FOLIAGE LEAVES : Fl'XCTION, STRUCTURE, ETC. 31 



our liinirs in lu'eatliintr, and also comes off from burning 

 wood or coal. It is a common wa.ste product, being a com- 

 bination of carljon and o.'^ygen so intimate that the two 

 elements are separated from one another with great dif- 

 ficulty, rturing the process of photosynthesis it has been 

 discovered that carbon dio.xide is being absorbed from the 

 air by the leaves. As this gas is absorbed cliiefly by green 

 parts and in tlie light, in just the conditions in which oxy- 

 gen is being given off, it is natural to connect the two, and 

 to infer tliat 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. 



Wlien we ol>serve 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 liv the green parts of the plants in the presence of 

 light, the retention of the carbon, and the elimination of 

 the oxvgen. 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. 



20. 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 dnwn 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 



