PHOTOSYNTHESIS 203 



Boussingault, also in 1868, tried the effect of growing 

 plants in an atmosphere of pure carbon dioxide, and in 

 mixtures of that gas with neutral gases Uke nitrogen and 

 hydrogen. He found that oxygen exhalation ceased 

 when carbon dioxide alone was present, and attributed 

 this result, not to the absence of oxygen, as De Saussure 

 held, but to the density of carbon dioxide, seeing that 

 assimilation recommenced as soon as the carbon dioxide 

 was diluted by one of the neutral gases above mentioned. 

 He also controverted De Saussure's statement that 

 plants give off nitrogen during photosynthesis, and 

 established the constancy of the volumes of carbon 

 dioxide absorbed and oxygen exhaled. He further 

 expressed the view that the pathway of gaseous exchange 

 was not by the stomata but by the cuticle, a view held 

 for many years, and one in which he was supported by 

 Barthelemy, who held that stomata were valves for the 

 regulation of the exit of gases and not for their entry. 



In 1870 Baeyer pubhshed a paper which had a profound 

 influence on the trend of research during the years to 

 follow, right up indeed to the present day. His theory 

 was based on Butlerow's discovery in 1861, that a 

 substance possessing some of the properties of a sugar 

 could be obtained by heating a condensation product 

 of formaldehyde in an alkaUne medium. Baeyer 's 

 hypothesis has been so often referred to in the text- 

 books that I think it advisable that you should have 

 it presented to you in his own words, as translated by 

 Jorgensen and Stiles. " The general assumption in 

 regard to the formation in the plant of sugars and related 

 bodies is that in the green parts carbon dioxide under 

 the action of light is reduced and by subsequent synthesis 

 transformed into sugar. Intermediate steps have been 

 sought for in organic acids — formic, oxaUc, or tartaric — 

 which may be regarded as reduction products of carbon 

 dioxide. According to this view, when the green parts 

 of plants are most strongly subjected to the action of 



