258 SCIENCE PROGRESS 



green parts of plants. In the same 5^ear Ingen-Houss showed 

 that this only occurred during illumination and that in darkness 

 the green parts produced carbon dioxide. Later on Ingen-Houss 

 concluded that the oxygen exhaled in light was derived from the 

 atmosphere, which he thought to be the main source of the 

 carbon in plants. 



A further step was taken by De Saussure at the beginning of 

 the nineteenth century. By quantitative analysis he found that 

 the increase in dry weight could not be wholly accounted for by 

 the amount of carbon gained, but that the elements of water 

 were retained by the plants and fixed simultaneously with the 

 carbon. From this time, until the discovery by Wohler, in 1828, 

 that " organic" could be prepared from " inorganic " compounds, 

 the comparatively slow progress in the theory of nutrition 

 of plants was mainly due to the prevailing belief in the action of 

 an unknown " vital force " in all the phenomena of life. 



But with the rise of " organic " chemistry and Dutrochet's 

 discovery of endosmose it became possible to find chemical and 

 physical explanations of some of the hitherto incomprehensible 

 processes carried out by plants. For example, it was suggested 

 by De Candolle, in 1832, that a kind of gum was formed in the 

 leaves from the union of water and its contained salts with 

 the carbon derived from the atmospheric carbon dioxide. He 

 thought, too, that this gum was easily alterable into starch, 

 sugar and lignine, and travelled in the rind and wood, apparently 

 through the spaces between the cells. Except for the path 

 assigned to the assimilates this view of De CandoUe's approxi- 

 mates very nearly to the theory at present in vogue. 



The chemical activity of leaves was still more clearly recog- 

 nised after 1840 when Liebig wrote his Organic Chemistry in its 

 relation to Agricultitre and Physiology. In this work he showed, 

 as the result of calculations, that the atmosphere contained 

 enough carbon dioxide to support the vegetation of the world 

 for countless generations. Until this time botanists had always 

 felt some difficulty in believing that a plant could obtain the 

 carbon forming so large a proportion of its substance from the 

 air, which only contains about "03 per cent, of carbon dioxide. 

 But the experiments of Sachs, Garreau, Boussingault, and the 

 critical work of F. F. Blackman have shown that the carbon is so 

 derived and that the amount assimilated bears a direct relation 

 to the number of " stomata " (minute openings leading from the 



