioxide is 
not only beneficial to plants in sunlight, but is also essential to their very existence, 
De Saussure proved by introducing an absorbent of this gas into the vessel 
containing a plant or the branch of a tree rooted naturally in the soil. Under 
these conditions the portions of the plant enclosed always died. He also ascer- 
tained by experiment the increase in dry weight of a sunflower plant during 
four months of natural growth; and knowing approximately the amount of 
water transpired during that period, and the maximum amount of solids which 
this transpired water could possibly introduce into the plant, he calculated that 
these solids, and the carbon dioxide in solution in the transpiration water, fell 
far short of accounting for the observed increase in the dry weight of the plant. 
This increase must, therefore, be mainly due to the fixation of atmospheric carbon 
dioxide and water. 
It is certainly a remarkable fact that the rigid experimental proofs which De 
Saussure brought forward in support of his views did not carry conviction to the 
minds of every one. His book, however, suffered the fate of many others which 
have appeared in advance of their time. It is true that De Saussure’s doctrines 
were always kept alive by the advanced physiologists of the French school, such as 
De Candolle and Dutrochet, but when Liebig first turned his attention to the 
subject he found the field in possession of the humus theory of Treviranus, a theory 
? Although clearly indicating that no change of volume occurred in the mixture 
of air and carbon dioxide so treated, his tinal analytical results show a small apparent 
evolution of nitrogen. This was due to the eudiometric methods he employed— 
methods, it is true, far superior in point of accuracy to those of his predecessors, but 
still necessarily imperfect. 
