FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 31 
our lungs in breathing, and also comes off from burning 
wood or coal. It is a common waste product, being a com- 
bination of carbon and oxygen so intimate that the two 
elements are separated from one another with great dif- 
ficulty. During the process of photosynthesis it has been 
discovered that carbon dioxide is being absorbed from the 
air by the leaves. As this gas is absorbed chiefly by green 
parts and in the light, in just the conditions in which oxy- 
gen is being given off, it is natural to connect the two, and 
to infer that the process of photosynthesis involves not only 
the green color and the light, but also the absorption of 
carbon dioxide and the elimination of oxygen. 
When we observe that carbon dioxide is a combination 
of carbon and oxygen, it seems reasonable to suppose that 
the carbon and oxygen are separated from one another in 
the plant, and that the carbon is retained and the oxygen 
given back to the air. The process of photosynthesis may 
be partially defined, therefore, as the breaking up of carbon 
dioxide by the green parts of the plants in the presence of 
light, the retention of the carbon, and the elimination of 
the oxygen. The carbon retained is combined into real 
plant food, in a way to be described later. We may con- 
sider photosynthesis as the most important function of the 
foliage leaf, of which the absorption of carbon dioxide and 
the evolution of oxygen are external indications ; and that 
light and chlorophyll are in some way essentially connected 
with it. 
26. Transpiration. One of the easiest things to observe 
in connection with a working leaf is the fact that it gives 
off moisture. A simple experiment may demonstrate this. 
If a glass vessel (bell jar) be inverted over a small active 
plant the moisture is seen to condense on the glass, and 
even to trickle down the sides. A still more convenient way 
to demonstrate this is to select a single vigorous leaf with 
a good petiole ; pass the petiole through a perforated card- 
board resting upon a tumbler containing water, and invert 
