16 niE PLAJST. 



the plant in two ways — through the roots in the 

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

 which absorb it from, the air in the form of gas. 

 The leaf of the plant seems to have three (»fficcs : 

 absorbing carbonic acid from the atmosphere — as- 

 sisting in the chemical preparation of the sap — and 

 evaporating its water. If we examine leaves with a 

 microscope we shall find that some liave as many as 

 170,000 openings, or mouths, in a square inch ; othera 

 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 

 w'hen 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 eft'ect. Young green shoots 

 and sprouts doubtless have the power of absorbing 

 and decomposing carbonic acid. 



The roots of plants, by their absorbent surfaces, or 

 through the spongioles at tlie 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 

 carho/i 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 oflice of uniting with carbon. This 



