1998 Volume 48 Issue 3 Pages 257-264
Shoot morphology and growth characteristics of two morphologically similar submerged plants, Ranunculus nipponicus var. submersus and R. nipponicus var. okayamensis, were compared to clarify their morphological and ecological differences. Under both field and experimentally equalized conditions, these two plants were clearly distinguishable from each other on the basis of leaf and peduncle size by separating the reproductive and vegetative shoots, and also by leaf color during winter. Both plants showed continuous shoot growth in thc field, and seasonal change was higher for R. nipponicus var. okayamensis than for R. nipponicus var. submersus, possibly because of fluctuations of water temperature in the field. While the flowering of R. nipponicus var. submersus occurred all year round with a peak during June and July, that of R. nipponicus var. okayamensis was seen only during May and November. As to the effect of temperature on shoot growth, both plants showed thermophobic traits, and no significant difference. The elongation rate of R. nipponicus var. okayamensis was higher than that of R. nipponicus var. submersus in both of the water sampled from the Hongu River and the Utsumiya Rive, which are the natural habitats of R. nipponicus var. submersus and R. nipponicus var. okayamensis, respectively. The present results provide support for the suggestion that these two plants may be varieties rather than ecotypes.