﻿LLO YD— A B SCISSION 



probably by hydrolysis (text fig. i). This remaining membrane 12 

 it is which continues, during elongation of the cell, to invest the 

 protoplasm, and which, because of the disappearance of the 

 remainder of the wall, comes into intimate contact with those of 

 neighboring cells (text fig. i, c). Being very soft and delicate 

 membranes, on coming into contact with each other they cling 

 together and appear as a single membrane, which is optically 

 scarcely resolvable as double, so that its composition must be ar- 

 gued from the occasional separation of its components (pi. fig. n), 

 and from their conjoint greater thickness. These changes are 

 for obvious reasons, more prolonged where the walls, or por- 

 tions of the walls, are most thickened, as in the collenchyma and 

 prosenchyma sheath, and are therefore more readily seen in such 

 tissues (pi. figs. 5, 6, 12, 13). 



Not only are these thin membranes proper to neighboring cells 

 separate from each other, but they become separated also at their 

 upper ends (in some cases at their lower ends) from the thick 

 membrane of the cells in the next tier above, only the lower ends 

 of which are chemically altered, but enough for this. The iodine 

 reaction demonstrates this to be the case. That this separation 

 of walls actually obtains can be proved by taking rather thick 

 sections of suitable material and, after a slight treatment with 

 5 per cent sodium hydrate, or weak hydrochloric acid. 1 - 5 pulling 

 the portions separated by the abscission layer apart on the slide, 

 when the separation cells pull away in many cases without breaking. 

 Investing their free ends one usually finds a thicker portion of 

 wall (pi. fig. 8) evidently derived from the chemically altered but 

 unstretched membranes with which the ends of the abscission 

 cells were in contact (text. fig. 1, d, g). It is more difficult to 



