Chromosome-exchange Sites inTradescantia PaludosaMicrospores

SummaryThe distribution of chromosome-exchange aberrations between cells, induced by radiation in Tradescantia paludosa microspores four days before pollen-grain mitosis, has been investigated. Gamma-, x- and neutron-radiations given at a variety of dose-rates, and under different environmental gas conditions, are represented in the data.Dicentric plus centric ring types conform to a binomial distribution (p + q)n with n = 2·4, over the whole yield range investigated. If n is equivalent to the average number of ‘sites’ available for exchange, this is strong evidence that these are pre-determined physical entities.When dicentric and centric rings are considered separately, each type conforms to a binomial with n = 2·0. Non-additivity of the n values suggests that differences in probability exist at the two types of site, and a spurious n value arises from pooling.Some possible reasons for this difference between sites are considered.