44 BOTANICAL GAZETTE [JULY 
to the stem, and therefore are not included in the analyses. The 
data in the table, therefore, represent only the general chemical 
changes during the life of the leaves, while the detailed chemical 
changes occurring in the abscission layer itself will be given under 
microchemical analysis. Should one desire to make a macrochem- 
ical analysis of the abscission layers alone in Coleus no less than 
40,000 plants would be needed. 
The data in table IV show an increase in dry weight in the 
abscission end of the petiole at the time of abscission; also an 
increase in alcohol-ether soluble material, but no increase in water 
soluble material. The significance of these changes is uncertain. 
In the older petioles there is a slight decrease in polysaccharides 
and an increase in reducing substances. There is no increase in 
ammonia and amino acids, as might be expected if the protoplasm 
were breaking down. Oxalates and total calcium remain fairly 
constant, but there is a slight decrease in the amount of calcium in 
solution and in the acidity. A more detailed discussion of the 
chemical changes in the abscission layer is given under micro- 
chemical analysis. 
In the older blades there is a decided decrease in the amount 
of accumulated starch at the time of abscission, but the amount 
of reducing substances remains fairly constant. Attention has 
already been called to the fact that the formation of the abscission 
layer is completed while the leaf is still in an active photosynthetic 
condition. Both photosynthesis and the translocation of foods 
continue for several days or weeks later. The data clearly show 
that the presence of the abscission layer does not prevent the 
movement of water and foods between leaf and stem. 
Microchemical analysis 
A microchemical investigation of Coleus showed a striking 
localization of physical and chemical changes in the abscission layer 
shortly before and at the time of abscission. The formation of the 
abscission layer usually in the third pair of leaves and the occurrence 
of abscission usually in the eighth pair of leaves (when the plants are 
grown in 4-inch pots in a greenhouse) make it possible to study the 
whole history of ‘the abscission layer by investigation of only 6 
