136 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
differ with the age of the tree from which the cuttings were taken. As the 
tree grows older, the epidermal and cortical cells of the roots become smaller, 
while the xylem and meristematic cells of these roots become larger. The 
endodermal and phloem cells of the roots seem to become smaller through 
their tangential diameters and larger through their radial diameters. 
Table 26 shows that epidermal and cortical root cells seem to become 
shorter with the increasing age of the parent tree. 
The noted increase in size of the xylem cells in cross section is particularly 
interesting in the light of unpublished studies by H. M. Benedict, who 
found that in the leaves of Vitis vulpina L. the amount of vascular tissue 
not only increases, but also that the xylem cells become larger as the vine 
grows older. 
Concerning Salix nigra, one may conclude from the material examined 
that the age of the tree seems to affect the size of root cells of cuttings. 
Collection and Care of Leaves 
During the fall of 1917 and the fall of 1918, leaves were collected from 
about seventy black willow trees. The age was determined as stated above 
Table 27. Average Areas of Vein-islets of Leaves of Trees of Salix nigra, Group A 
Ave. Areas of 
Number of Tree 
Diameter of Tree 
20 Vein- islets 
(Inches) 
(Sq. Mm.) 
8 
••• I| 
.244 
II 
1 
.440 
16 
2 
•305 
17 
.440 
23 
I 
.423 
24 
I 
.458 
25 
li 
.423 
27 
2 
.268 
29 
2 
•323 
30 
1 
.354 
31 
I 
.314 
32 
2 
.423 
37 
2 
.611 
38.... 
2h 
.478 
39 
a 
•323 
49 
2 
.578 
51 
I| 
.458 
52 
2h 
.440 
70 
X 
.550 
71 
.550 
72 
1 
.458 
73 
1 
.610 
74 
3 
•550 
75 
16 
.393 
77 
.440 
78.. 
1. 
.687 
79 
1 
.354 
80 
1 
8 
.407 
81 
3 
16 
.407 
82 
1 
8 
.440 
83 
3 
16 
.354 
84 
1^ 
.478 
85 
1 
.407 
86 
1 
8 
.478 
4369 
