With the use of the light and electron microscopes, the chromosomes of Fritillaria lanceolata and Scilla sibirica are shown to differ in respect of the heterochromatin they contain. In root meristems of the former, the heterochromatic regions (H-segments) were recognizable at all phases of the mitotic cycle by their slighter opacity to electrons than that of euchromatic parts. This was due both to less tight packing of the chromatin fibrils and lower opacity of the fibrils themselves, even though both had the same diameter, about 3 nm.

In root tips of the Scilla, the heterochromatin was invariably of similar opacity to euchromatin and thus only recognizable at telophase and interphase as large chromocentres. The opacity to electrons in the heterochromatin of the 2 species, was at all times closely paralleled by the staining behaviour seen with the light microscope in sections (0.07-0.5 µm in thickness) stained with toluidine blue.

The disparity in the Fritillaria, as seen in sections with the light microscope, in respect of the stainability of the hetero- and euchromatin, was masked in Feulgen squash preparations of root tips from plants grown at 18-20 °C; at metaphase by the thickness of the chromosomes and at interphase by the density of the chromocentres. When, on the other hand, the plants were grown for 4 days at 2 °C, the masking effect of thickness was circumvented in metaphase chromosomes by differential super-contraction in euchromatin.

The implications of these findings in respect to previous interpretations of the differential reactivity of H-segments to low temperature, as well as the phenomenon of enhanced and reduced fluorescence in these segments with fluorochromes are discussed.

*Present address: School of Biological Sciences, University of East Anglia, Norwich England.