156 



VEGETABLE PHYSIOLOGY 



The apparatus shown in fig. 88 will enable this inter- 

 change of gases to be seen. Into a glass jar is poured some 

 water containing carbon dioxide in solution. Some aquatic 

 plant is put into the water and a funnel inserted above it, 

 the end of which rises into a burette filled with water and 

 closed by a stopcock. The whole apparatus being placed 

 in sunlight, bubbles of oxygen will be given off by the 

 leaves and will rise into the burette. If no carbon dioxide 

 is in the water, no oxygen will be given off. 



There is little certainly known at present as to the details 

 of the changes which connect these two phenomena. It 

 has been suggested by Baeyer that the carbon dioxide is 

 decomposed with the formation of carbon monoxide and 

 oxygen, according to the equation 2COo = 2CO + Og. At 

 the same time there is a decomposition of water, possibly 

 in the way denoted by the equation 2H 2 = 2H 2 + Og- 

 The oxygen is given off, the volume being found, when care- 

 fully measured, to be equal 

 to the volume of carbon 

 dioxide undergoing de- 

 composition. The carbon 

 monoxide and the hydro- 

 gen are then thought to 

 unite, producing form- 

 aldehyde, a body repre- 

 sented by the formula 

 CHoO, or preferably 

 1 HCOH. This suggested 

 series of reactions agrees 

 fairly closely with the ob- 

 served facts, but it must 



not be regarded as anything more than an hypothesis. Indeed 

 there are considerable difficulties in accepting it as it stands. 

 There is no evidence that carbon monoxide is formed. 

 Experiments have shown that this gas is quite useless to 

 most plants ; if it is supplied in the place of the dioxide, 

 the formation of carbohydrates does not take place. Nor has 



FIG. 88. APPARATUS TO SHOW THE EVOLU- 

 TION OF OXYGEN BY A GREEN PLANT 

 IN SUNLIGHT. 



