158 VEGETABLE PHYSIOLOGY 



HCOH + 2 , 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 = C 6 Hi 2 6 . 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 



