GENETIC AND CYTOLOGICAL EFFECTS 273 



Since the ring-chromosome studies in maize also revealed that breakage 

 by ultraviolet is frequent, a phenomenon obscured in rod chromosomes by 

 restitution, it has been assumed that ultraviolet must have, in addition, a 

 marked influence on the matrices of the chromosomes. Coagulation of 

 the matrices by ultraviolet would thus not only prevent the realization of 

 rearrangements from X-ray-induced breaks but also from those breaks 

 which it itself induces, providing in this manner a mechanism which would 

 lead to an apparent qualitative difference in behavior of the X-ray- and 

 ultraviolet-induced breaks. 



SPECTRAL RELATIONS 



Significant comparison of the relative effectiveness of different wave 

 lengths can be made only on the basis of rather precise estimates of the 

 amount of energy reaching the site of the mutagenic reaction. Among 

 higher organisms suited to the genetic analysis of the induced alterations, 

 this is a serious difficulty, and all wave-length comparisons must be 

 interpreted with due regard for the approximations involved in estimating 

 the dose actually applied to the chromosomes whose reactions are 

 determined. 



With Dr-osophila, using the technique of irradiation of the adult fly, the 

 comparison of effectiveness of different wave lengths is not feasible. 

 Mackenzie and Muller (1940) estimate that about 99.9 per cent of the 

 ultraviolet energy is absorbed before the radiation reaches the germ plasm. 

 Even slight differences in the relative loss for different wave lengths could 

 make tremendous differences in their relative intensity at the site of 

 genetic action. With the polar cap technique the absorption loss is very 

 much less, but the technical requirements for the identification of the 

 individual mutation make the method unsuitable for experiments on the 

 scale required for wave-length comparisons. 



With seed plants adapted to genetic analysis, irradiation of the pollen 

 presents some opportunity for the comparison of wave-length effective- 

 ness. Extensive data on the effects of monochromatic radiations have 

 been reported for Antirrhinum and for maize. But even within the single 

 pollen grain, internal filtration results in large and variable differences in 

 the penetration of different wave lengths to the site of the nucleus. 



The maize pollen grain is approximately spherical, with a diameter of 

 about 93 n. Uber (1939) measured ultraviolet transmission, at different 

 wave lengths, for the pollen grain wall and pollen grain contents of maize. 

 The results indicated that, with equal incident energy at wave lengths 

 297 and 265 m^u, for example, the dose penetrating to a point 16 fx beneath 

 the wall is three times as large for the longer wave length as for the shorter, 

 and at a depth of 32 n it is about fifteen times as large. In most of the 

 pollen grains the sperm nuclei are located within this depth range. But, 



