CHAP, ii.] of Plants. De Saussure. 499 



the style therefore, as might be expected, is dry and unattrac- 

 tive; the writer seems to confine himself too anxiously within the 

 limits of what is given in experience, and there is no doubt that 

 many errors in later times might have been avoided if the 

 inductive proof of de Saussure's doctrines had been accom- 

 panied with a deductive exposition of them of a more didactic 

 character. 



The processes of vegetation examined by de Saussure were, 

 for the most part, the same as those which Ingen-Houss and 

 Senebier had studied at length and correctly described in 

 their general outlines. But de Saussure went beyond this, 

 and by means of quantitative determinations struck a balance 

 between the amount of matter taken up and given off by the 

 plant, thereby showing what it retains. In this way he made 

 two great discoveries : that the elements of water are fixed in 

 the plant at the same time as the carbon, and that there is no 

 normal nutrition of the plant without the introduction of 

 nitrates and mineral matter. But we cannot form a due idea 

 of de Saussure's services to physiology without going further into 

 the detail of his work. 



We will first consider his investigations respecting the as- 

 similation of carbon in plants. Here we have the important 

 result, that larger quantities of carbon dioxide in the atmo- 

 sphere surrounding the plants are only favourable to vegetation 

 if the latter are in a condition to decompose them, that is, if 

 they are in sufficiently strong light ; that every increase in the 

 amount of carbon dioxide in the air in shade or in darkness is 

 unfavourable to vegetation, and that if that increase is greater 

 than eight times in the hundred it is absolutely injurious. On 

 the other hand he found, that the decomposition of carbon 

 dioxide by the green parts in light is an occupation that is 

 necessary to them, that plants die when they are deprived of 

 it. The first clear insight into the chemical processes which 

 accompany the decomposition of carbon dioxide in the interior 

 of the plant was obtained by perceiving, that plants by appro- 



K k 2 



