1918] SAMPSON—ABSCISSION 49 
and pectic enzymes also play a réle in these changes is uncertain. 
Hydrolytic action may underlie some or all of these changes, but 
this must remain an open question until the molecular composition 
of these compounds is definitely known. At any rate, the trans- 
formation of the cellulose and pectose leads to the formation of an 
excess of pectic acid in the cell walls of the abscission layer. 
The most important question still open is the cause of the final 
breaking down of the calcium pectate of the middle lamella. There 
appear to be but two possibilities. Either the calcium ion of the 
pectate is captured by some anion, liberated in the cells of the 
abscission layer, and held in solution or precipitated, thus freeing 
the pectic acid, or the breaking down of the cellulose and pectose 
may lead to such an excess of pectic acid that the available calcium 
is no longer able to hold a sufficient proportion of the pectic acid 
as a salt, and thus maintain the solidity of the middle portion of 
the cell wall. 
The fact that calcium is not found in solution in the cells of the 
abscission layer, nor in crystalline forms either in the cells or in 
the cell walls, is decidedly against the first view, which is simply 
WIESNER’s organic acid theory stated in slightly diferent terms 
and which has already been discussed in detail. 
_ The second view is more easily understood when we recall the 
well known law of physical-chemical equilibrium. As soon as an 
excess of pectic acid is present in contact with the calcium pectate 
of the middle lamella there is undoubtedly a diffusion of calcium 
ions from the middle lamella and a diffusion of pectic acid into the 
middle lamella until an equilibrium of distribution of the two ions 
is established. A critical proportion of pectic acid to calcium would 
be reached in the middle lamella when the excess pectic acid breaks 
the continuity of the calcium pectate layer. This second view has 
the further advantage of being in accordance with all the experi- 
mental facts so far known, particularly the formation of excess 
pectic acid in the cell walls and the paucity of calcium in solution 
in the cells of the abscission layer. 
The fact already stated, in the discussion of calcium, that there 
is an abundance of calcium in solution in cells within 5 cell layers 
of the line of cleavage in abscission, indicates either that the process 
