1. Relation between knobs and chromocenters of interkinetic nuclei. - Resting nuclei of maize stained with Feulgen contain discrete, deeply-staining bodies in addition to diffuse chromatic material. These deeply-staining bodies are called chromocenters. A good correlation was found between the number of chromocenters in the interkinetic nuclei and the number of knobs present in the pachytene chromosomes. In strains free from conspicuous knobs but possessing B chromosomes a good correlation was found between the number of B chromosomes and the number of chromocenters. The chromocenters derived from B chromosomes are not as large as those from some of tho larger knobs -- evidently all of the heteropycnotic material observed in the B chromosomes at pachytene is not represented in the chromocenter. That portion of the B chromosome immediately adjacent to the centromere of the B is more knob-like in appearance than other portions of the chromosome and it is believed that it is this proximal portion which forms the chromocenter. Plants free from conspicuous knobs and B chromosomes have a great majority of their interkinetic nuclei free from any structures which might be interpreted as chromocenters (except for the two nucleolar organizer regions on chromosome 6). That chromocenters often fuse is indicated by the range in number and size. Strains with knobs of approximately uniform size have chromocenters of uniform size -- barring fusion -- while strains with different sized knobs have a marked range in size of chromocenters. The data obtained are summarized in the following table.
| Tissue studied |
Strain | Knob No. at pachytene |
B chrom. No. | Number nuclei counted |
Mean No. chromo-centers |
Range in number |
Modal class |
| root | A | 9 | 0 | 100 | 8.16 | 4-11 | 8 |
| style | A | 9 | 0 | 100 | 8.00 | 4-12 | 8 |
| root | B | 6 | 0 | 100 | 5.05 | 2-8 | 5 |
| root | C | 6 | 0 | 100 | 5.22 | 2-6 | 5 |
| root | D | 0 | 4 | 100 | 3.27 | 1-5 | 4 |
| root | E | 0 | 0 | 100 | 0.22 | 0-1 | 0 |
Occasionally the number of bodies classified as chromocenters was greater than the number of conspicuous knobs. This may be due to the misclassification of diffuse heterochromatin as chromocenters or more likely to the failure to distinguish small knobs at pachytene. Fusion of two or more of these small knobs might give rise to recognizable chromocenters. In every strain studied the number of chromocenters was determined before that of knob number. All preparations were stained with the Feulgen reaction.
D. T. Morgan, Jr.