360 FALL OF THE LEAF. 



portion of the leaves and leaflets, which rapidly increase by division, and in a short 

 time form a zone, readily to be distinguished from the thick older tissue by its 

 lighter tint and by the fact that it is somewhat transparent. Usually this zone 

 is formed in the petiole, and at those places where the vascular bundles become 

 narrowed in passing from the twig to the leaf-blade, there to divide up into the ribs 

 and veins. The growing tissue is inserted just at this place; it actually presses and 

 tears the other older cells apart, and even causes a rupture between them. As soon 

 as the separation -layer has attained its proper thickness, its thin- walled cells 

 separate from one another, but so as not to injure or burst their membranes in 

 any way. It seems that the so-called middle lamella of the cell-wall is dissolved by 

 organic acids, and that thus the continuity between the cells of the separation-laj^er 

 is destroyed. The most trifling cause will now effect a splitting in the loose tissue 

 and a fracture between the cells of the separation-layer; and when no other external 

 shock follows, the detachment ultimately takes place of itself, the weight of the 

 leaf helping to bring about a complete severance. As a rule, however, the fall of 

 the leaf is hastened by external influences. Every gust of wind brings down the 

 leaves; the alterations in volume dependent on the frost and chill and the subse- 

 quent thawing of the cell-sap, aid the severance and also hasten the tearing of 

 vascular bundles which are still entire; and thus it happens that thousands of 

 leaves fall to the ground even in the absence of wind, especially when, after a 

 frosty night, the rising sun illuminates the autumn-tinted leaves, and dissolves the 

 frozen sap. 



The region where the separation is effected is usually sharply marked off", and it 

 looks as if the leaves and leaflets had been cut through with a knife. The severed 

 surfaces present a variety of contours, according to the shape of the leaf-stalk. 

 Sometimes it is horseshoe-shaped, sometimes triangular or rounded, or it reminds 

 one of a trefoil-leaf, and sometimes it has an annular shape. The stalk of the 

 plane-tree lei„f has at the base a conical swelling which incloses a bud; when the 

 leaf falls a fissure is formed entirely going round it. Many of the separation 

 surfaces of the leaf-stalks are like the articular surfaces of the long bones in 

 the human skeleton (of the radius, tibia, and at the elbow). Vine leaves form 

 two layers of separation, one close to the stem at the base of the leaf-stalk — 

 the other at the upper end of the leaf-stalk immediately below the blade. In the 

 palmate leaves of the Horse-chestnut and Virginian Creeper (ATiipelopsis), in the 

 compound leaves of Spiraea Aruncus, in the pinnate leaves of the Chinese Tree of 

 Heaven {Ailanthus glandulosa), and in the bipinnate leaf of the North American 

 GyTYinocladus Canadensis, a small separation layer arises below each leaflet, and a 

 larger one, in addition, at the base of the leaf-stalk. Such leaves, consisting of 

 several leaflets, collapse like houses built of cards when touched, and under the trees 

 late in the autumn lies a confused heap of leaflets and leaf-stalks, the latter some- 

 times looking Kke long rods (as, for example, in the Ailanthus and Gymnocladus), 

 sometimes almost like long bones (as in the Horse-chestnut, fig. 93). 



Frequently the layer of separation is so situated on the leaf -stalk that after the 



