1918] SAMPSON—ABSCISSION 41 
The failure of calcium to show a specific effect was unexpected. 
Later work, however, has thrown some light upon the matter, and 
it will be discussed under microchemical analysis. 
Oxygen pressure and leaf-fall 
Mo tiscu (14) grew plants half submersed in water and found 
that the aerial portions dropped their leaves sooner than the 
submersed portions. From this fact he concluded that low oxygen 
pressure retarded leaf-fall. This surmise proved to be correct. 
Coleus plants grown in a hydrogen atmosphere under bell jars with 
only sufficient oxygen to maintain a slow growth retain their leaves 
much longer than plants in normal air.. Under conditions of the 
experiment the plants in normal air usually retain 8 pairs of leaves. 
In the hydrogen atmosphere the plant retained 11 pairs of leaves. 
Inception of decay at the base of the stem destroyed the experiment 
at this point. Similarly, petiole fall, after amputation of the blade, 
is greatly retarded in very low concentrations of oxygen. Plants 
grown in o.1 normal oxygen pressure showed no retardation of 
leaf-fall. Whether the effect of oxygen in such cases is that of an 
essential factor influencing the general metabolism of the plant, or 
of a formative factor influencing directly the oxidase activity in the 
abscission layer, or of both acting simultaneously, is a problem still 
to be investigated. Likewise a critical investigation of the possi- 
bility of a double effect of carbon dioxide on leaf-fall might throw 
more light upon the causes underlying abscission. 
Macrochemical analysis 
In order to follow the chemical changes leading up to abscission, 
both macrochemical and microchemical methods of analysis were 
employed. About 2500 plants grown under greenhouse conditions 
were used for the macrochemical analysis. These plants were all 
grown at the same time under the same conditions, and collection 
of material was made at the same time each day. Series C was 
collected between February 17 and March 3, collections being taken 
from day to day as the lower leaves began to absciss. Series D 
and E were collected from these same plants on March 3 and 4. 
