46 BOTANICAL GAZETTE [JULY 
As a result the abscission layer has a lower percentage of reducing 
sugars throughout its entire history than the adjacent regions of the 
petiole. This difference is most marked in the fifth and sixth pairs 
of leaves near the close of the formation of the abscission layer, but 
it.is still quite evident at the time of abscission. In the cell walls 
of the abscission layer the change in form of the carbohydrates is 
still more pronounced and significant. During the process of 
abscission the first evident change in the cell walls is a conversion 
of cellulose of the secondary cell membranes to pectose. The second 
step is a conversion of some of this pectose to pectic acid and 
pectin. This is followed by the breaking down of the middle lamella 
of calcium pectate and the separation of the cells. The changes 
from cellulose to pectose can readily be followed by differential 
staining and crystallization methods, and by solubility tests. The 
evidence of the conversion of pectose to pectin and pectic acid is 
based upon solubility tests. Pectin is soluble in water, pectic acid 
is insoluble in water but soluble in dilute alkalies, while pectose is 
insoluble in both water and dilute alkalies. When an abscission 
layer at the time of abscission is treated with 3 per cent ammonium 
or potassium hydroxide or with 5 per cent sodium carbonate, the 
free pectic acid is dissolved. If the walls are then again examined 
a considerable portion of the secondary membrane, bordering the 
middle lamella which is still intact, is seen to have disappeared. 
A discussion of the changes in the calcium pectate is postponed 
until all the remaining facts have been stated. 
OxipAsEs.—In the stem and petioles oxidases are found in the 
epidermal and phloem tissues. In the abscission layer oxidases 
are found in all tissues except the xylem. Not only is this distri- 
bution peculiar to this region, but the increase in oxidases with 
the age of the abscission layer is also quite pronounced. Quanti- 
tative tests of the increase in oxidative activity are still to be 
made. 
Iron.—Slight traces of iron (Fe++*) are usually found through- 
out the plant, especially where chlorophyll is present. It is most 
abundant in the xylem tubes and in the epidermal region until a 
few hours before abscission, when it becomes extraordinarily abun- 
dant in the cells of the abscission layer. The path of diffusion of 
