166 VEGETABLE PHYSIOLOGY 



green cell such as one of a filament of Spirogyra, the plant 

 cannot make any use of it. It has been found, however, 

 that if Spirogyra is fed with a compound of formaldehyde 

 and sodium-hydrogen-sulphite, which slowly evolves the 

 former in the presence of water, a formation of carbo- 

 hydrates occurs. This cannot, however, be accepted as proof 

 that formaldehyde normally subserves this purpose. 



If 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 C0 2 + H 2 = 

 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 

 condensation of several molecules. Such a condensa- 

 tion of formaldehyde would lead to the formation of sugar 

 thus : 6HCOH = C 6 H 12 G . 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 



