528 



greatly in age. Accordingly the [)r(jce.ss of leaf fall is not always the same. 

 The most usual case consists in the formation of a tissue zone at the base of 

 the leaf which is a characteristic abscission layer. W'e repeat here the 

 illustration of the autumnal abscission layer in the leaf of .Icsciihts Hippu- 

 castanum (cf. Fig. io6). The illustration gi\es a section made longitudin- 

 ally through the joint at the base of the petiole, a is the bark parenchyma of 

 the branch; b, the layer of plate-corR cells which remains when the petiole 

 has fallen and thus forms a protection for the bark tissue; c indicates the 

 cells at the base of the petiole which at e pass over into the firmer paren- 

 chyma of the broadened bases of the i)etioles, provided with abundant accu- 

 mulations of calcium oxalate, netween c and e takes place the process of 

 separation, since at </ the cells round off and l)egin to separate from one 

 another. If now the leverage of the leaf, mo\ed b}- the wind, makes itself 

 felt, the i)etioles break off at the loosened cell layer. 



Fig. lOtj. Autumnal abscission layer of a horse chestnut leal'. 

 (After Dobncr-Nobhe.) 



The riper the leaf is at the time of the final autumnal frost, the more 

 easily it falls; on this account, the old leaves of the branch arc found to be 

 the first ones broken off by the wind in the autumn. The greater the life 

 energy and the quantity of plastic inaterial, the more resistent the youthful 

 leaf seems to frosts which are not killing frosts. 



If killing degrees of frost occur in the autumn at a time when the leaf 

 has not yet sufficiently matured its abscission hner, i. e., the tree is still far 

 distant from its dormant period, then the dead foliage remains on the 

 branches o\ er winter (beech and oak). The beeches in which the foliage 

 remains hanging often leaf out later in the spring than do normally matured 

 specimens'. 



At the time of the first night frost, it is found in the early morning, if 



the frost still lies on the ground and even in windless weather, that, as soon 



as the sun comes up, the simple leaves of the trees break off and the leaflets 



of composite leaves fall from the common spindle, v. Mohl- found in such 



1 dc Candollc, A., in Contralbl. t. Agrikulturchemie 1879, I, p. 159. 

 - Bot. Zeitung- 1860, p. 16. 



