174 



TIIK LEAVES. 



ence further enlarges. But in most Endogenous plants, where the 

 leaves are scarcely, if at all, articulated with the stem, which in- 

 creases little in diameter subsequent to its early growth, they are 

 not thrown off, but simply wither and decay ; their dead bases or 

 petioles being often persistent for a long time. 



311. The Death of the Leaf, however, in these and other cases, is 

 still to be explained. Why have leaves such a temporary exist- 

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

 or a single summer ? An 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 (44, 93) is at 

 once deposited in the woody tissue of the stem ; but a larger por- 

 tion is carried into the leaves, where, as the water is exhaled 

 pure, all this earthy substance, not being volatile, must be left be- 

 hind 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, must grad- 

 ually choke the tissue of the leaf, and finally unfit it for the per- 

 formance of its offices. Hence the fresh leaves most actively fulfil 

 their functions in spring and early summer ; but languish towards 

 autumn, and erelong 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 (224, 231), 

 and for similar reasons ; although their situation is such that they 

 become part of a permanent 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 direct 

 microscopical observation. Li Fig. 223, 224, some superficial paren- 

 chyma thus obstructed by long use is represented; and similar 

 illustrations may be obtained from ordinary lea^■es. 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 



