The Death and Fall of Leaves. g 2o 



carried into the leaves, where, as the water is exhaled or dis- 

 tilled perfectly pure, all this earthy matter must be left behind 

 to incrust the delicate cells of the parenchyma, much as the 

 vessels in which water is boiled for culinary purposes are in 

 time incrusted with an earthy deposit. This earthy accumu- 

 lation gradually chokes the tissue of the leaf, obstructs the ex- 

 halation, and finally unfits it for the performance of its offices. 

 Hence the fresh leaves most actively fulfil their functions in 

 spring and early summer; but languish towards autumn, and 

 ere long inevitably perish. Hence, although the roots and 

 branches may be permanent, the necessity that the leaves 

 should be annually renewed. 



The general correctness of this view may be tested by di- 

 rect microscopical observation. That this deposit consists in 

 great part of earthy matter is shown by carefully burning 

 away the organic matter of an autumnal leaf over a lamp, and 

 examining the ashes by the microscope ; which will be found 

 very perfectly to exhibit the form of the cells. The ashes 

 which remain when a leaf or other vegetable substance is 

 burned in the open air, represent the earthy material which it 

 has accumulated. A vernal leaf leaves but the minutest quan- 

 tity of ashes; an autumnal leaf yields a very large proportion, 

 from ten to thirty times as much as the wood of the same spe- 

 cies ; although the leaves contain the deposit of a single season 

 only', while the heart-wood is loaded with the accumulations 

 of successive years. 



