Carbon Assimilation. 
81 
seems certain that various monosaccharides can be produced from 
formaldehyde under certain conditions. But these conditions, 
generally high temperature and alkaline medium, are not the 
same as those existing in the plant, so that it is impossible to 
argue from these experiments “ in vitro ” as to the possible con¬ 
densation of formaldehyde to sugar in the leaf. Nor do we know 
of any photochemical or enzymatic reactions which could bring 
about this change. 
3. It has been urged that carbon assimilation should proceed 
in absence of carbon dioxide if an intermediate product were 
given as nutrient. Thus Loew (1889) and Bokorny (1888-1911) insist 
that Spirogyra in absence of carbon dioxide, but in presence of 
the sodium bisulphite compound with formaldehyde can form 
starch, while Grafe (1909, 1911) and Miss Baker (1913) 
have urged that plants can so utilise gaseous formaldehyde itself 
if this is present in the air in a concentration sufficiently low to 
prevent toxic effects. This only takes place in the light; in the 
dark formaldehyde is toxic. Spoehr established that formaldehyde 
vapour mixed with air is quickly oxidised to formic acid in sun¬ 
light, so that Grafe’s and Miss Baker’s experiments could only be 
used in favour of a formic acid theory of assimilation. 
Moreover, the utilisation of a substance by the leaf is no proof 
that that substance is an intermediate product in carbon assimila¬ 
tion, as we know of several substances which can be utilised by 
the plant, such as glycerine and sugars not normally found in the 
leaf, but which nevertheless are not generally supposed to be 
intermediate products. 
Again it has not been found possible to utilise carbon-mono¬ 
oxide in assimilation, as has been shown for example by de Saussure 
(1804), Boussingault (1868) and Krasheninnikoff (1909). 
Thus it is seen, as Spoehr expresses it, that Baeyer’s hypothesis, 
“ though alluring on account of its simplicity, is by no means as well 
established as many writers on the subject would have us believe.” 
Indeed it seems to us that the words of Sachs written 35 years 
ago (1882, 1887) are as applicable now as on the day when 
they were written; “whether it is right to claim, with Bertlelot 
and Kekule, formic acid or some other member of the formyl group 
as the first product of assimilation, on account of its simple con¬ 
stitution, I hold as at least very questionable; and it has hitherto 
been proved by nothing.” 
