1918] SAM PSON—ABSCISSION 35 
conditions of forced leaf-fall all processes are greatly accelerated. 
The data in table I show that all processes, including the process of 
abscission, may be completed in the third pair of leaves in a period 
of 2 or 3 days as a result of the amputation of the blades. On the 
other hand, amputation of the blades of the first and second pairs 
of leaves before the beginning of the formation of the abscission 
_ layer inhibits its formation entirely. 
Method of abscission 
UNDER ORDINARY GROWING CONDITIONS.—The method of 
abscission has received much attention, but a critical survey of the 
exact changes in the cellulose and pectic compounds is wanting. 
An attempt to follow these changes in Coleus led to the discovery 
of certain facts which have a direct bearing upon the existing 
theories of the cause of abscission. 
Microchemical analyses show that there is a breaking down 
not only of the calcium pectate of the middle lamella of the cells 
of the separation layer, but also of the cellulose of the secondary 
membrane, leaving only a thin layer of cellulose surrounding the 
lumen of the cells. This cellulose is first changed to pectose, which, 
according to Cross (1), ToLLENS (16), and EuLER (3), contains 
more oxygen than cellulose and probably is an oxidized form. The 
pectose is then further changed to pectin and pectic acid; the 
excess pectic acid becomes gelatinous and is no longer able to hold 
the cells together, and the leaf falls. The changes taking place in 
the walls of the xylem tubes are still to be investigated. During 
this process there is not a disappearance of calcium from the cell 
walls, but the excess of pectic acid produced renders the amount of 
calcium present’ insufficient to maintain the solidity of this portion 
of the cell wall. The excess pectic acid appears to come from the 
transformation of the pectose rather than from the breaking down 
of the calcium pectate of the middle lamella. Since this description 
differs from all previous accounts of the method of abscission, 
further discussion is postponed until all the facts are brought 
together under the topic of microchemical analysis. 
FORCED LEAF-FALL.—Leaf-fall was accelerated by treatment 
with ethylene, amputation of the blade, and by allowing the soil 
