Spores of Chaetomium globosum have been subjected to monochromatic ultra-violet irradiation and colonies have been grown from single spores which were irradiated dry. Special attention bas been given to experiments in which The mercury lines at 265 m^u, 313 m^u, and 334 m^ were used. In all these cases many saltants wore produced as tbe result of the treatment of the spores. The primary object of the investigation was to observe any selective appearance of saltants at different wavelengths. Evidence for such a selective effect bas been given by one of ns, McAulay (1938") and the present experiments were made with more rpfined methods to obtaiii further information on this question, A very marked selective effect was found in which a certain easily recognized type of change was produced in large numbers of colonies grown from spores irradiated by short wavelengths, but in a very few irradiated by long wavelengths. This cbann'e is more in the nature of a growth modification than of a saltant, A part of the colony is normal but a sector appears whose vertex usually is not at the centre of the circular growing colony. Tbe sector is nearly clear of aerial mycelium and is frequently edged with dense brown in the substratum, Tbe aerial mycelium is often dense at the edge of thp sector and may form white knots. The modification has been designated by the letter "K."There is a particular feature of interest about the K type. The colony, grown from a single spore, comprises distinct parts eacb with quite different characteristics. It would appear tbat the mycelium growing from the single spores is tinstable in the sense that a colony derived from it may have two quite distinct forms.On several occasions tbe same single spore gave rise to three or more types, normal, K and a saltant (Fig, 2, No. 6),A very large number of saltant types is produced in Ch/ietomium glohosum by irradiation with wavelengths over the range 230 ma to 834 m/i. The percentage of colonies saltating is roughly constant over this range provided the energy per sq, cm. applied to the spore is a constant proportion of the lethal dose, in striking contrast to the "K" growth modification which appears selectively at short wavelengths. EXPERIMENTAL METHOD,The monochromator used in these experiments was constructed in the laboratory, and has been described by McAulay and Taylor (1939), A 125 watt mercury discharge lamp, a commercial unit with outer envelope removed, was used as a source of ultra-volet light.Spores were carefully spread on a eoverslip in a narrow are within the limits of thp wavelengths and separated from one another RO that all would have an equal opportunity of being irradiated. The spectrum was focussed on the spores by fluorescence of an underlying uranium 1 The funds required for thia work came from the Commonwealth Research Grant to the University.

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