16 , PHOTOSYNTHESIS 



oxygen than young ones ; that all plants in darkness, especially such parts 

 as roots, flowers, fruits render the air impure; that high concentrations 

 of carbon dioxide are poisonous even to plants and that the sunlight itself 

 has no effect on the air without acting in conjunction with the plants. 

 Besides, Ingen-Housz established a number of minor points which were 

 all most logically presented in his publication of about 150 pages. The 

 contradictory results of Priestley and of Scheele were explained and thus 

 did Ingen-Housz grasp the very fundamentals of the process. 



In 1784 Lavoisier established the composition of carbon dioxide and 

 the nature of combustion. At this time the battle of opinions regarding 

 this process was at its height, and the tremendous importance of Lavoisier's 

 discoveries was at first not realized even by Ingen-Housz. But in his last 

 important publication : "An Essay on the Food of Plants and the Renova- 

 tion of Soils" (London 1796) he had quite freed himself from the phlogis- 

 ton conceptions and succeeded in interpreting his discoveries according to 

 the newer school of Lavoisier. Ingen-Housz now saw clearly the cosmical 

 function of green plants, the relation between animal and plant nutrition, 

 the closed cycle of carbon dioxide and water -» organic matter + oxygen, 

 of the organic matter as food for animals + oxygen -^ carbon dioxide + 

 water. The source of the oxygen was the carbon dioxide ; the combustible 

 matter remained in the plant and served as food for plant and animal, 

 when it was again converted into carbon dioxide by the combustion 



process. 



There were at this time also some contributions from other workers ; 

 none of them, however, had the same clarity of vision as Ingen-Housz and 

 could distinguish between the two functions proceeding simultaneously, 

 photosynthesis and respiration, nor did they make use of the correct con- 

 ception of the composition of carbon dioxide. Thus there arose heated 

 arguments and polemics, more especially between Ingen-Housz and Sene- 

 bier. The historical writings regarding these discussions have in general 

 favored the contentions of Ingen-Housz. The publications of the latter 

 are marked by a clear style of writing, succinct expression and logical 

 sequence of reasoning. Senebier, on the other hand, while an industrious 

 experimenter, accumulated a large mass of observational data which was 

 published in an exceedingly prolix and poorly ordered manner and does not 

 support a logical sequence of thought. He repeated many of the experi- 

 ments of Ingen-Housz, and succeeded in demonstrating that only the green 

 portions of leaves were photosynthetically active. He also showed that 

 it was the light and not the heat from the sun which induced the emission 

 of oxygen, and by use of his well kiKJwn double walled bell-jars filled 

 with colored solutions, he was able to ascribe the chief action to the red 

 rays of the spectrum. 



Thus Priestlev. Ingen-Housz and .Senebier may be considered the 

 pioneers who discovered this new domain and made the first surveys 

 thereof. Considering the status of the knowledge of chemistry at the time, 

 their accomplishments were highly meritorious. 



