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BOTANICAL GAZETTE 



The purpose of this paper is to support a challenge of the cor- 

 rectness of Hannig's views and of the evidence brought to support 

 them, and to substitute evidence to show that abscission in Mirabilis 

 does not represent a new type. To do this will entail a detailed 

 account of the process as understood by me, and to compare this 

 with that of Hannig as exhaustively as possible. 



Digest of Hannig's view 



Hannig recognizes in general two methods of abscission of 

 flowers (that is, of the supporting axis), namely, (a) by the 

 solution of the middle lamella of the cells of the abscission zone; 

 and (b) by the complete dissolution of an entire layer of cells. 

 The latter constitutes the new type in question, and is, accord- 

 ing to Hannig, "of a very peculiar sort." His account runs as 

 follows: 



Two or three cell layers of the abscission zone, which is 12-20 

 cells thick, are destroyed and go completely over into solution. 

 This destruction proceeds first by the thinning of the membranes 

 of the affected cells. These membranes become more strongly 

 refringent, while the cell contents take on a granular character. 

 The process is first recognized by the absence of intercellular spaces, 

 followed by granular degeneration of the cell, ending finally in the 

 liquidation of the whole layer of tissue. The process begins at 

 a particular point under the epidermis, which is soon broken. 

 The tear then extends around the whole cortex and finally inwardly 

 into the pith. The vascular bundles appear to be broken across 

 mechanically. The persistence of starch in the abscission cells, 

 while the surrounding tissues, except the endodermis, lose it, is 

 asserted, but even those cells which are reduced to extremely thin 

 membranes, simply because of their too rapid destruction, also 

 retain it. Starch in the abscission cells is therefore of no par- 

 ticular significance. Hannig was unable to find any evidence, 

 by means of suitable reagents, of chemical alteration of the cell 

 walls to distinguish them from those of neighboring cells. From 

 these conclusions the further one is drawn that the entire cells of 

 the abscission layer, without recognizable previous alteration of 

 the cell membranes, go into solution (p. 430). 



