Somatic mitosis in the root tip ofAllium cepa—A review and a reorientation

Abstract

Despite the obvious truism that a proper understanding of somatic mitosis is basic to the whole field of biology, and particularly to cytology, embryology and genetics, such understanding is confined, for the most part, to the professional cytologist and cytogenetist, and is not to be found in elementary texts, which are about a quarter of a century behind the frontiers of research. Furthermore, there are several important parts in the picture which have remained to the present time unfilled by the cytologists themselves. Three forms are generally used to illustrate mitosis in elementary courses and in advanced histology and cytology-somatic cells of whitefish, gonial cells of grasshopper follicle, and meristematic cells of onion-root tips. Whitefish preparations are excellent for an overall picture showing the asters and spindles, and the four classical stages, viz., prophase, metaphase, telophase and interphase, but the chromosomes are too small for detailed study. The situation in the grasshopper will be treated in another series of papers. My present topic is onion-root tip mitosis, for which there is at present no complete and up-to-date account in the literature, despite the great importance of this form in both pedagogy and research. Chromosome morphology and karyokinesis have been described in the genus by various older workers (1, 2, 26, 27, 57-78), and latterday authors (16, 42, 43, 80) employ Allium in studies of the structure and behavior of the chromonemata, which are particularly clear in many members of the Liliaceae. Belar (3) illustrates his text with a sequence in Allium cepa, and Heitz has investigated the nucleolus