24 
BULLETIN 1038, U. S. DEPARTMENT OF AGRICULTURE. 
size of the leaf. In other words, the total size of vein in rosetted 
leaves of reduced size tends to be as great as that normally developed 
to support the full-sized leaf, but the development of vascular tissue 
within the vein becomes reduced with the severity of attack and 
consequent reduction in the size of the leaf blade. 
Table V. — Relation of vascular tissue in leaves to rosette. 
Diameter of large 
side veins. 
Area of cross section, 
large side veins. 
Area of 
cross sec- 
Description of material. 
Total. 
Vascular 
part. -. 
Total. 
Vascular 
part. 
tion of 
vascular 
part to 
total 
veins. 
Frotscher variety, Thomasville, Ga.: 
Normal leaf, south side 
Microns. 
347 
359 
346 
259 
Microns. 
227 
202 
202 
83 
Square 
microns. 
94, 006 
100, 641 
94,006 
52,269 
85, 472 
Square 
microns. 
40, 107 
32,041 
32, 041 
5,280 
25, 442 
Per cent. 
43. 
Rosetted leaf, about half size — 
North side 
32 
34 
10 
30 
Linear leaf, smith sidp (mi rl vein) 
487 1 80 
An examination of the vein islets of healthy and diseased leaves 
(secondary stage) has revealed striking differences in size, shape, 
and arrangement. Over the entire normal leaf blade these tiny areas 
bounded by the small, anastomosing veinlets tend to be isocliametric 
and of uniform size (PL X, fig. E). In the yellow areas, on the 
contrary, great differences in size and shape are the rule (PL X, figs. 
A to D). At the center of these spots the vein islets are smallest and 
become larger and larger with increasing distance from the center 
until often in the neighboring green parts they are considerably 
larger than in the healthy leaf. Their appearance suggests an in- 
hibitory influence generated from the center, which largely prevents 
normal growth and differentiation there, but acts as a poison more 
and more feebly with receding distance from the focal center until in 
the neighboring green parts it has become sufficiently attenuated to 
function as a stimulant rather than as an inhibitory factor. This 
theory is also borne out by the writer's histological studies. 
Not only are the vein islets highly variable in size, but they are 
often greatly distorted in shape. In many cases they are linear in 
outline, and with reference to the spot they approximate the arrange- 
ment of spokes in a wheel (PL I, fig. 1; PL X, fig. B). Thus it 
will be seen that the direction of their greatest expansion may or 
may not parallel the direction of greatest expansion in the leaf blade 
as a whole. That is, the size, shape, and arrangement of the vein 
islets in these chlorotic spots of the secondary stage are controlled 
from the focal center of the spot rather than by the normal mor- 
phogenic forces of the leaf. 
