72 The Ottawa Naturalist. [Aug. -Sept. 



being supplanted by the other even in the same organ. Thtis, 

 in Gossypium (the flower-peduncle), normally one to three 

 divisions occur, previous to separation, but they ma}" be quite 

 absent in the peripheral tissues; and when rapid abscission 

 follows a suitable stimulus, none at all are to be observed. 



The cells of the abscission layer may separate from their 

 secondary walls at their distal extremities, and, in addition, the 

 outer daughter cells may separate also from each other, one 

 clinging to the distal rejected wall, but from which it is usually 

 free, and one (or more) remaining permanently incorporated 

 with the proximal wall. The separation in younger or thin- 

 walled tissues may follow the plane of the middle lamella, but 

 more frequently it occurs between a verv thin innermost 

 membrane {i.e., that which immediately surrounds the lumen) 

 and the earlier formed secondary membranes; or this membrane 

 may have been laid down anew, during growth, against, but not 

 incorporated with, the earlier formed secondary membranes. 

 The evidence favotirs the former alternative; but this I cannot 

 at present decide. 



The crux of the problem of abscission lies in the manner of 

 separation. It is conceivable that the mere increase of turgor is 

 sufficient to cause separation along the place of the middle 

 lamella. The smaller the cells, the more efficient can the energy 

 at the disposal of the cell be applied; and small cells are found 

 to characterize the abscission layer in many cases. Fitting 

 favours this view in the light of the rapidity of reaction which, 

 he believes, allows too short a time for swelling of the membranes, 

 and because he could not see anv such change in them. Renewed 

 growth in these and increased turgor he regards as the responsible 

 factors. (Fitting 21, p. 244-5). Strasburger (41), Tison (except 

 in cases of rhexolysis) Lee (42) and others have taken a quite 

 different position, seeing in the chemical alteration of the 

 membranes a factor of primary importance. Tison calls the 

 product of this change a pecto-cellulosic mucilage. The 

 membrane lining the cell lumen does not take part in this. 

 Growth contributes a relatively small part to the process. 

 Hannig, too, takes the same position, except in the case of the 

 peduncle of Mirabilis, in which occtirs what he considers to be 

 an undescribed method — a lysigenous one, involving the total 

 destruction of the membranes. The appearances seen by him 

 could, I believe, be reconciled with a method already known. 

 This will, however, be discussed elsewhere. 



Between these extreme views lie those of Wiesner, Kubart 

 and Loewi, who, without denying a role to chemical alteration 

 (by the excretion of organic acids, or perhaps enzymes), see in 



