78 NATURE TEACHING 



characteristic colour to the leaves,* and in some cases 

 the stems of plants, and (2) sunlight. 



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 

 mentioned above. Whilst a plant is in the sunlight the 

 oxygen given out masks the breathing process, and it is 

 only when plants are in darkness, either artificial or that 

 ordinarily occurring at night, that the fact that a plant 

 does really breathe out carbon dioxide like an animal 

 can be detected. When later we try experiments on the 

 breathing of plants, it is essential to remember that the 

 plants must be kept in the dark. 



The Food of Plants. 



As the result of the building-up processes which go 

 on in the leaf, we find that starch is formed. In the 

 practkal work at the end of this chapter, experiments 

 are described which enable us to prove (i) that starch is 

 actually formed in leaves ; (2) that for this formation of 

 starch, by the living substance of the plant, leaf-green 

 and sunlight and the presence of carbon dioxide are 

 necessary conditions. 



* In some plants for instance, copper beech, coleus the colour 

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

 always there nevertheless. 



