OVULES PER LOCULE
The number of ovules present in each locule varies from 1 to several to numerous per locule. Mayo reported numbers as high as 51 per locule for P. subg. Meconostigma but in P. subg. Philodendron the highest number found for any Central American species is 36 per locule reported for P. fragrantissimum. Only four other species, P. antonioanum, P. panamense, P. squamipetiolatum and P. verrucosum have 30 or more ovules per locule and only 17 species have 20 or more ovules per locule (but fewer than 30) (see Appendix 2 TECHNICAL DATA ON PISTILS). Eighteen species have between 10 and 19 ovules per locule.
In all 56 species (62 taxa) have fewer than ten ovules per locule. Most species with 10 or more ovules per locule have axile placentation, the exceptions being P. brenesii, which may have up to 12 ovules per locule and P. davidsonii with up to 10 ovules per locule, (both having sub-basal placentation). Alternatively, a number of species with fewer than 10 ovules per locule also have axile placentation. These include P. brevispathum (with as few as 6 ovules per locule), P. chirripoense (with as few as 8), P. coloradense (with 4-7 ovules), P. crassispathum (7-10), P. dressleri (3-4), P. gigas (3-4), P. jefense (6), and P. roseospathum var. angustilaminatum with 6-9 ovules per locule.
The remaining species have both few ovules per locule and basal or sub-basal placentation. This group is a large, apparently unnatural assemblage constituting 47 species (48 taxa). Two species in the group, P. ferrugineum and P. sagittifolium, are also reported with axile placentation.
Along with the morphology of leaves, the number of ovules per locule has long been used as a principal means of assigning sectional affinity to Philodendron (Engler, 1878, 1879, 1899; Krause, 1913; Mayo 1989). Central American members of P. subg. Philodendron generally fall into three categories in terms of numbers of ovules per locule. Many species, including most members of P. sect. Calostigma, P. sect. Tritomophyllum and P. sect. Baursia, have one to few ovules (rarely to 4 or 5 but always with basal or sub-basal placentation) per locule (marked with code "a" in Appendix 2. Forty-four taxa in 47 species belong to this category. A second group (marked with code "b" in Appendix 2) with axile or sub-basal placentation, generally has 4-10 ovules, but sometimes up to 14 ovules (but if so with ovules varying down to 6 per locule). Nineteen species fall into this category. The third group has exclusively axile placentation, mostly with many ovules per locule, usually 15 or more, sometimes as few as 12 per locule and rarely as few as five when other locules in the same spadix have up to 20 ovules per locule. The groups may not be natural ones (though it is more likely to have resulted from inadequate sampling owing to lack of material) since in two species, P. davidsonii and P. roseospathum, one variety of each species belongs respectively to both group "a" and group "b".