ABSCISSION AND LEAF-FALL 217 



they are quite dead. This applies to the leaves of many deciduous and 

 evergreen trees, and petals, stamens, and unfertilized flowers often separate 

 from the parent plant while still turgid l . 



The plant itself prepares for the abscission of such organs, and does 

 so in various ways. A young leaf or an unripe apple can support a con- 

 siderable load, but can withstand less and less stress as they grow older. 

 Owing to the loosening of the tissues, the internal strains gradually 

 disappear, and in the capsule of Impatiensti\\.s occurs suddenly at the moment 

 of dehiscence. This does not happen when an unripe fruit is killed ; nor 

 do the young leaves fall when a branch is cut and rapidly dried. Leaves 

 which die before they are ready to fall are, however, gradually removed by 

 the wind and other mechanical agencies. Dead branches may be removed 

 by the same means, but in some cases the tree is kept clean and its foliage 

 open by an active abscission of twigs or even buds 2 . 



The dry stems and withered leaves of many herbaceous plants show 

 that active abscission does not occur in all cases. Similarly the older parts 

 of rhizomes simply die and decay away. Hollow trees are produced by the 

 decay of the duramen, which under normal conditions persists owing to its 

 supporting function. 



The partial or total separation of contiguous cells is produced by the 

 softening or swelling of the middle lamella, and the abscission of leaves, 

 fruits, and conidia is produced by similar means, although special changes 

 are in many cases involved in the process. Thus Mohl showed that in 

 leaves and many fruits, separation is provided for by a resumption of 

 growth in certain resting cells which produce the abscission layer. Even 

 after this had separated Mohl 3 found that the vascular bundles in the 

 petiole of Catalpa were still unbroken, whereas Wiesner 4 has found that 

 the energy of growth is sufficient to cause the vascular bundles to snap 

 at the line of abscission. This is not always the case, for the dead leaves 

 of beeches and oaks wither on the tree and are gradually torn off by wind 

 and rain. Frost is also of importance, and the first cold night of autumn 

 may cause all the remaining leaves of Robinia, Aesculus, and Fraximis 

 to fall simultaneously. 



Abscission is not primarily due to the formation of a layer of cork, 

 which in fact usually occurs after the organ has fallen 5 . In some cases, 



1 Mohl, Bot. Ztg., 1860, pp. i, 273; Hofmeister, Allg. Morph., 1868, p. 551; Wiesner, 

 Sitzungsb. d. Wien. Akad., 1871, I. Abth., Bd. LXIV, p. 465 ; Biologic, 1889, p. 67; R. v. Hohnel, 

 Ueber d. Ablosungsvorgange d. Zweige einiger Holzgewachse, 1878; Bretfeld, Jahrb. f. wiss. Bot., 

 1880, Bd. xii, p. 133 ; Reiche, ibid., 1885, Bd. xvi, p. 684 (flowers) ; Molisch, Sitzungsb. d. Wien. 

 Akad., 1886, Bd. xcvi, I, p. 148 ; Dahmen, Jahrb. f. wiss. Bot., 1892, Bd. xxm, p. 476 (seeds) ; 

 M. Biisgen, Bau und Leben unserer Waldbaume, 1897, pp. 19, 148; Fouilloy, Rev. gen. d. Bot., 

 1899, T. XI, p. 304 (leaves). On the fall of members in algae cf. Schimper, Pflanzengeographie, 

 1898, p. 835. 



8 Biisgen, 1. c., p. 19. 3 Mohl, 1. c., p. 5. * Wiesner, 1. c., p. 505. 



5 Cf. Mohl, Bretfeld, 1. c. ; Staby, Flora, 1886, p. 113. 



