136 ELEMENTS OF BOTANY 
16i. Details of the Work of the Leaf.1— A leaf has four 
important functions to perform: 
(1) Fixation of carbon, or (3) Excretion of water. 
starch-making. 
(2) Assimilation.? (4) Respiration. 
162. Absorption of Carbon Dioxide and Removal of its 
Carbon. — Carbon dioxide is a constant ingredient of the 
atmosphere, usually occurring in the proportion of about 
three parts in every 10,000 of air, or one thirty-third of one 
per cent. It is a colorless gas, a compound of two simple 
substances or elements, carbon and oxygen, the former 
familiar to us in the forms of charcoal and graphite, the 
latter occurring as the active constituent of air. 
Carbon dioxide is produced in immense quantities by 
the decay of vegetable and animal matter, by the respira- 
tion of animals, and by all fires in which wood, coal, gas, 
or petroleum is burned. 
Green leaves and the green parts of plants, when they 
contain a suitable amount of potassium salts, have the 
power of removing carbon dioxide from the air (or in 
the case of some aquatic plants from water in which it is 
dissolved), retaining its carbon, and setting free part or all 
of the oxygen. This process is an important part of the 
work done by the plant in making over raw materials into 
food from which it forms its own substance. 
1 See Kerner and Oliver’s Natural History of Plants, Vol. I, pp. 871-483. 
2In many works on botany (1) and (2) are both compounded under the 
term assimilation. 
