158 VEGETABLE PHYSIOLOGY 
HCOH + 0,, 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 con- 
densation of several molecules. Such a condensation 
of formaldehyde would lead to the formation of sugar 
thus :—6HCOH = C0;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 
appears to be the first carbohydrate to be formed ; it is not 
very readily detected, being freely soluble in the cell-sap. 
Almost as quickly as the formation of sugar we have the 
appearance of starch in the substance of the chloroplasts, 
and as this is easily visible, it was long thought that starch 
was the culminating product of the photosynthetic process. 
We shall find reasons shortly for suggesting a wholly different 
meaning to the appearance of the starch, that it is indeed 
only a temporary store of carbohydrate in an insoluble 
condition, due to the production of sugar being in excess 
of the quantity which the cell can dispose of by immediate 
consumption or translocation. 
If we accept the view of the polymerisation of formalde- 
hyde to give rise to the sugar, we cannot withdraw this 
operation also from the activity of the chloroplast. Sugars 
are what are called optically active compounds : that is, they 
possess the power of deflecting a ray of polarised light to 
the right or to the left as the latter is made to pass through 
either crystals or a solution of them. Formaldehyde has no 
such power. There is no process known by which an 
optically active compound is formed from an optically 
inactive one without the intervention of living substance. 
Consequently we must suppose that the polymerisation is 
