824 
Journal of Agricultural Research 
Vol. XXXI, No. 9 
this sheath. Usually the blackening is due to a staining of the tissue 
by the invading fungus, or it may be due to the crusts of the mycelium. 
Examinations show that the cytoplasm and nucleus begin to dis¬ 
integrate as the fungus enters a cell. In fact, the nucleus was often 
seen breaking up before the fungus actually entered the cell, and by 
the time the hyphae were well distributed through the cells no sign of 
the nucleus could be seen. The cell wall was also affected by the 
presence of the fungus, but the exact nature of the change was not 
determined. On staining the tissues with saffranin and light green, 
the cellulose cell walls took up the light-green stain until the fungus 
entered the cell, or the adjoining cell, then the walls took up and 
retained the saffranin. It appears from this that enzymes or toxic 
substances penetrate the host cells in advance of the fungus. The 
fact that the cell contents of the host showed disintegration before 
any hyphae entered the cell also substantiates this view. 
SUMMARY 
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The name Ophiobolus graminis Sacc. is used in this paper to desi- 
nate the fungus causing the take-all disease of wheat. 
Perithecia of the New York strain of this parasite were formed in 
pure cultures on artificial media and on wheat plants growing in soil 
that had been inoculated with the fungus. Ascospores and para- 
physes were formed in abundance. 
When conditions were favorable the ascospores germinated rapidly, 
sending out from 1 to 3 germ tubes which branched and rebranched, 
forming a network of hyphae. Numerous cultures of the New York 
strain, made from single ascospores, produced perithecia and mature 
ascospores, thus showing that this strain of the fungus is homothallic. 
The Oregon and Arkansas strains studied were never induced to 
sporulate. 
Although the three strains of the parasite reacted similarly to 
temperature, they showed some variation in this regard. In all cases 
growth took place between 4° and 33° C., but it was very slight at the 
extremes. The optimum temperature for the growth of the New York 
strain was found to range between 19° and 24°, whereas the optimum 
for the Oregon and Arkansas strains was between 23° and 24°. 
Light checked mycelial growth slightly and greatly stimulated 
sporulation in the New York strain of the parasite. 
The strains of the parasite used in these studies grew within the 
P H range of 3.0 to 10+ . The optimum hydrogen-ion concentration 
for the New York strain was found to be P H 6.0 and that of the Oregon 
strain was P H 6.8. The optimum for the Arkansas strain was less 
limited, in that it extended between P H 6.8 and 7.6. 
The ascospores and probably the mycelium of the parasite were 
found to overwinter in plant refuse in the soil. Mycelium over¬ 
wintered readily in agar cultures in test tubes. 
The varieties of wheat used in these investigations (Goldcoin, 
California Club, and Marquis) were all found to be susceptible to 
infection by Ophiobolus graminis during the entire growing period of 
the plants. All of the studies made thus far show, however, that 
plants are most susceptible in the seedling stage. 
Infection was never obtained in any part of the plants much above 
the soil line. 
