560 
Journal of Agricultural Research 
Vol. XIV, No. 12 
plate shows the halves of another woody cylinder that are being forced 
farther and farther apart. A longitudinal view of one of these wedges 
is given in Plate 76, A. 
The above examples of ray infection have in most cases shown the 
wood split into two approximately equal halves. Such cases are easier 
to show in a photograph than where the splitting is more complete. 
Plate 77, A, shows a portion of a woody cylinder that is being much more 
thoroughly split up by medullary infections. By further growth of the 
infected medullary rays the bundles become more and more separated 
from each other. Through unevenness in the growth of different por¬ 
tions of diseased tissues they become twisted and distorted in various 
ways. Sometimes the twisting is so great that a portion of the bundle 
that was originally nearest the center of the woody cylinder is turned 
toward the surface of the club, while the portion originally adjacent to 
the cortex is turned toward the center of the root. 
It is interesting to note the further development of the bundles when 
they are separated from each other in this way. No longer held together 
in a compact mass, they broaden out so that in cross section they appear 
fan-shaped rather than wedge-shaped. Figure B of Plate 77 shows a 
bundle that is beginning to broaden out in this way. Plate 78, A, shows 
another strand in which the spreading has progressed until the bundle 
is semicircular in cross section. Figure B of this plate gives a view of a 
still later development. Here the bundle has grown until it is almost 
cylindrical and is being further split up by later infections. 
The vascular strands, separated from each other and distorted in 
various ways, are apparently no longer able to function normally. The 
diseased tissues that separate them probably use up a large part of the 
water which they transport. In this way the parasite is not only capa¬ 
ble of hindering the further development of the conducting system but 
is also able, by^means of medullary infection, to interfere with the func¬ 
tioning of those elements that are already present when it makes its 
attack. If roots and stems become infected while still young, the injury 
to their vascular systems consists in hypoplasia of cell differentiation. 
This also occurs when old organs are attacked, but here, in addition to 
arresting the development of new xylem and phloem elements, the vas¬ 
cular tissues already present are tom apart through hyperplasia in the 
medullary rays. For this reason late infection does not prevent the 
disease from causing the host plant to wilt. 
In the above paragraphs emphasis has been placed on the changes 
wrought by the parasite in the vascular tissues because these changes 
are so striking and conspicuous. It would be wrong to suppose, how¬ 
ever, that these are the only factors concerned in bringing about death 
or even in producing wilt. The plasmodia in all probability give out 
substances deleterious to the plant. The yellowing of the leaves during 
