CHAPTER VI. 



LEAVES AND BUBS. 



162. Summary of Chief Functions of the Green 

 Leaf. These functions may be thus briefly summarised : 



(1) Leaves are the respiratory organs of the plant, since 

 they allow a free exchange of gases with the atmosphere by 

 means of their stomates. 



(2) The chlorophyll of the leaf, which, as we have proved, 

 is itself formed only under the influence of light, has the 

 power, under this same influence, of decomposing the carbon 

 dioxide of the atmosphere, and of building up with the 

 carbon so obtained, together with the dissolved mineral 

 substances absorbed by the roots from the soil, the sugar, 

 starch, and other organic compounds of the plant. Some of 

 the material thus formed in the leaves is transmitted to the 

 stem and root in a soluble state, and often stored in these 

 organs in large quantities. 



(3) Water absorbed by the roots passes up the stem to 

 the leaves, where a large proportion of it is lost by evaporation 

 through the stomates. Water lost by evaporation in this 

 way is being continually replenished by the roots, and thus 

 there is a steady upward current by means of which new 

 mineral matter is constantly supplied. The stomates are 

 reduced in size by the collapse of the guard-cells when 

 transpiration is too active, and when the water supply in 

 the soil is deficient. They also close in darkness, when no 

 carbon assimilation can take place ; and by so doing they also 

 prevent the plant from becoming too cold, for evaporation is 

 always accompanied by a reduction in temperature. 



The leaf primarily acts as an organ for absorbing radiant 

 energy, and storing it up as potential energy in the form of 

 food. It also promotes gaseous exchange in general, and 

 usually forms the main transpiring surface of the plant. 



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