1T> TIIK PLANT. 



the plant in two ways through the roots in the 

 water which goes to form the sap, and at the leaves, 

 which :il>sorb it from the air in the form of gas. 

 The lent' of the plant seems to have three offices: 

 absorbing carbonic acid from the atmosphere as- 

 Hsting in the chemical preparation of the sap and 

 evaporating its water. If we examine leaves with a 

 microscope we shall find that some have as many as 

 170,000 openings, or mouths, in a square*inch ; others 

 have a much less number. Probably the pores on 

 the under side of the leaf generally absorb the car- 

 bonic acid. This absorptive power is illustrated 

 when we apply the lower side of a cabbage leaf to a 

 wound, as it draws strongly the other side of the 

 leaf has not an equal effect. Young green shoots 

 ami sprouts doubtless have the power of absorbing 

 and decomposing carbonic acid. 



The roots of plants, by their absorbent surfaces, or 

 through the Bpongiolee at the ends of their roots, ab- 

 sorb from the soil water, which contains carbonic 

 acid and other substances required for their nutrition. 

 How large a proportion of the carbonic acid is ab- 

 sorbed in this manner is not definitely known. It 

 probably depends on various circumstances, but is, 

 no doubt, always important. 



Carbonic acid, it will be recollected, consists of 

 carbon and oxygen, while it supplies only carbon to 

 the plant. It is therefore necessary that it be divided, 

 or decomposed, and that the carbon be retained while 

 the oxygen is sent off again into the atmosphere, to 

 perform again its office of uniting with carbon. This 



