The Effect on the Growth of Sclerotinia fructigena of Alternating Periods of Light and Darkness of Equal Length

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Abstract
Apparatus has been designed to enable Sclerotinia fructigena to be grown at a constant temperature and under various equal alternating periods of light and darkness ranging from alternations of 0·3 second to twelve hours. Diameters of the fungal colonies were determined after three days' growth in the apparatus. In darkness and in twelve-hourly alternations the fungus produced an irregular colony, in the latter case showing some zonation. Under all other conditions the growth was regular. Growth was slowest in total darkness, and faster in continuous light than in twelve-hourly alternations. The rate of spread under twelve-hourly alternations was slower than with any other of the alternating treatments. With the shortening of the period of alternation the growth increased. It reached a maximum at about one-minute alternations, decreased to a minimum at five-second alternations, and subsequently rose again. The effect of alternating light of different periodicities on the growth rate of the fungus is compared with its effect on chlorophyllous plants. It is concluded that the effect of alternating light on the growth of plants cannot be interpreted solely by its influence on the photosynthetic mechanism, but that in addition there is an action on some photochemical mechanism present in non-green plants or there is some direct effect of light on the protoplasm. In conclusion it gives me much pleasure to express my appreciation of the interest and criticism which Professor V. H. Blackman has shown during the course of these experiments.