SUMMARY: Growth of colonies of and two strains of were studied. The radial growth rate ( ) of these colonies was found to be a function of the length () of the leading hyphae spanning the colony's peripheral growth zone and the specific growth rate (α) of these hyphae. Thus = ωaL . The peripheral growth zone is the region in which hyphae are able to contribute protoplasm to the apical extension of the colony's leading hyphae. The width of the peripheral growth zone remained constant with time but varied from 423 μm. for to 8660 μm. for . The specific growth rate of the hyphae in the colony's peripheral growth zone appeared to be identical to the organism's maximum specific growth rate in submerged culture. The width of the peripheral growth zone was not influenced by temperature or by adding an inhibitor to the medium but did vary with glucose concentration. An observed difference between the radial growth rates of the two strains of , which had almost identical specific growth rates in submerged culture, was found to be correlated with a difference between the widths of their respective peripheral growth zones. Although there was a significant difference in the length of the apical cells and hyphal compartments of the leading hyphae of these strains, the growth zone of the hyphae of each strain contained 16 septa; it is suggested that the strains differ in the frequency of septum formation but not in the rate at which the septa become plugged.


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