80 NATURE TEACHING. 



life would become impossible. Plants are the 

 means whereby this accumulation is hindered. 

 When carbon dioxide comes in contact with 

 the living substance of the plant, under cer- 

 tain conditions, it is split up into its consti- 

 tuent parts, carbon and oxygen. The carbon 

 is kept by the plant and built up into its 

 tissues and the oxygen set free. The condi- 

 tions referred to above are the presence of 

 (1) the green colouring matter (leaf -green or 

 chlorophyll] which gives the characteristic col- 

 our to the leaves,^ and in some cases the 

 stems of plants, and (2) sunlight. 



2. The process which goes on in the leaf 

 whereby the carbon dioxide is broken up in this 

 way and the carbon used by the plant is known 

 as assimilation. Assimilation must be very 

 carefully distinguished from the respiration or 

 breathing of plants, in which, exactly as in 

 that of all animals, ' oxygen is taken in and 

 carbon dioxide given out. A plant is always 

 breathing but can only carry on the process of 

 assimilation under the special conditions men- 

 tioned above. Whilst a plant is in the sun- 

 light the oxygen given out masks the breathing 



* In some plants, for instance crotons and coleus, the colour 

 of the leaf -green is hidden by other colours. But the leaf -green 

 is always there nevertheless. 



