22 DISCOVERY OF PHOTOSYNTHESIS CHAP. 2 



The equation (2.1), by which we have summarized the discoveries of 

 Ingen-Housz, was thus improved by Senebier in one point, and could now 

 be written as follows: 



plant 



(2.2) Fixed air + light > something phlogisticated in the plant + 



dephlogisticated air 



6. The Assimilation of Carbon 



Priestlej^ and Ingen-Housz assumed that the plants, in the process of 

 transforming "impure air" into "purified air," acquire nourishment for 

 themselves. (If they would have adhered strictly to the phlogiston 

 theory, they should have concluded that this nourishment is pure phlo- 

 giston.) Senebier, who discovered that the substrate of transformation 

 is fixed air, could have gone a step further, and used Lavoisier's theory of 

 the composition of fixed air to deduce that carbon is the acquisition which 

 the plants make by absorbing carbon dioxide and exhaling oxygen. How- 

 ever, ■ Senebier again showed himself no match for the quick-witted 

 Ingen-Housz. In 1779. the Dutchman had reaped the seeds sown b}' 

 Priestley's experiments, and left to the Swiss pastor the work of gleaning; 

 in 1796, he was the first to harvest the fruits of Lavoisier's theory. He, 

 who as late as 1784, denied that fixed air has anything to do with photo- 

 sj-nthesis, wholeheartedly espoused this doctrine in 1796, in a new book 

 Food of Plants and Renovation of the Soil. He hinted that he had believed 

 in this doctrine since 1779, and left Senebier and his polemics with him 

 about this point unmentioned. Nevertheless, this last book of Ingen- 

 Housz is another remarkable achievement, and has been a milestone in 

 the development of the science of plant nutrition. The mechanism of 

 photosynthesis is presented in this book, for the first time, in terms of the 

 new chemical theory, calling "fixed air" — carbonic acid, and "dephlogis- 

 / ticated air" — oxygen, and proclaiming that plants acquire their carbon 

 ' (whose important role in the composition of organic matter was well recog- 

 nized by then) by the decomposition of carbonic acid from the air. Hales' 

 doctrine of "aerial nourishment" of plants has thus received its first 

 concrete interpretation. While Senebier thought that carbon dioxide 

 from the air is first dissolved in soil water and reaches the leaves through 

 the roots, Ingen-Housz suggested that plants receive only their "juices" 

 from the soil, and obtain both their carbon, and their oxygen, from the air. 

 He had the erroneous idea that while carl)()n is obtained from carbonic 

 acid during the day, oxygen is derived from the same source during the 

 night. (This is one example of the many "wild guesses" which can be 

 found in the writings of Ingen-Housz.) One question which worried 

 Ingen-Housz was the supply of carbon dioxide. While de Saussure 

 thought that carbon dioxide is a permanent, although minor and variable 

 component of the atmosphere, Lavoisier found no carbon dioxide at all 



