Abstract: Reported in the present paper are chromosome numbers and karyotypes of two monospecific genera, Disanthus Maxim. and Mytilaria Lec. in the Hamamelidaceae. Disanthus cercidifolius subsp. longipes (H. T. Chang)K. Y. Pan, distributed in China, was found to have the same chromosome number as the subsp. cercidifolius in Japan, i. e. 2n = 16, with x=8. The interphase nuclei were of the simple-complex chromocenter type, and the mitotic prophase chromosomes were of the proximal type. Remarkable differences in karyotype were found between the two subspecies: the former with 2n = 2x = 16 = 6m + 10 sm, while the latter with 2n=2x=16=6m + 6sm + 4st (t). The absence of st ( or t) chromosomes in subsp. longipes, together with gross-morphological evidence, suggest that this subspecies be more primitive than the other. Mytilaria laosensis Lec. was found to have interphase nuclei of the simple-complex type, and mitotic prophase chromosomes of the proximal type. The chromosome number was 2n=26 with x=13 and the karyotype was 2n =2x= 26=22m +4sm (1SAT). This is the first chromosome report for this genus. The basic chromosome number, x= 13, might be a new one for the whole Hamamelidaceae. The present authors support the treatment of Mytilaria as an independent subfamily, Mytilarioideae, considering its morphological characters and relationships with the genus Exbucklandla.

Key words: Disanthus Maxim., Mytilaria Lec. Chromosome number, Karyomorphology, Systematics