136 ESSENTIALS OF BOTANY 



161. Details of the Work of the Leaf .^ — A leaf has four 

 important functions to perform : 



(1) Fixation of carbon, or (3) Excretion of water. 



photosynthesis. 



(2) Assiinilation.2 (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 Plant.t, Vol. I, pp. 371-483. 



2 In many works on botany (1) and (2) are both compounded under the 

 term assimilation. 



