THE LEAF 133 



decay ; generally speaking, indeed, the occurrence of com- 

 pounds of lime indicates increase of age. It is only when 

 the cells become old that the cell-walls become strengthened 

 by salts of lime, and become less permeable ; and it is then 

 also that crystals of oxalate of lime are deposited. 



Thus we find yellow autumn leaves rich in lime but poor 

 in plastic substances. They have served their purpose. The 

 yellow colour is due to the dissolution of the chlorophyll, the 

 xanthophyll still remaining in the tissues. In the falling 

 leaves the protoplasmic framework of the chlorophyll corpuscle 

 is also absorbed, and the nitrogenous compounds make their 

 way back into the stem. After the fall of the leaves, decom- 

 position alone takes place within their tissues, and among the 

 products of decomposition we have, in the first place, carbonic 

 acid, i.e., the substance which it has been the function of the 

 leaf to absorb and to transform into organic matter. 



