462 
C. H. CRABILL 
4. Boiled extract was used likewise. 
In no case was the growth of the fungus hindered by the presence 
of the extract. It appears then that it is a lack of food rather than a 
toxic secretion which keeps colonies of C. pirinum from growing 
together. 
A microscopic examination of thin Petri dish cultures shows that 
the mycelial strands exhibit a similar chemotaxis. They tend to 
Fig. 13. Two colonies of a plus strain somewhat enlarged. The dark zone 
is olive green in color and just inside the white advancing margin. Inside of this 
zone the mycelium is hyaline. The colored oil has been withdrawn from it and 
deposited in the spores. The two colonies are antagonistic and do not grow together 
on adjacent sides. 
diverge continually. If somewhat crowded they grow parallel but 
seldom converge or cross. It is doubtful therefore if plus mycelium 
ever crosses over into minus sectors and vice versa. In this way the 
integrity of the plus and minus sectors is preserved. 
Some experiments were conducted to throw light on this question. 
