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 lea-ves to rosette. 





Diameter of large 

 side veins. 



Area of cross section, 

 large side veins. 



Area of 

 cross sec- 

 tion , rela- 



Description of material. 



Total. 



Vascular 

 part. 



Total. 



Vascular 

 part. 



tion of 



vascular 



part to 



total 



veins. 



Frotscher variety, Thomasville, Ga.: 



Microns. 

 347 



359 

 346 



259 

 487 



Microns. 

 227 



202 



202 



83 



180 



Square 

 microns. 

 94,006 



100,641 

 94,006 

 52,269 

 85,472 



Square 

 mzcrons. 

 40, 107 



32,041 



32, 041 



5,280 



25,442 



Per cent. 

 43 



Rosetted leaf, about half size- 



32 





34 





10 





30 











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 isodiametric 

 and of uniform size (PL X, fig. E), In the yellow areas, on the 

 contrary, great differences in size and shape are the rule (PI. X, jfigs. 

 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 largelj^ 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. 



