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 Treboux 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. Even in these experiments, however, it 

 is not certain how it was produced. In any except very 

 dilute solutions it is intensely poisonous to plants. 



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 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 



