KEY TO SUBGENERA
OF PHILODENDRON
1. Stem of mature flowering plants
with a succession of many leaves terminated by a solitary, or rarely several
inflorescences; petioles with long sheaths encircling the stem at their base.
P. subg. Pteromischum
1. Stem of mature flowering plants
with a succession of short sympodial segments each bearing a cataphyll and
a single leaf with the inflorescence(s) 1 to many and appearing to be borne
in the leaf axils; petioles of adult plants with short, usually inconspicuous
petiole sheath and borne on the side of the stem, not encircling it at the
base.
2. Stems often arborescent;
staminodial zone between staminate and pistillate zones of the spadix
subequal or longer than fertile zone; stamens usually at least 3 times
longer than broad. P. subg.
Meconostigma
2. Stems rarely arborescent,
often scandent; staminodial zone between staminate and pistillate zones
of spadix much shorter than the fertile staminate zone; stamens less than
3 times longer than broad. P.
subg. Philodendron
There are also a number of anatomical
characteristics separating the subgenera. Vegetative buds of Philodendron
subg. Philodendron are always located below the point of overlap in the
sheath margins of the cataphyll whereas they are lacking in P. subg. Pteromischum
(Ray, 1987b). Philodendron subg. Pteromischum is also distinct in
having a style with a shallow compitum with a subepidermal concentration of raphide
crystals (Mayo, 1986,1989) and a total lack of tannin cells in the stamens (Mayo,
1986). In addition, while hypophyllous stem segments are typical for P.
subg. Philodendron they are ambiphyllous, hyperphyllous or peraphyllous
in P. subg. Pteromischum. In addition, P. subg. Philodendron
is characterized by having continuous parenchyma from the cortex to the center
of the stem. In contrast P. subg. Pteromischum has a central cylinder
with a solid ring of fibers around the central cylinder.
While not definitive, there are a
number of other features that normally are useful to separate P. subg.
Pteromischum. Its blades are typically more or less oblong, moderately
thin and typically more inequilateral than P. subg. Philodendron.
The stems of P. subg. Pteromischum are commonly less than 1 cm
diam and frequently with rather long internodes. While there are many members
of P. subg. Philodendron with more or less oblong leaf blades,
these are often cordate or subcordate at the base in P. subg. Philodendron.
In addition many members of P. subg. Pteromischum have stems that
branch and spread away from their support before flowering. This behavior is
rare in P. subg. Philodendron.
Because of the usually conspicuously
sheathed petioles for P. subg. Pteromischum, the subgenus is more
likely to be confused with sterile specimens of Rhodospatha Poepp. than
with the oblong-bladed species of P. subg. Philodendron (and in
such a case the presence of trichoschlereids beneath the epidermis of Rhodospatha
easily distinguish it from Philodendron).