COMPOSITION OF PLANTS 31 



largest constituents of the plant are carbon, about 45 per cent, or 

 more, and oxygen, about 45 per cent, or less. Since the carbon 

 is not necessarily present in the water it must be derived from 

 some other source. Carbon is present in the atmosphere in 

 small quantities, combined with oxygen in the form of carbon- 

 dioxide (CO 2 ), and in the absence of this gas the plant will not 



FIG. 6. The preceding experiment may be performed more satisfactorily by 

 substituting an iron capsule for the hard glass test-tube and connecting the 

 delivery tube with a Leibig condenser. Such an arrangement is represented in 

 Fig. 6. 



thrive. Consequently we must assume that the carbon is 

 absorbed from the atmosphere by the stem and leaves. This 

 conclusion may be verified by experiment. 



67. The oxygen taken up by the plant may be, and as a 

 matter of fact is, taken up in part as free oxygen from the 

 atmosphere, in part in combination with carbon as CC>2, and in 

 part in combination with other elements absorbed by the roots. 



