53 
leaves. It seems most probable that the primary inocula in the spring 
are the conidia from the twigs and cankers or the ascospores in the over- 
wintered leaves. 
Pathogenesis. The fungus passes the winter in the living twigs. 
In autumn, fruit-bodies begin their development on the affected twigs 
and reach maturity early in the spring. In prepared sections of develop- 
ing fruit-bodies on oak twigs, NoTE:— 
18. That, while in the lower half of the fruit-body an opening 
has formed, the upper half is made up of mycelium and is still covered 
by the cork-layer of the bark. 
Before early spring the mycelium in the upper part of the fruit-body 
disappears and the cork-layer is ruptured so that the conidia may escape. 
Examine with the hand-lens the twigs with the mature Myxosporium 
stage. Make freehand sections at right angles to the twig. Stain with 
eosin, mount and OBSERVE :— 
19. That the top of the fruit-body is open. Has it a definite 
ostiolum? 
20. The abundance of conidia. 
21. The size, shape and color of the conidia. 
These conidia are produced in great abundance and ooze out of the fruit- 
bodies in creamy drops or strings. In the presence of abundant moisture 
they may be washed or splashed to the young unfolding leaves. It seems 
that the conidia thus disseminated in early spring from the fruit-bodies in 
the twigs are the most efficient of the various possible primary inocula. The 
other types of primary inocula undoubtedly play some réle and must be 
considered. In the overwintered leaves, specimens of which are provided, 
NOTE :— 
22. The small pimple-like bodies on the surface. 
Remove one of these fruit-bodies toaslide, mount and crush. OBSERVE :— 
23. The abundance of conidia coming from the pustules. 
24. The remains of the walls which enclosed the spores. 
25. The size, shape and color of the conidia. Compare with the 
conidia obtained from fruit-bodies on the twigs. 
Make prawincs of this stage. 
The pycnidia-like fruit-bodies thus formed on overwintering leaves 
come about by growth of the vegetative hyphae around the margin of the 
acervulus, arching-up over the conidia until they are enclosed. This is 
the Sporonema stage and is formed on leaf-petioles as well as on the blade. 
Study the Sporonema stage on old petiole-lesions if material is available. 
DRAW. 
The conidia in the Sporonema pustules on dead leaves remain viable 
through the winter and may cause primary infection in the spring if they 
reach susceptible parts of the hosts. 
In the same overwintered leaves showing the Sporonema stage, search 
for the perithecia. NOTE:— 
26. Small hair-like spines protruding from ruptured places in 
the leaf-epidermis. 
27. On the opposite side of the leaf, that a distinct bulge in the 
surface is evident. — 
Cut out small bits of the leaves containing perithecia. Mount in water 
and tease away the leaf-tissues. NOTE:— 
