92 
The Primary Cycle is initiated in the spring. The source of inoculum 
is, without much doubt, the sclerotia on overwintered host-debris, in the 
soil, or on resting organs of the host such as tubers or roots. Solien 
Pathogenesis. Exactly how the primary infections are initiated 
does not appear to have been observed. The following studies suggest 
the probable phenomena involved. Tease apart, in a drop of water on the 
slide, one of the sclerotia from the material provided (on potato tubers 
or in pure culture). Examine and OBSERVE :— ; ; 
21. The barrel-shaped cells in profusely branched chains which 
make up the sclerotium. ; 
22. The color of the walls and the granular protoplasmic con- 
tents of many of the cells. 
23. That these cells break away readily in groups of several each. 
Make prawines of several of these cells together. . : 
Many of these cells germinate like conidia. Study the slides provided 
showing germination. .OBSERVE:— 
24. The different types of germination presented, polar and equa- 
torial. 
25. The diameter of the germtubes as compared with that of the 
parent-cell and the number of germtubes from each. 
26. The granular and vacuolate character of the contents of the 
germtubes. 
27. The difference in the thickness and color of cell-walls in 
germinating cells and germtubes. 
DRAW several germinating cells to show the different types observed. 
These barrel-sbaped cells constitute the inoculum. Just how they find 
their way to the infection-courts is not always clear. The germtubes 
may be capable of growing for considerable distances through the soil 
to the susceptible stem or root which they penetrate and infect. 
The vegetative mycelium, developed from these germtubes, is quite 
different in character from that composing the sclerotia. Mount some 
of the growing mycelium from the advancing margin of the agar culture 
provided. OBSERVE :— 
28. The long parallel hyphae, much branched and frequently 
anastomosing. 
29. The characteristic manner of branching: the acute angle 
formed by the branch; the construction of the branch at the point of 
origin; and the septum laid across it a short distance above the point of 
origin. These characters usually hold for the actively growing mycelium 
in the tissues of the host and serve as a diagnostic sign of the disease. 
30. The colorless character of the young mycelium; the yellow- 
ing and browning with age. 
Make thin longitudinal sections of diseased stems (or use prepared 
slides) and oBsERVE:— 
: 31. The mycelium in relation to the cells of the host-tissue; 
inter- or intracellular? What tissues are invaded? How does the my- 
celium here compare with that studied from the culture? 
DRAW a portion of the section in detail to show the character of the 
mycelium and its relation to the cells. 
 . Hyphae may grow out from a diseased plant and, spreading through the 
soil, reach and infect nearby healthy plants. It is probable that external 
mycelium and sclerotial cells, formed in the soil, may be carried in cultiva- 
