This work has followed the gynoecial classification defined by Mayo (1989) in which six distinct style types were described and illustrated (Fig. 469., Style types in Central America; see Appendix 2: Technical Data on Pistils which is a table for a summary of the style types for P. subg. Philodendron of Central America). Only two species, P. niqueanum and P. utleyanum, have style types which are not yet known. The table also includes the number of locules per ovary, placentation type, number of ovules per locule, ovary size, disposition of ovary, and the nature of the ovular sac when present.
Seemingly the most important morphological features of the pistil from the standpoint of systematic importance are the quantity and distribution of the ovules and the type of style. The number of locules per ovary is of significance but it does not correlate well with sectional differences. See above section on "LOCULES PER OVARY" Other features, such as the shape of the ovary, size of respective parts, shape of the stigma, ovule size and shape, and funicle length have not proven to be important taxonomically.
Placentation type seems to be highly correlated with the number of ovules and therefore plays a role taxonomically along with the number of ovules per locule.
While the stigma is moderately uniform in its superficial appearance, the style is highly diverse in morphological features. Unfortunately these features are largely masked by the stigmatic papillae when the pistil is fresh. Despite the shape of the style, the stigmatic papillae, which cover all or part of the style, may form a stigma of more or less uniform shape.
Style Type A has a compitum (common funnel into which the pollen may be packed, defined as the space between the upper stigmatic papillae and the level at which the stylar canals emerge onto the style surface) with ridged inner walls and a lobed apex with each lobe corresponding to the apex of one carpel. Style Type A is restricted to P. subg. Meconostigma and thus will not be considered further here.
Style Type B (Fig. 469-A) lacks a compitum. Instead the stylar canals open into relatively broad concavities on the style apex. These concavities are arranged in a ring with one hole per locule. Style Type B also has the stylar canal (tubes which lead from the stylar surface to each locule) entering directly into the apex of the locular cavity. Although the style apex may be completely flattened or broadly concave, it is sometimes weakly ridged between the pores of the stylar canals. These ridges meet in the middle of the style and may even form a weak central beak. Some styles also have well developed stigmatic papillae associated with the stylar canals causing the surface to be at least weakly lobed with a single lobe for each locule. In dried condition Style Type B sometimes appears as a button-like structure, somewhat resembling Style Type D. It is therefore important in determining style type to make the comparisons of material at or near anthesis. Style Type B seems to be most closely related to Style Type D and shares with that style type the relatively large stylar canal pores on the surface of the style apex relatively near its periphery. Style Type B is the most common type in Central America, known in at least 70 species. One species, P. tripartitum, though usually having Type D styles, also has both Type B and E styles in some populations of the species (see that species for a discussion of its style type.)
Style type C (Fig. 469-B) is characterized by being decidedly concave or funnel-shaped at the apex with no lobes on the margins of the rim and with the stylar canals arising in a narrow cluster at the base of the funnel. Since the stylar canals are closely clustered there is no central dome (defined as any stylar tissue that lies above the level at which the stylar canals emerge onto the style surface). In contrast to style Types B and D, both of which have rather prominent stylar pores, the stylar pores of Type C differ in being small, sometimes barely visible, and in a generally smaller circle nearer the middle of the style apex. This style type is rare in Central American P. subg. Philodendron, but rather known primarily in P. subg. Pteromischum. Only five species, namely P. correae, P. cotonense, P. ligulatum var. heraclioanum, P. straminicaule, and P. warszewiczii have exclusively Type C styles. Though their pistils are funnel-shaped at the apex, these species have funnels that are generally not so deeply funnelform as are those of P. subg. Pteromischum as illustrated by Mayo (1989). One other species, P. radiatum, has at least one collection with the style type that also has a funnelform apex and is believed to be a Type C style. While the dried style on Croat & Hannon 63414 is distinct and button-like the pores are central in a shallow concavity.
Style Type D (Fig. 469-C) is similar to Type B in that it lacks a compitum and has thick stylar canals emerging in a circle on a flat stylar apex relatively close to the margin of the stigma apex. It differs from Type B in that the style is constricted around the circumference to form a protruded flat "style boss" (defined as a more or less domed, circular, stigma-bearing projection that extends beyond the main part of the style and is separated from it by a short neck) that rises above the general level of the style apex. Thus the style appears to have a short flat neck at the apex. It is from the "style boss" that the stylar canals emerge. The stylar pores are relatively large and borne relatively near the margin of the style apex. With at least 23 species of P. subg. Philodendron in Central America having Style Type D, it is the second most common style type.
It is easy to confuse or misinterpret these two style types if the specimens are not well preserved, especially if the material studied is not from fresh material but rather from rehydrated herbarium material. Species with Type B styles sometimes have styles which dry with a button-like apex resembling those of Type D. At least one collection of P. advena with a Type D style also has a Type B style. Some populations of P. tripartitum have not only Type D styles but Type B and Type E as well (see the discussion of that species for details).
Style Type E (Fig. 469-D) has a slender funnel-shaped to cylindrical compitum with a distinct raised annulus around the upper rim of the funnel. The stylar canals arise in a small cluster at the base of the funnel just as in Type C styles. The latter differs, however, in lacking the rim on the style apex. Type E styles are rare in Central American P. subg. Philodendron, found in only P. granulare and perhaps P. smithii. Philodendron granulare has such an unusual form of the Type E style that it should perhaps warrant its own status. In P. granulare the stylar funnel actually protrudes well above the surface of the style (at least in its dried state). Philodendron smithii was reported by Mayo (1989) as having a Type E style but no rim is obvious with fresh material of the species. It is more appropriately a Type C style. Philodendron tripartitum, though usually with Type D styles, has Type E styles in some parts of its range (see the discussion following that species for details).
Style Type F is narrowly funnel-shaped with a small dome at the base of the funnel around which the stylar canals arise. It is not known among the Central American Philodendron. It is known only from P. burle-marxii, a member of P. sect. Baursia (Mayo, loc. cit.) from Amazonian Brazil.