84 
phores. When ripe these pycnospores are disseminated and initiate 
secondary cycles. 
Make a DRAWING of a section through the pycnidium. (See Cornell Bul. 
379, fig. 22. 
sles The fungus readily infests the dead bark of a 
great many plants. (See list in Cornell Bul. 379: 95 and 98.) Pycnidia 
may be formed on such plants. Occasionally perithecia are also formed 
from the saprogenic mycelium. The conditions under which they are 
produced are not clearly understood. . : 
Study sections through a perithecium under the demonstration micro- 
scope. Make a pRawING to show its structure. (Compare Cornell Bul. 
379, fig. 18.) 
Secondary Cycles are not different from the primary except as to 
the time of year at which they occur. 
Pathological Histology. The lesions produced on the host are necrotic. 
Laboratory studies of histological changes will be confined to the more 
striking features of the canker. (Read Cornell Bul. 379:111-116, and 
study figures 34-37.) 
The mycelium attacks the cortex, phloem, cambium, medullary rays, 
vascular bundles and pith. The outward changes evident to the naked 
eye, however, are the results of a necrosis of the cortex and phloem. These 
tissues are affected most generally and most extensively. Study slides 
showing sections cut through the center of limb-cankers and MAKE :— 
17. Diagrammatic DRAwINGs of both cross- and longisections 
through the center of the canker, showing healthy tissue above and below 
the lesion. : 
The fungus breaks away from the lesion at different points and spreads 
in a streak up and down the limb. If this is attempted in the cortex, 
the advance of the fungus is halted by the development of a cork-layer. 
There results, then, a pocket. If the pathogene leaves the cortex and 
advances in the sap-wood, no check on its progress is made by a cork- 
layer as just described above. On the other hand it enters the sap-tubes 
of the xylem and travels a considerable distance above and below the canker. 
As it does so the wood-cells, adjacent to the xylem-ducts, are discolored 
and filled with a brownish deposit. Consequently a streak is visible to 
the unaided eye where the mycelium is advancing. sHow this streak in 
the diagram of a longitudinal section through the canker. 
Study prepared sections through the margin of a young canker. 
NOTE :— 
18. The appearance of the tissues involved. Study the margin 
between healthy and diseased tissues. What sort of tissue is evident? 
Find the mycelium. Make a DRAWING including a portion of healthy and 
diseased tissues and the mycelium. . 
Study prepared sections cut through the stem at some distance above 
the canker. pDRaw to show points observed. 
REFORT 
1. Discuss the parasitism of Physalospora Cydoniae Arnaud. 
