87 
development may be arrested at. any stage by winter conditions and be 
resumed in the spring. Study the material provided and oBSERVE:— 
13. The long, twisted, yellow, tendril-like masses of pycno- 
spores as they are pushed out from the subcortical pycnidia which appear 
as “pimples’’ on the thin-barked twigs. Study either fresh material in 
moist-chambers or photographs. These strings of spores are composed 
of thousands of the pycnospores stuck together by a mucilaginous substance. 
(See illustration photographs 8, 9 and 10.) pRaw. 
Make a mount of a portion of one of these spore-threads in a drop of 
water on a slide and NoTE:— 
14. That the spores in the yellow string separate immediately. 
Place under the microscope and OBSERVE:— 
15. The small hyaline pycnospores. They are among the smal- 
lest pycnospores of the fungi. (See illustration photograph 13, d and e, 
and photographs 13-14.) 
Some conception of the large number of pycnospores produced from one 
canker may be obtained by noting the number observed in the mount 
from only a portion of one small thread. These spores, on account of 
their clinging together, cannot be blown about by the wind. Then too, 
the fact that they separate when water is present, would indicate that 
eventually these conidial strings find lodgment very near the place of 
formation, if not carried away by insects, birds or other animals. The 
possibility of bark-boring insects and insectivorous birds acting as in- 
oculating agents in transferring these conidia to a healthy tree is plausible. 
These pycnospores will not germinate in pure water but require the 
presence of substances from the bark in solution. The process of germina- 
tion begins with enormous swelling of the minute spore. Study pycno- 
spore-germination, either from slides or from Cornell Bulletin 347 : 566. 
DRAW to show three or more stages in spore-germination. : 
Ascospores or conidia, when introduced into a wound of any sort in the 
bark which exposes the tissues beneath the cork, germinate and develop a 
mycelium. (See illustration photograph 12.) 
The further developments of the pathogene are the same regardless 
of whether ascospores or conidia initiate the cycle. The period of incuba- 
tion is an uncertain one depending on many factors; weather conditions 
(especially temperature), depth of inoculation and thickness of bark.- 
In general, however, in the thin bark of twigs, definite lesions may be 
discerned three weeks after inoculations made with conidia, and four 
weeks after inoculation with ascospores. 
Wounded tissue and a brief saprophytic growth, sufficient to produce 
mycelial fans, are necessary before living tissues can be invaded and infec- 
tion accomplished. Examine material and illustration photograph pro- 
vided. OBSERVE :— 
16. The form, size and color of these mycelial fans; relation 
to the host-tissue. 
17. In mounts under the microscope, the structure of the fans, 
branching of hyphae and relation to each other. 
pRaw to show general appearance of the fans and also their structure. 
After infection is accomplished and the mycelial fans begin to invade 
and kill living bark-tissues, the canker develops rapidly. Pycnidia are 
produced abundantly just back of the advancing edge of the canker. 
Study material or photographs of young cankers and NOTE:— 
