38 
Sometimes in very old artificial cultures or where the ascospores germinate 
in non-nutrient media, so-called microconidia are produced. (See text.) 
These have never been observed to germinate. _ 
When the mycelium has killed the tissues, it rapidly extracts and appro- 
priates the food-substance therein. As this food-supply begins to dwindle 
the mycelium forms gnarls of short intertwining hyphae at different points 
in or on the lesion. These rapidly enlarge, forming the mature black 
sclerotia. 
Make thin sections of one of the sclerotia and OBSERVE :— 
19. Its structure; (a) the black outer layer or rind made up of 
about two or three layers of cells whose walls are thick and brown to 
black; (b) the inner white medulla made up of densely interwoven thick- 
walled hyphae. These sclerotia are storage-organs and the food-substance 
stored is largely in the thickened walls of these medullary hyphae. 
DRAw to show structure of rind and medulla much enlarged. (See de Bary, 
Morph. and Biol: Fungi p. 30-32, fig. 14.) ; 
During a drought or cold weather all the mycelium, except that in these 
sclerotia, dies. The sclerotia pass the winter in the soil or old host- 
debris, in a dormant condition. In the spring they germinate; that is, 
from them develop the apothecia in which are produced the ascospores. 
(See de Bary, Morph. and Biol. Fungi p. 52-53, 218-219.) 
Make thin sections through the apothecium provided or use prepared 
slide. MAKE OUT:— 
20. Asci with discharging-pore at apex; ascospores, paraphyses, 
and hypothecitum; also structure of stipe and excipulum. DRAW a 
portion of the section to show these structures in the proper relation to 
one another. 
Secondary Cycles. These may originate from mycelium which 
spreads through the soil from diseased to neighboring healthy plants. 
In this case the mycelium serves as inoculum. Aside from this the secon- 
dary cycles duplicate the primary. 
Pathological Histology. Mount a bit of the diseased tissue from just 
back of the advancing margin. DETERMINE :— 
i 21. The effects of the pathogene on the host-cells. Draw to show 
this. 
This mycelium secretes a toxine which kills the host-cells. De Bary 
believed he had shown this to be oxalic acid. When the sclerotia are 
forming, large drops of liquid are extruded upon their surfaces. This 
also contains the toxine. 
If cultures showing these drops on the sclerotia are available, remove 
with the end of a needle-handle one of the drops to a clean slide; quickly 
place a small drop of calcium nitrate solution beside it, place under the 
low-power and OBSERVE :— 
22. The formation of calcium oxylate crystals as one causes the 
two drops to come in contact, using a needle. Cover and study these 
crystals with the high-power. 
The mycelium also secretes a cytolytic enzyme which dissolves the 
middle-lamella from between the host-cells. 
__ Make thin sections of the young succulent portion of the host and mount 
in a large drop of the liquid from a bouillon-culture of S. Libertiana. 
Mount similar sections in water. Cover in a petri-dish to prevent evapora- 
tion. After an hour or so, EXAMINE :— 
