﻿i 9 i6] LLOYD— ABSCISSION 215 



He further holds that there is present in all plants which shed 

 their leaves in laboratory air a preformed primary abscission zone 

 (" Trennungsschicht ") which is more or less sharply set off from 

 the neighboring tissues. It is pronounced in some species {Salvia, 

 Fuchsia. Impatiens, etc.), but less so in Begonia and Mirabilis. 

 This zone is to be seen also at the base of the internode. It is the 

 less marked the older and thicker the internode, but still can be 

 recognized, since the cells are smaller and display new transverse 

 cell walls. In Impatiens, however, these new walls are so infrequent 

 that the abscission zone is recognizable thereby with difficulty. 

 The manner of separation is the same in leaf, stem, and flower 

 (pedicel, peduncle). 



The larger nodes offering the best opportunities for observation, 

 Hannig further says that it is not the entire abscission zone which 

 is dissolved, but a layer ("Ldsungsschicht") of a few cells which 

 are not distinguishable beforehand in any way. This dissolved 

 layer is placed usually in the middle, but sometimes toward the base 

 of the abscission zone, and without reference to the direction of the 

 cell layers. Contrasting Mirabilis with forms like Impatiens (in 

 which the cells of the abscission zone are set free by the dissolution 

 of the middle lamella), it is pointed out that, while in these the 

 abscission surfaces exposed after rupture are granular or pulveru- 

 lent in appearance, due to loosened cells, those in Mirabilis are 

 mucilaginous. In this plant then the abscission zone is only a more 

 or less broad band of tissue, in the approximate middle of which 

 the separation layer (''Losungsschicht"), itself not in any way 

 differentiated as to its cells, occurs. This zone is conceived by 

 Haxxig only as a zone of tissue capable of choristic response, to 

 adopt Fitting's term. 



The present account of abscission in Mirabilis 



The preceding account, being a digest of Hannig's statement, 

 has been set out with some fulness and detail in order to obviate 

 the possibility of a criticism which may be in any way superficial 

 or gratuitous. Hannig's resulting contention that in Mirabilis 

 (and in the allied Oxybaphus, which I have not examined), a 

 type of abscission obtains which is "quite new and distinct," is 



