Endomitosis in tapetal cells of some cymbidiums (Orchidaceae)

True endomitosis in the anther tapetum of the three Cymbidium species viz. C. aloifolium, C. devonianum and C. tigrinum is described. In these Cymbidiums, most tapetal cells go through endomitosis instead of either normal mitosis or so called "inhibited" mitosis. The nuclear membrane does not disappear, but during metaphase the chromosomes are considerably condensed far more than in normal mitosis, and nucleolus persists throughout the endomitotic cycle. Endomitosis may not be unusual to the tapetal cells of these peculiar Cymbidiums but may have a wider application and explain many of the cytological phenomena occurring in the tapetal cells of other plants. Further, physical localization of 45S rRNA genes during endomitotic divisions may help to confirm their atypical activities as well as provide insight into the course and degree of tapetum polyploidization. Analysis of DNA-methylation levels is also recommended for understanding the role of nucleolus for spindle formation during endomitosis.