68 HISTORY OF BOTANY 



keeps up a constant polemic. Thus he finds that the 

 quantity of gas given off by a plant cannot be accounted 

 for by the amount present in the plant body, and takes 

 great pains to prove that the gas evolved from green 

 parts differs entirely from that exhaled by roots and 

 flowers. He only confirms his contemporary when he 

 states that young leaves are unable to make full use of 

 the carbon dioxide supplied to them. He always seems 

 to miss the fundamental generalisation " only green 

 plant organs carry out carbon assimilation." He finds 

 that leaves perform their functions when isolated as well 

 as when attached to the plant. By a long series of 

 experiments he shows that it is light and not heat that 

 is the effective agent in the assimilatory process. He 

 finds also that oxygen is given off only when carbon 

 dioxide is present, but comes to the extraordinary con- 

 clusion that " fixed air " is changed into oxygen and 

 that this change is brought about by its acid character 

 acting as a stimulant on the leaf. Impressed with this 

 idea he tried to produce the same results by using other 

 acids, but needless to say the plants died. He said that 

 aquatics give off the more oxygen the more the water in 

 which they are cultivated is impregnated with " fixed 

 air/' a conclusion at variance with that arrived at by 

 Ingen-Housz, who, after describing a similar experiment, 

 says : "It is also true that leaves thus placed in water 

 impregnated with 'fixed air/ do not yield that fine 

 dephlogisticated air which they yield when placed in 

 common pump water." 



Although Senebier has a vague idea that the " fixed 

 air " has an important relation to the vital economy of 

 the plant he fails to grasp the fact that the oxygen comes 

 from the carbon dioxide, for he thinks that the leaf in the 

 sunlight can change nitrogen into oxygen and that 

 " fixed air " may also be transformed into oxygen. 

 In spite of very many experiments Senebier persists 

 in regarding the soil as the source of the " fixed air " 



