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 2CO, = 2CO + Os. At 
the same time there is a decomposition of water, possibly 
in the way denoted by the equation 2H,O = 2H, + O:. 
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 
gS). = CH,O, = or__ preferably 
= = ===. HCOH. This suggested 
Fia. 88,—ArraRatTus To sHow tHE Evotu- geries of reactions agrees 
sy hel lia iia acini 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 
