PLACENTATION

Placentation type for P. subg. Philodendron in Central America is usually axile or sub-basal with 50 species having sub-basal placentation and an additional five species with basal placentation. Those with basal placentation include P. dwyeri, P. granulare, P. smithii (also sometimes sub-basal), P. sousae (also sometimes sub-basal), and P. zhuanum. Forty-seven species have axile placentation and two additional species, P. ferrugineum and P. sagittifolium, with mostly sub-basal placentation, sometimes have axile placentation when they have larger numbers of ovules per locule.

Some general rules regarding placentation type and number of ovules per locule are the following: species with basal placentation have only a solitary ovule per locule and tend to have a few large seeds per berry. Species with sub-basal placentation usually have only one to a few ovules per locule (averaging 2.1 to 3.5 respectively for the minimum and maximum number in the range) and typically fewer than six but rarely to ten, as in the case of P. davidsonii var. davidsonii and P. fortunense or to 12 as in the case of P. brenesii or P. davidsonii var. bocatoranum. These species tend to have rather large seeds as well, although sometimes seeds are quite small. Alternatively, species with axile placentation usually have ten or more ovules per locule (averaging 14-18 respectively for the minimum and maximum in the range) but sometimes as few as three per locule in P. dressleri and P. warszewiczii, as few as 4 in P. coloradense and P. knappiae, 6 in P. brevispathum and P. llanoense, or 7 per locule in P. crassispathum.

For those species with only a few ovules per locule the disposition of the ovules seems to be unorganized or digittate, but for those species with more than a few ovules, the arrangement on the placenta may be in a single series or more generally in two or less frequently in 1-2 or 2-3 series (See Appendix 2. TECHNICAL DATA ON PISTILS). Mayo (loc. cit.) reports that, based on his studies of the gyneocial morphology and anatomy of 15 mostly South American species, the placenta of even those species whose ovules appear to be in a single row might actually be biserriate with the funicles inserted alternately along the placental ridge. This is also true in Central American species. Forty Central American taxa of Philodendron proved to have a biserriate placenta, while much smaller numbers appeared uniseriate (13), 1-2-seriate (9) or 2-3-seriate (11) or digitate (4). Twenty-six species lacked any appreciable organization owing to the small number of ovules and one species, P. niqueanum, was not studied owing to inadequate material.

Species with sub-basal placentation having two or more ovules per locule often have funicles of different lengths. This enables the ovules to be spaced, a critical feature, since maturing fruits would impinge on one another's space as they enlarge. In addition, the funicles sometimes get longer or are stretched in fruit to accommodate the great increase in the size of the berries. The funicles of species with both sub-basal and axile placentation is generally only loosely attached to the wall of the ovary and often the funicle can easily be pulled loose to or near to the base. Mayo (loc. cit.) states that it is assumed that basal and sub-basal placentation was derived from axile placentation by reduction, but the observation that the funicle on many of those species with axile placentation can be easily pulled free to the base would argue against this. It would seem from the standpoint of economy alone, that the stylar canals would enter the locules from higher up on the locule wall to avoid traveling an unnecessary distance.

Funicles often bear a band of glandular trichomes at or near the base. These were well illustrated by Engler & Krause (1913) who reported them commonly in P. sect. Baursia and P. sect. Calostigma but also with a few in P. sect. Philodendron as well. These small glands can only be seen under high magnification and probably serve to secrete mucous into the ovary to prevent the ovules from drying out.

Funicles are often fused into a thickened, sometimes ramified, more or less translucent placenta. This structure serves as a tough membrane which allows the entire placenta and its associated funicles and ovules to be removed, making counting them less difficult. Sometimes however, adjacent locules share a common trunk so that care must be taken to insure that one is not removing the contents of two locules.

Funicles are typically about as long as the ovules themselves, sometimes slightly longer or shorter. The free portion of the funicle on species with axile placentation seems to be proportionately shorter than the free portion of those with sub-basal placentation. This is perhaps because those species with axile placentation have somewhat longer locules allowing greater spacing of the ovules. Those species with sub-basal placentation and three or more ovules per locule have the funicles arising from one small area near the base of the axile wall and often have much smaller locules than those with axile placentation. Some of the ovules need to be more separated for proper development, and funicles of different lengths allow for this. Thus in many cases there are longer funicles for those species with sub-basal placentation. The generally smaller locule size for species with sub-basal or basal placentation is confirmed by a survey of the locule size. Species with basal or sub-basal placentation have locules ranging in length from 0.32 mm in P. jefense to 9.5 mm in P. warszewiczii and the average minimum length is 1.9 mm while the average maximum length is 3.5 mm. Alternatively, locules range in length 0.40 in P. bakeri, P. sousae, P. sulcicaule, and P. wilburiivar. longipedunculatum to 0.7 mm in P. advena and the average minimum length is 1.12 mm long while the average maximum length is 1.75 mm long.