INFRAGENERIC RELATIONSHIPS WITHIN PHILODENDRON
Philodendron is currently divided into three subgenera. A subgeneric system of classification for Philodendron was proposed as early as 1832 by Schott (Schott, 1832) who recognized four groups which remain without rank, Euphilodendron, Calostigma, Meconostigma and Sphincterostigma. The latter two were combined by Engler (1899) as P. subg. Meconostigma. Schott's Calostigma was later called P. sect. Oligospermium Engl. (Engler, 1878) and is once more called P. sect. Calostigma [(Schott) Pfeiffer] (Mayo, 1990). Euphilodendron became P. sect. Polyspermium in Engler's Flora Brasiliense treatment in 1878 and must now be treated according to the rules of nomenclature as P. sect. Philodendron (Mayo, loc. cit.).
It was not until Kunth's 1841 treatment for `Enumeratio Plantarum' that members of what are now called P. subg. Pteromischum were removed from Monstera Adans. and placed in Philodendron. Schott recognized Pteromischum as a grex in his 1860 Prodromus and Engler first recognized the species occurring in this group as P. sect. Pteromischum in his Flora Brasiliensis treatment (Engler, 1878).
Phylogenetic and phenetic analyses by Mayo (1986, 1988) have shown Philodendron to have three distinct subgenera which are distinct in vegetative and floral morphology, floral anatomy and to some extent by distribution. Philodendron subg. Meconostigma, with a predominantly southeastern South American distribution is highly apomorphic but cladistically primitive in the genus (Mayo, loc. cit.). Based on a study of gynoecial morphology Mayo considers that the P. subg. Meconostigma evolved in eastern Brazil as a group adapted to open habitats and later spread into the more humid Amazon basin. By the same standard he assumed that P. subg. Philodendron and P. subg. Pteromischum also arose later and became predominant as hemiepiphytes in humid forests. He considered P. subg. Pteromischum to be a sister group to P. subg. Philodendron and that P. subg. Philodendron is the most advanced of the three subgenera. The geological history of the continent would probably agree with this since eroded mountain plateaus of eastern Brazil are much older than the current land surfaces of the Amazonian basin. Most of the speciation of the genus, now so rich on the Andean slopes of northern and western South America surely must have evolved since the Andes arose during the late Cenozoic.
Mayo elevated P. sect. Pteromischum to the status of subgenus (Mayo, 1989) and Grayum (1996) has subdivided the subgenus into two sections, P. sect. Radicantia Grayum with sylleptic sympodial growth and P. sect. Fruticosa Grayum with proleptic sympodial growth (Ray, 1987b) a growth form that is rare in the family, known only in Alocasia (Schott) G. Don and a few species of Monstera (Grayum, in press).
The P. subg. Philodendron is difficult to define and though very distinct at the generic level (See section on "Suprageneric Relationships" for separation from similar genera Dieffenbachia and Homalomena) it is primarily defined by its negatives, i.e., it lacks the specific characteristics of P. subgenera Pteromischum and Meconostigma (see key to subgenera below). There are relatively few members of P. subg. Philodendron with a pachycaulous habit common to so many members of P. subg. Meconostigma, i.e., with very stout, generally erect stems and possessing conspicuous leaf scars. Philodendron subg. Philodendron also lack the conspicuous, more or less triangular scales borne in the leaf axils of P. subg. Meconostigma. Though sometimes obvious (Fig. 11) in P. subg. Philodendron, they are usually small and inconspicuous and fall early. The species most similar to P. subg. Meconostigma is P. warszewiczii but another species, P. basii is similar in being large with a thick, erect stem.
There are species in P. subg. Philodendron that may have somewhat elongated blades while in juvenile condition, but adult plants, even those with oblong to elliptic, mostly non-cordate blades (which are so typical of P. subg. Pteromischum), never have conspicuously sheathed petioles. Species of P. subg. Pteromischum have suptle characteristics that to the expert permit immediate recognition. These characters include the slender, somewhat woody, brittle stem, a conspicuous petiole sheath, thinner blades with rather pronounced primary lateral veins, the presence of interprimary veins, the frequent presence of raphide cells or stitch-like markings. Another feature which is often useful in separating P. subg. Pteromischum from P. subg. Philodendron is the much higher incidence of assymetrical leaf blades in the former. Assymetrical blades, though sometimes present in P. subg. Philodendron, especially at the base, are not common.
The three subgenera of Philodendron in general can be most easily separated by the characters presented in the following key (modified after Mayo, 1991):