1918] SAMPSON—ABSCISSION 47 
the iron leading to its accumulation in the abscission layer has not 
been traced. No manganese was found. 
CALCIUM AND OXALATES.—The data in table IV show no marked 
difference in the distribution of oxalates in the leaves. Only occa- 
sional crystals of calcium oxalate are found in the cells, and none 
are found in the cell walls of the abscission layer at any time. 
Likewise the total calcium has a fairly constant distribution 
throughout the plant, but slight variations of the calcium in solu- 
tion are to be noted. The most striking and significant changes 
of the amount of calcium in solution, however, are shown by 
microchemical tests. Treatment of sections with 50 per cent 
- sulphuric acid or with 3 per cent oxalic acid or ammonium oxalate 
show an abundance of calcium in solution in all living cells of the 
petiole except those of the abscission layer at the time of abscission. 
The crystals of calcium sulphate or of calcium oxalate obtained by 
these treatments were very numerous in the cells of the abscission 
layer before the time of abscission, the latter averaging 30 crystals 
per cell, while during abscission only an occasional crystal was 
obtained. This decrease of calcium in solution is not always 
confined to the abscission layer, but breaks off rather abruptly 
in the first few layers of cells of the adjacent region of the petiole. 
In some cases cells not more than 5 cell layers distant from the line 
of cleavage showed no decrease in the number of crystals. These 
facts show that the calcium in solution in the abscission layer dis- 
appears during abscission, and it should be further stated that the 
disappearance takes place in the- first stages of the process. 
Summary of microchemical analysis 
1. A pronounced increase in nitrates always occurs in the 
abscission layer at the time of abscission. This increase may be 
gradual, extending over the entire life-history of the abscission layer, 
or it may appear somewhat suddenly a short time before abscission. 
2. A gradual increase in the amount of reducing sugars accom- 
panies the aging of leaves and stem. This increase is initiated 
last and is least pronounced in the abscission layer. 
3. During the process of abscission the cellulose of the secondary 
membrane of the cell walls of the abscission layer is converted into 
