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.

1. A research is described using different methods of experiment on two contrasted types of apical meristems with the object of measuring the permanent electrical field inside and outside the plant in the neighbourhood of the apical meristem under conditions of specified control. 2. Most of the experiments were performed on 3-5-day-old maize seedling roots and turnip seedling hypocotyls. The conditions of measurement were such as to produce minimum stimulation of the plant. The general results were checked by experiments with other plants. 3. The measured p.d. is a result of e.m.f.'s which are unchanged when supplying current of the order of 10-8 amp. through external conductors. Both turnip hypocotyls and sections of couch grass stems generate power of the order of 10-9 W. without polarizing (see text). In these experiments the equivalent resistance of the plant materials obeyed Ohm's law. 4. The average potential gradient along the surface of thirty-four turnip hypocotyls in the neighbourhood of the apical meristems was found to be 7.1±3.8 mV./mm. (over 6-8 mm.). The surface of the meristematic region is positive to that of older tissue. The corresponding average for twenty-five maize roots was 5.7±2.4 mV./mm. (over 6-8 mm.). 5. There are transverse e.m.f.'s between the outside and inside of the plants distributed radially from the stele to the root exodermis and hypocotyl epidermis. The inside is always negative with respect to the outside. The radial e.m.f. is larger in the meristematic region than elsewhere: In turnip hypocotyls, the radial e.m.f. at the meristematic region varies between 44 and 103 mV.; 5-10 mm. from the apex it is 17-60 mV. In maize roots, the corresponding figures are 17-77 and 6-33 mV. There is a small but possibly not significant potential change along the axis in both the organs considered, possibly 1 mV./mm. Under the conditions of the experiment, the interior of the meristem was found to be negative with respect to the rest of the axis. Points 4 and 5 are summarized in Fig. 4A. 6. The theory that the interior of a mass of meristematic tissue is electrically negative to older tissue is discussed in the light of the experimental evidence and with regard to the limitations of the technique used.

1. Mutants produced by x-irradiation of fungal spores of Chaetomium globosum have been compared with those produced by ultraviolet irradiation. 2. The most striking difference between the mutants produced by x-irradiation and ultraviolet irradiation is the absence in x-ray experiments of the K mutant which is produced in large numbers at short ultraviolet wave lengths. 3. A comparison is made of the relation between x-ray dose and numbers of lethal mutants, and the relation between the short ultraviolet wave length 2804 dose and numbers of lethal mutants. Both are compared with theoretical curves for 1, 2, 5, and 8 quantum hits. 4. The production of lethal mutants by x-rays is shown to be consistent with the theoretical curve for five quantum hits on the sensitive spot of the spore, whereas the production of lethal mutants by the ultraviolet wave length 2804 Å.u. is consistent with two quantum hits.