The loops and ultrastructure of the nucleolus of Ipheion uniflorum.

Abstract

1. The nucleolus of Ipheion uniflorum was studied in root tips and endosperm in respect of structure of internal loops made visible with phase-contrast by means of the detergent “Tween 80”, and with the electron microscope in ultrathin sections of root tips, in respect of ultrastructure. 2. Two loops were observed in each large fused nucleolus in the diploid root tissue and three in those of the triploid endosperm. 3. The loops in this species, which are confined for the most part toward the centre of the nucleolus, attain a length of about 20 μ in nucleoli 15 μ in diameter, and irrespective of nucleolar size a thickness of about 1.4 μ during interphase. 4. Strands, containing DNA as shown by application of DNase to ultrathin sections, and whose staining behaviour with “electron stains” was similar to chromatin, were observed with the electron microscope in a channel of irregular width which evidently runs throughout each loop axis. 5. These strands, which were sometimes seen in large clusters up to 0.13 μ across, appeared to have a common diameter of about 200 A, derived from coiling of a fibre slightly less than 25 A. 6. The strands were flanked by two other loop elements: a unit 20 A-wide coiled singly in strands 100 A in diameter and another of duplex character 50 A-wide consisting of loosely coiled 20 A subunits; the duplex units were often seen running in closely parallel groups. 7. Nucleolar vacuoles, common to all species, appeared spasmodically in the loop mass, seemingly caught between the loops. 8. Particles, averaging 250 A in diameter and containing a tightly coiled fibril slightly thicker than 20 A, were seen outside the channel and vacuoles in the loop mass, in all or most of the nucleoli of sections of root tips from two fixations. 9. In sections of most fixations, particles (nucleolar ribosomes) averaging 150 A in diameter were seen in association with 100 A-wide threads which merged with the loop elements at the periphery of the nucleolus; both the particles and threads contained fibres 15–20 A in diameter, coiled to give their respective structures. No clear evidence was found in support of the premise that these nucleolar particles represent segments of the threads with which they are associated. 10. In sections of root tips which had been RNased between fixation and dehydration, the 150 A peripheral particles and 50 A duplex loop elements were not seen. It was not certain whether some other major elements were affected.