152 VEGETABLE PHYSIOLOGY 
The formation of formaldehyde, again, is very difficult 
of proof. It very readily undergoes change, and therefore 
is difficult to detect in a plant. It has been found, how- 
ever, that if Spirogyra is fed with a compound of form- 
aldehyde and sodium-hydrogen-sulphite, which slowly 
evolves the former in the presence of water, a formation 
of carbohydrates occurs. This cannot, however, be accepted 
as proof that formaldehyde normally subserves this purpose. 
There is, however, a certain amount of evidence that 
formaldehyde plays some part in photosynthesis. Bouilhac 
and Tréboux have succeeded in getting plants to grow in a 
very dilute solution of it. Moreover, formaldehyde has 
been obtained from plants by distilling leaves which have 
been exposed for a long time to light and subsequently 
soaked in water. FEiven in these experiments, however, it 
is not certain how it was produced. In any except very 
dilute solutions it is intensely poisonous to plants. 
Ii we concede that formaldehyde is very probably the 
first stage in the photosynthetic process, a consideration of 
the probable decomposition seems to lead us to the view 
that the carbon dioxide and the water are made to interact 
without the liberation of carbon monoxide, and that the 
reaction may be represented by the equation CO, + H,O = 
HCOH + O,, which agrees equally well with the observed 
facts. 
The formaldehyde may give rise without much difficulty 
to a form of sugar. It is a property of the aldehydes to 
undergo readily what is known as polymerisation, or 
condensation of several molecules. Such a condensa- 
tion of formaldehyde would lead to the formation of sugar 
thus :—6HCOH = C,H,,0,. There are many sugars of 
this composition in the plant, especially glucose or grape 
sugar, and fructose or fruit sugar. 
That some such process takes place is extremely probable, 
for sugar is present in the mesophyll cells very speedily 
after the absorption of the carbon dioxide and the begin- 
ning of the exhalation of oxygen. Sugar of some kind 
