Abstract
In dark-field photomicrographs 1 of microincinerated pollen mother cells of the lily, the chromosome ash is most conspicuous against the black background which results from the paucity of ash in the karyolymph or nucleoplasm. Animal cells yield a similar picture (Scott, 2 Kruszynski 3 ). This marked concentration in the chromosomes of elements of comparatively high atomic weight, as potassium, calcium and iron probably and phosphorus certainly, may be related quantitatively to X-ray phenomena in at least 3 ways.
First, rough calculations based on microincineration evidence and on the composition of nucleic acid indicate that the mass absorption coefficient of chromatin is more than double that of air and, hence, that of nucleoplasm. This relatively greater absorption of radiant energy by the chromosomes may account in large measure for the greater sensitivity of the nucleus compared to the cytoplasm, an experimental fact confirmed by a number of investigators.
Second, the amount of chromatin in a “sensitive volume”, a concept associated with the quantum hit theory of radiation injury, would be but a fraction of that computed for it on the ordinary assumptions, i. e., those which underlie the definition of the roentgen as a dosage unit. Were the validity of the theory established and relevant measurements precise enough, this size factor might prove important in assigning the “sensitive volume” to a definite cytological entity.
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