THEIR DEATH AND FALL. 177 



ing a clean scar (Fig. 127, 130) at the insertion. The solution of 

 continuity begins in the epidermis, where a faint line marks the 

 position of the future joint while the leaf is still young and vigor- 

 ous : later the line of demarcation becomes well marked, internally 

 as well as externally ; the disintegrating process advances from 

 without inwards until it reaches the woody bundles ; and the side 

 next the stem, which is to form the surface of the scar, has a 

 layer of cells condensed into what appears like a prolongation 

 of the epidermis, so that, when the leaf separates, " the tree 

 does not suffer from the effects of an open wound." " The pro- 

 vision for the separation being once complete, it requires little to 

 effect it; a desiccation of one side of the leafstalk, by causing 

 an effort of torsion, will readily break through the small remains of 

 the fibre-vascular bundles ; or the increased size of the coming 

 leaf-bud wilt snap them ; or, if these causes are not in operation, 

 a gust of wind, a heavy shower, or even the simple weight of the 

 lamina, will be enough to disrupt the small connections and send the 

 suicidal member to its grave. Such is the history of the fall of the 

 leaf. We have found that it is not an accidental occurrence, aris- 

 ing simply from the vicissitudes of temperature and the like, but a 

 regular and vital process, which commences with the first formation 

 of the organ, and is completed only when that is no longer useful ; 

 and we cannot help admiring the wonderful provision that heals 

 the wound even before it is absolutely made l and affords a covering 

 from atmospheric changes before the part can be subjected to 

 them." * Leaves fall by an articulation in most Exogenous plants, 

 where the insertion usually occupies only a moderate part of the 

 circumference of the stem, and especially in those with woody 

 stems which continue to increase in diameter. When they are 

 not cast off in autumn, therefore, the disruption inevitably takes 

 place the next spring, or whenever the circumference further en- 

 larges. But in most Endogenous plants, where the leaves are 

 scarcely, if at all, articulated with the stem, which increases little 

 in diameter subsequently 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- 



* Dr. Inman, in Henfrey's Botanical Gazette, 1. p. 61. 



