1918] SAMPSON—ABSCISSION 45 
pairs of leaves in each plant. The fact that the leaves are opposite 
is also of advantage. Abscission of the pair may occur simul- 
taneously, or one of the pair may absciss long before the other 
begins, or it may occur at any stage in between. Since both 
abscission layers at each node may readily be obtained in a single 
free-hand section, it is possible to contrast all stages of abscission 
under exactly the same treatment. A study of the changes induced 
by forcing abscission in one of the leaves at each node is likewise 
facilitated. 
The investigations completed include a study of the distribution 
and amount of nitrates, carbohydrates, oxidases, iron, manganese, 
calcium in solution, and oxalates. 
NITRATES.—The data in table IV show a great increase of 
nitrates in the abscission end of the petiole as compared with the 
remainder of the leaf. Furthermore, the nitrates in this part of the 
petiole are least abundant at the time of formation of the abscission 
layer and most abundant a short time before leaf-fall. As already 
noted, these figures cannot be taken to represent the percentage of 
nitrates in the abscission layer. Microchemical tests show some 
interesting variations. In many plants the increase in nitrates is 
confined almost entirely to the abscission layer, while in others the 
petiole or the neighboring part of the stem may also show a like 
increase. In all cases studied there is an increase in nitrates in the 
abscission layer just before and at the time of abscission. In some 
plants this increase is gradual from the time of the formation of the 
abscission layer to the time of abscission. In other cases only 
traces of nitrates appear in the abscission layer until a short time 
before abscission, when they increase rather suddenly. 
CARBOHYDRATES.—The data in table IV show that the free 
reducing sugars, like the nitrates, increase in the abscission end 
of thé petiole with the increase in the age of the leaves, but, unlike 
the nitrates, they are less abundant in this part of the petiole than 
in the remainder of the leaf. This correlation of the amount of 
reducing sugars and the age of the tissue is still more striking when 
studied microchemically. From the terminal bud to the oldest 
leaves there is a gradual increase in reducing sugars in both stems 
and leaves. This increase is initiated last in the abscission layer. 
