72 The Science of Life. 



and only by the green parts that oxygen is given off, 

 that this is quite distinct from another (respiratory) pro- 

 cess in which carbonic acid gas is liberated, and that the 

 chief if not the only source of the carbon in plants is in 

 the carbonic acid gas of the atmosphere. 



In 1800 Senebier (1742-1809) corroborated Ingen- 

 Houss's discovery of the decomposition of carbon di- 

 oxide. Much more important, however, was the work 

 of Theodore de Saussure (1767-1845), son of the famous 

 explorer of the Alps, who introduced the quantitative 

 method of estimating a plant's income and expenditure, 

 and thereby showed that the elements of water are fixed 

 in the plant as well as the carbon of the carbon dioxide, 

 that respiration is essential to growth and is related to 

 the internal heat (measurable in flowers), that plants are 

 unable to use the nitrogen of the atmosphere, and that 

 there is no normal nutrition apart from nitrates and 

 similar salts in the soil. 



The chief representatives of vegetable physiology 

 about 1840 were De Candolle (better known as a sys- 

 tematist), Treviranus, and Meyen, but none of them 

 made any new step of importance. Two impeding 

 theories had to be got rid of, the theory of vital force 

 and the theory of humus. The former could only die 

 hard, but the latter was cut short by Liebig. According 

 to the " humus-theory " it was believed that plants feed 

 upon prepared organic matter (or humus) in the soil, 

 and this was regarded as a source of both carbon and 

 nitrogen. Liebig showed, however, that (Fungi apart) 

 plants derive from the soil only water, ammonia, and 

 inorganic salts, and corroborated the already established 

 conclusion that all the carbon supplies are in the CO 2 

 of the air. As plants die down they necessarily enrich 

 the soil with humus, but this humus as such forms no 

 part of the food-supply. There is no doubt that 1840, 

 when Liebig published the first edition of his Organic 

 Chemistry in application to Agriculture and Physiology ', 

 is one of the red-letter dates in the history of biology. 

 It marks the first concrete realization of the " circulation 

 of matter". 



What Liebig had shown in a general way was con- 



