Chemotropism in Rhizopus nigricans

Abstract

The following results apply to Rhizopus nigricans Ehrenb. in particular, but experimental evidence is at hand that the general principles involved apply also to Botrytis cinerea Pers. and Penicillium no. 24 Thom. Most of the data have been derived from work with two layers of medium separated by a perforated mica plate. 1. The fungus shows a marked negative chemotropic reaction to a medium in which it has been growing for some time. 2. The hypothesis brought forward by Clark and Fulton, that this negative chemotropism is a reaction of the fungus toward its "staling substances," is conclusively substantiated by the following evidence: (a) the hyphae in one layer of medium turn away from another layer without spores if it contains their own staling products; (b) the hyphae always show a marked turning from the medium in which they are growing to any medium which is free of hyphae, no matter what the composition of that medium may be, unless the second medium contains the substances which exert a negative chemotropic stimulus; (c) when approximately equal amounts of mycelium occur in two similar layers of medium, no turning of the hyphae from the one to the other results. 3. These staling substances are formed as a result of the vital activities of the fungus itself. They consist probably of excreted products of metabolism. 4. The staling substances appear to be either thermolabile or volatile, for boiling a solution containing them reduces markedly their negative chemotropic influence. 5. Positive chemotropism toward the substances tested (turnip juice, cane sugar, and glucose) also exists; but under ordinary conditions of growth this positive chemotropism is very much weaker than the negative chemotropism previously mentioned. 6. Turnip juice exerts a much stronger positive chemotropic stimulus than the other simple chemical substances tested, much stronger than, for example, 5 per cent cane sugar. This suggests the possibility that plant juices in general evoke a stronger positive chemotropic response than the simple chemical substances heretofore experimented with. The nature of the attractive substance or substances in turnip juice has not yet been determined. 7. It is impossible in the present stage of our knowledge to compare accurately the strength of two chemotropic stimuli; but,using the number of hyphae turning as a test of the relative action of the two stimuli, we certainly may conclude that in comparison with the stimulus exerted by negative chemotropism, that due to positive chemotropism is very much less. For example, in a given preparation with two layers of media separated by a perforated mica plate, and with the hyphae at a certain stage of growth, when 90--100 per cent of them are turning toward the holes from plain agar to turnip juice agar, the stimulus due to the positive chemotropic effect of the turnip juice has, very approximately, one-third of the effect of the stimulus due to the negative chemotropism from the staling products of the fungus itself. In other words, positive chemotropism is responsible at this stage and under these conditions for about one-quarter of the whole reaction. With the sugars the part played by positive chemotropism is very much less and is easily overlooked. 8. In contrast to the increase in strength of the negative chemotropic stimulus with the age of the preparation, the attraction resulting from positive chemotropism remains fairly constant. 9. If the preceding conditions are true of fungi in general, it is probable that the distribution of a fungus in its host is influenced mainly, not by positive chemotropism, but by the dominant negative chemotropism due to its own staling products. 10. The part played by osmotropism in these reactions must, if any, be a very small one.