Biosystematic studies on the genus Isoetes (Isoetaceae) in Japan. III. Variability within qualitative and quantitative morphology of spores

Qualitative and quantitative characteristics of mega- and microspores from all the cytotypes of Japanese Isoetes are described based on the voucher specimens whose chromosome numbers were known. In I. japonica, the hexaploid possessed reticulate megaspores and levigate microspores, while the octaploid and the heptaploid had echinate microspores. Mega- and microspores of the hexaploid and the octaploid were of normal appearance, while those of the heptaploid displayed polymorphism. The tetraploid and the hexaploid of I. sinensis resembled each other, since they both possessed cristate megaspores and echinate microspores. Echinate megaspores and levigate microspores characterized the diploid I. asiatica. The spore size was largely variable within each cytotype, while the size of the megaspores varied more than that of the microspores. The microspore length was closely correlated with polyploid level. In I. sinensis, the mean microspore length of the tetraploid was 27.6 μm while that of the hexaploid was 31.9 μm, hence these two cytotypes were easily distinguishable. In the hexaploid I. japonica, variations in mega- and microspore size displayed geoclinal variation showing a positive correlation (r=0.43-0.55) with the longitude and the latitude of the populations. A palynotogical key for cytotypes is presented.