io SOIL CONDITIONS AND PLANT GROWTH 



of salts in plants. De Saussure grew plants in air or in known 

 mixtures of air and carbon dioxide, and measured the gas 

 changes by eudiometric analysis and the changes in the plant 

 by " carbonisation ". He was thus able to demonstrate the 

 central fact of plant respiration the absorption of oxygen and 

 the evolution of carbon dioxide, and further to show the de- 

 composition of carbon dioxide and evolution of oxygen in 

 light. Carbon dioxide in small quantities was a vital neces- 

 sity for plants, and they perished if it was artificially removed 

 from the air. It furnished them not only with carbon, but 

 also with some oxygen. Water is also decomposed and fixed 

 by plants. On comparing the amount of dry matter gained 

 from these sources with the amount of material that can enter 

 through the roots even under the most favourable conditions, 

 he concludes that the soil furnished only a very small part of 

 the plant food. Small as it is, however, this part is indispens- 

 able : it supplies nitrogen une partie essentielle des vegetaux 

 which, as he had shown, was not assimilated direct from the 

 air ; and also ash constituents, qui peuvent contribuer a former, 

 comme dans les animaux, leur parties solides ou osseuses. Fur- 

 ther, he shows that the root is not a mere filter allowing any 

 and every liquid to enter the plant ; it has a special action and 

 takes in water more readily than dissolved matter, thus effect- 

 ing a concentration of the solution surrounding it ; different 

 salts, also, are absorbed to a different extent. Passing next 

 to the composition of the. plant ash, he shows that it is not 

 constant, but varies with the nature of the soil and the age of 

 the plant ; it consists mainly, however, of alkalis and phos- 

 phates. All the constituents of the ash occur in humus. If a 

 plant is grown from seed in water there is no gain in ash : the 

 amount found at the end of the plant's growth is the same as 

 was present in the seed excepting for a relatively small amount 

 falling on the plant as dust. Thus he disposes finally of the 

 idea that the plant generated potash. 



After the somewhat lengthy and often wearisome works of 

 the earlier writers it is very refreshing to turn to de Saussure's 

 concise and logical arguments and the ample verification he 



