138 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
Measurement of Vein-islets 
After the leaves were taken to the laboratory, they were cut transversely 
at their broadest point and the cut ends of the veins and veinlets were 
counted. The width of the leaves was measured at the place where the 
veinlet count was made. All data were tabulated. 
The number of cut ends of veins in the twenty leaves from each tree 
was added and averaged to find the average number of veinlets for the 
entire tree. This number was then divided by the average width of the 
twenty leaves to obtain the average area of the vein-islets for the whole 
tree. This method was used for each of the seventy trees. A grand 
average was then made of vein-islet areas of trees in groups A, B, and C. 
These areas were compared and tabulated as shown in tables 27-30. 
It will be seen from an examination of table 30 that the average area of 
vein-islets in the leaves from trees in group C is smaller than in leaves from 
trees in groups A and B. The average area of vein-islets of trees in group 
C is 71 percent smaller than that of vein-islets of trees in group A. 
Summary and Conclusions 
1. Cuttings from younger trees of Salix nigra Marsh, rooted in less time 
than those from older trees. 
2. Leaves appeared on cuttings of younger trees before they did on 
those of older trees. Age seems to be correlated with a decrease in rate 
of growth. 
3. The number of xylem strands coincides with the number of large 
cortical air spaces. 
4. Epidermal and cortical root cells of cuttings seem to become smaller 
with the increasing age of the parent tree. 
5. Xylem and meristematic root cells of cuttings seem to become larger 
as the parent tree becomes older. 
6. Endodermal and phloem root cells tend to decrease in size through 
their tangential diameters and to increase in size through their radia4 
diameters. 
7. The age of the parent tree apparently affects the size of root cells of 
cuttings from that tree. 
8. The average area of vein-islets in leaves from older trees is smaller 
than average vein-islet areas of leaves from younger trees. With the onset 
of senility the amount of vascular tissue seems to increase, thus reducing the 
average area of vein-islets. 
9. Large air spaces were found in the cortical tissue of the willow roots. 
The number of these spaces is not constant. Four is the average number. 
The writer wishes to acknowledge gratefully the help so willingly given 
by many friends and colleagues in the preparation of this paper. My 
thanks are especially due to Prof. Harris M. Benedict of the University of 
