178 THE LEAVES. 



ence ? Why in ordinary cases do they last only for a single year, 

 or a single summer ? The answer to this question is to be found 

 in the anatomical structure of the leaf, and the nature and amount 

 of the fluid which it receives and exhales. The water continually 

 absorbed by the roots dissolves, as it percolates the soil, a small 

 portion of earthy matter. In limestone districts especially, it takes 

 up a sensible quantity of carbonate and sulphate of lime, and be- 

 comes hard. It likewise dissolves a smaller proportion of silex, 

 magnesia, potash, &c. A part of this mineral matter is at once 

 deposited in the woody tissue of the stem (210) ; but a larger por- 

 tion is carried into the leaves (40, 92), where, as the water is ex- 

 haled or distilled perfectly pure, all this earthy substance 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 incrustation, in con- 

 nection with the deposition of organic solidified matter (39), gradu- 

 ally chokes the tissue of the leaf, obstructs the exhalation, and 

 finally unfits it for the performance of its offices. Hence the fresh 

 leaves most actively fulfil their functions in spring and early sum- 

 mer ; 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. But the 

 former are, in fact, annually renewed likewise ; and life abandons 

 the annual layers of wood and bark almost as soon as it does the 

 leaves they supply (216, 217, 228), and for similar reasons ; al- 

 though their situation is such that they become part of a perma- 

 nent structure, and serve to convey the sap even when no longer 

 endowed with vitality. 



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

 rect microscopical observation. In Fig. 185, 186, some superficial 

 parenchyma thus obstructed by long use is represented ; and sim- 

 ilar illustrations may be obtained from ordinary leaves. That this 

 deposit consists in great part of earthy matter is shown by care- 

 fully burning away the organic materials 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 materials which it has accu- 

 mulated. A vernal leaf leaves only the minutest quantity of ash- 

 es ; an autumnal leaf yields a very large proportion ; from ten to 



