THEIR DKATII AND FALL. 173 



must therefore distinguish between the death and the fall of the 

 leaf 



310. The Fall of the Leaf is owing to an organic separation, through 

 an articulation, or joint, which fonns between the base of tlie 

 petiole and the surface of the stem on which it rests. The forma- 

 tion of the articulation is a vital process, a kind of disintegration of 

 a transverse layer of cells, which cuts off the petiole by a regular 

 line, in a perfectly uniform manner in each species, leaving a clean 

 scar at tlie insertion (Fig. 153, 155). 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 vigorous : later, the 

 line of demarcation becomes well marked, internally as well as ex- 

 ternally ; the disintegrating process advances from without inwards, 

 until it reaches the Avoody bundles ; and the side next the stem, 

 which is to form the surface of the scar, has a layer of cells con- 

 densed into Avhat appears like a prolongation of the epidermis, so 

 that, Avhen the leaf separates, "the tree does not suffer from the 

 effects of an open wound." " The provision 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 fibro-vascular bundles ; or the in- 

 creased size of tlie coming leaf-bud will 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 ac- 

 cidental occurrence, arising simply from the vicissitudes of tempera- 

 ture and the hke, 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 wonder- 

 ful provision that heals the wound even before it is absolutely made, 

 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 oidy 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 circumfer- 



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

 15 * 



