PHILODENDRON SECTIONS WITH ENTIRE LEAF BLADES
Seemingly the most reliable way to separate species in P. subg. Philodendron is based on number of ovules per locule, a system first devised by Engler stemming from his first revision of Philodendron (Engler, 1878). Engler used the number of locules per ovule to separate two large groups which he called P. sect. Polyspermium and P. sect. Oligospermium (now P. sect. Philodendron and P. sect. Calostigma respectively) These two sections together comprise the largest percentage of species in the subgenus. As they are constituted they are very diverse morphologically and it is possible that the number of ovules per locule will not prove to be reliable for separation at the sectional level. It is possible that species with relatively few ovules or solitary ovules may have evolved independently several times from ancestors with numerous ovules having axile placentation. Since it has not yet been determined if this is the case, the classification system used here will in general be a conservative one.
Krause (1913) closely followed Engler's sectional revision. His P. sect. Philodendron (as P. sect. Polyspermium) consisted of species with axile placentation and "many" ovules per locule while P. sect. Calostigma (as P. sect. Oligospermium) consisted of those species with sub-basal (or less frequently basal) placentation with "1 or few" ovules per locule. Since the time of the last revision of Philodendron many species have been added and the number of ovules per locule provide a much wider spectrum than was known to Engler (that is to say that the number of ovules per locule provide a continuous distribution) but there is still a significant correlation between axile placentation and moderately large numbers of ovules per locule and the converse, basal and sub-basal placentation and relatively low numbers of ovules. Both P. sect. Philodendron and P. sect. Calostigma will be discussed below.
While these two groups, P. sect. Philodendron and P. sect. Calostigma, constitute the largest percentage of Central American species, several other sections are separated on the basis of leaf shape, venation and style type (see above key). Each of these will be discussed in turn. Krause (loc. cit.) treated ten sections in his revision of Philodendron. As previously discussed, P. sect. Pteromischum has been elevated to the status of subgenus and P. subg. Macrolonchium Engl. has been reduced to a subsection of P. subg. Philodendron.