302 The Physiology of Plants BOOK in 



an adequate quantity of water are split up, the former 

 yielding carbon monoxide and oxygen, the latter oxygen 

 and hydrogen ; the oxygen groups unite and are exhaled ; 

 the carbon monoxide and the hydrogen unite to form 

 formaldehyde ; six molecules of the formaldehyde poly- 

 merise to form a molecule of sugar ; the sugar by dehydra- 

 tion loses a molecule of water and becomes starch. 



This hypothesis was received with acquiescence in most 

 quarters, but not in all. Gerland, in 1871, and Sachsse, in 

 1877, suggested that chlorophyll itself is the primary pro- 

 duct of the synthetic process, while Pringsheim in 1880 

 held that hypochlorin fills that place, a view untenable 

 in view of the fact established by Hilburg, that the quantity 

 of hypochlorin obtainable from a leaf does not diminish 

 when no carbon dioxide is supplied to it. 



Baeyer's hypothesis was challenged in 1877 by Erlenmeyer, 

 who proposed certain modifications of the early stages in 

 the process. According to him, the first interaction of 

 carbon dioxide and water leads to the formation of formic 

 acid and hydrogen peroxide, and these are subsequently 

 decomposed, yielding formaldehyde, water, and free oxy- 

 gen. He made no suggestion as to the further changes. 



In the later years of the century other hypotheses were 

 advanced, which, like the former, were not at once based 

 upon experimental evidence. Indeed we may say in passing 

 that up to 1900 very little experimental evidence of any 

 kind bearing upon the early stages of construction was 

 forthcoming. 



Vines put forward a theory in 1886 which associated the 

 matter more closely with the plastid. Starting with the 

 observation that a carbohydrate substance, cellulose, is 

 produced or secreted by protoplasm in the process of the 

 thickening of cell walls, and noticing the formation of starch 

 grains in the chloroplast as soon as the absorption of carbon 

 dioxide has been established, he argued that the carbo- 



