No karyological studies were made for this revision but Petersen (1989) reviewed all chromosomal literature and made new studies. The chromosomes of P. subg. Philodendron are small, with counts of 2n=30, 32, 34, and 36 (rarely 26 and 48). Petersen (loc. cit.) speculates that the base number for the genus is 18. Coincidentally very few of the 29 species (other names were synonyms or hybrids) for which chromosome counts have been reported are members of the genus from Central America. Those Central American species for which chromosome counts have been reported are P. microstictum (as P. pittieri) 2n=34, P. radiatum 2n=32, P. hederaceum 2n=36, and P. verrucosum 2n=34. Philodendron wendlandii was counted at 2n=54 but Petersen believes this to be either an error in counting or not a Philodenron.