18 
18. The large globose or oval cortex-cells, closely packed next 
the cork-layer but with looser arrangement toward the wood. Their 
color and contents. ; 
19. Just beneath and partly surrounded by the cortex-cells, 
bundles of white densely packed sclerenchyma-fibers (bast). 
20. Toward the center in the following order, the phloem, 
cambium, and xylem. ; 
21. At the center of the cross-section,—the pith; the medullary 
rays distributed radially in the vascular cylinder. _ ; ; 
Study the diseased portion of the twig, comparing the blighted tissues 
with the healthy tissues already examined. OBSERVE :— 
22. The affected area. Which tissues are involved? How 
recognized? Are all the cells within the diseased region killed? 
283. The effect of the pathogene on the epidermis, cork, cortex, 
sclerenchyma-fibers, medullary rays, phloem, cambium, xylem and pith. 
Compare with the healthy condition of the cells as to form, size, color and 
contents. 
24. The relatively large cavities scattered throughout the affected 
portion. How may these be accounted for? 
25. The stratified appearance just outside of the phloem. To 
what due? 
26. The apparent absence of the bacillus. Why not evident? 
DRAWINGS :——(a) Make a diagrammatic drawing to show the healthy 
and diseased tissues. (b) A much larger and detailed drawing of a portion 
of a cross-section of the affected twig. Make the drawing include both 
healthy and diseased bark. 
Study closely the margin of the lesion. In case the section was taken 
from a twig in which the pathogene was inactive the diseased and healthy 
tissues will be separated by a cork-layer. If the pathogene was active 
no cork-layer will have yet been formed. Determine which was the 
condition in the twig from which the section provided wascut. In the apex 
of growing twigs, the parasite may invade the xylem-ducts and thus 
migrate some distance in them down the twig. 
REPORT 
1. In working out methods of control, of what importance are 
the following facts about fire-blight :-— 
a. It occurs only in North America. 
b. The bacteria causing the disease pass the winter in hold- 
over cankers in any of its numerous hosts. 
c. The bacteria get into the host through wounds. 
d. The chief agents of inoculation are certain insects. 
; e. The bacteria are usually introduced into the young and 
growing parts of the host where in these succulent tissues they multiply 
and develop the disease very rapidly. 
