1918] : SAMPSON—ABSCISSION 39 
sible after collecting, macerated in a mortar, made up to volume 
with distilled water, and after shaking for 30 minutes the solutions 
were filtered through a Buchner funnel and definite portions taken 
for titration. Fresh abscission layers treated in this way had an 
acidity of o.oroocc. per gram of wet weight, while that of the 
adjacent part of the petiole was o.0c0g5 cc. These two figures are 
not to be compared with those preceding, as the two sets of titra- 
tions were made at different times and on different plants. 
If the calcium pectate were being hydrolyzed by an organic acid, 
one would expect to find either an increase of calcium in solution 
in the cells of the abscission layer or an increase of crystals of 
calcium compounds in these cells. Such is not the case. Neither 
are there any calcium oxalate crystals in the middle lamella of these 
cells, such as one finds when the middle lamella is broken down by 
adding oxalic acid to sections under the microscope. 
Finally, the marked acidity of the abscission surface of a falling 
leaf was certainly not correctly interpreted by WiESNER. He 
ascribes the acidity of the abscission surface to the excretion of 
organic acids from the interior of the cells. This abscission surface 
is a continuous layer of pectic acid, formed during the abscission 
process, and the acidity of the abscission surface, therefore, is a 
result of the formation of pectic acid during abscission, and not of 
the escape of acids previously formed in the cells. This acidity 
of the middle lamella to neutral red may be seen in Coleus in any 
part of the plant, and it is increased in the walls of the abscission 
layer only after the formation of pectic acid during abscission. 
In conclusion, therefore, neither the turgor pressure theory nor 
the organic acid theory proposed by WIESNER to account for the 
cause of leaf-fall is in accordance with the facts observed in Coleus. 
Effect of salts on leaf-fall 
According to CZAPEK (2), the membranes of plant cells are 
colloidal in nature, and MANGIN has shown that the middle lamella 
is composed of pectic acid in combination with calcium. During 
the process of abscission the middle lamella undergoes a chemical 
alteration and the pectic acid present takes up water and swells. 
It was expected, therefore, that salts might show either a lyotropic 
