SUMMARY: An irreversible cessation of growth, accompanied by the death of the growing tips and the appearance of an intense brown pigment, is a regular feature of aged clones of . The frequency of this phenomenon, referred to as ‘vegetative death’, varies between clones of similar age and between identically maintained lineages of the same clone. It also varies with the method of propagation and with the other factors which affect the speed of ageing.

The heterokaryon and related tests show that vegetative death is cytoplasmic in origin while its segregation in the asexual progeny of colonies with mixed cytoplasms, and its infective spread via hyphal anastomoses, clearly define it as mutational in origin. Vegetative death involves a qualitatively different kind of cytoplasmic change from that previously found in the earlier stages of ageing.

In common with some other cytoplasmic mutants, those responsible for vegetative death suppress their wild type counterparts in cytoplasmic mixtures even though they are incapable of continued survival except in such mixtures. In this respect they differ from the generally reported behaviour of mixtures of mutant and wild type nuclei in heterokaryotic association. It is suggested, however, that the behaviour of either may be atypical, since most of the heterokaryons investigated will have been chosen for their stability, while a cytoplasmic mutant which was not suppressive is unlikely to be recognized and isolated by present techniques.


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