348 



plafanoides, Primus Padus, Robinia Pscndacacia. The cork layer should, 

 therefore, be considered only as a protective layer for the bark tissue ex- 

 posed by the falling of the leaf, often developed before the leaf has fallen. 



The real layer of separation, in fact, is formed above the cork layer 

 in the almost isodiametric parenchyma of the round-celled layer, not in the 

 brown- walled portion bordering directly on the cork, but in the adjacent 

 healthy portion, of which the walls are light colored. There, shortly before 

 the leaves fall, a zone is found running obliquely in front of the bud toward 

 the outer side of the petiole and composed of young, delicate walled cells with 

 intercellular spaces containing less air. Small starch grains are found in 

 these cells which otherwise do not occur in the enlarged end of the petiole. 

 In this newly formed tissue-zone, the cells separate from one another with- 

 out tearing, but by rounding ofif. as Tnmann^ has observed. One part re- 

 mains attached to the petiole as it breaks ofif, the other to the leaf scar 

 where it soon dries up. The leaf-fall, accordingly, is a z'ital act, not a me- 

 chanical one. Before the leaf falls, vascular bundles take no part in the 

 changes undergone by the cell tissue of the swollen end of the petiole. These 

 extend through the round celled layer and the cork layer without changing 

 their organization, even without turning brown. The cleavage in these 

 *"'kes place in a purely mechanical way after the split has extended through 

 the parenchymatous tissue. 



In many plants (Nuphar, many monocotyledons, herbaceous ferns') in 

 which there is no cork formation on the leaf scar, its outer dried cell layers 

 i>ass over directly into the healthy bark parenchyma and are thrown off 

 during later development. 



V. Bretfeld'"* arrives at the conclusion tliat the process of abscission of the 

 leaves is the same in monocotyledons and dicotyledons, only the shutting ofif 

 of the abscission surface dififers in dififerent genera. An essential difiference 

 lies, however, in the time of the formation of the tissue zone in which the 

 separating layer is produced. \\^hile in dicotyledons, the process of ab- 

 scission is the product of living activity, taking place shortly before the 

 leaves fall, this process in the tree-like monocotyledons, orchids and 

 Aroideae is exhibited as an act [^rcpared by the primordia of a definite 

 layer and advancing with the general tissue dififerentiation. 



The loss of leaves occurring in conservatory plants of the her- 

 baceous and bushy Begonias, of Cistus species and many Mytaceae and 

 Leguminoseae from New Holland must be mentioned in discussing leaf 

 fall due to an excess of water. The upward force of the sap is increased 

 excessively by an abundant watering of the plants at the time of minimalleaf 

 activity. The cleavage surfaces of the falling leaves at times are very 

 mealy, due to the loosened cells of the abscission surface. 



1 Bot. Zeit. 1850, p. 198. 



2 V. Mohl, tJber den Vernarbungsproze.ss bei der Pflanze. Bot. Zeit. 1849, p. 64.5. 

 p. 645. 



3 V. Bretfeld, tJber den Ablosung-sprozess saftig-er Pflanzenorganc Bot. Zeit. 

 1860, p. 273. 



