Monstera obliqua (Araceae)
Slender climbing epiphyte on trees, saplings, or shrubs. Juvenile: terrestrial creeper, the leaves entire, exserted and erect, the lamina membranaceous, ovate to lanceolate. Adult stem: green, smooth, terete, 2 – 7 mm thick, the internodes 1 – 6 cm long, occasionally producing hanging and creeping stolons with internodes 10 – 30 cm long. Petiole: 5 – 15 cm long, vaginate for most of its length, the sheath wings deciduous; geniculum 5 – 15 mm long. Lamina: membranaceous, quite variable in shape, ranging from lanceolate 35 cm long and 4 cm wide with an acute base to broadly ovate, 14 cm long and 12 cm wide with a truncate to subcordate base; mostly ovate, somewhat falcate, 8 – 15 cm long and 4 – 6 cm wide, the base oblique, unequal, with one side about twice as wide as the other; usually entire but sometimes perforated, the holes one many in a single series on each side of the midrib; primary lateral veins not prominent. Inflorescences: produced sympodially in groups of six to eight, rarely fewer. Peduncle: terete, 1 – 2 mm thick, 7 – 15 cm long, elongating throughout the development of the fruit. Spathe: green to white when immature, becoming a bright yellow at maturity, 4 – 7 cm long, 3 – 5 cm across when fully open, acuminate or mucronate for 3 – 8 mm at the tip. Flowering spadix: deep yellow, 5 – 10 mm thick, 2.5 - 6.0 mm long. Fruiting spadix: green to olive-green tinged with orange when immature, becoming lighter and finally deep orange at maturity, 10 – 15 mm thick, 4 – 8 cm long; the berries globose, 5 – 8 mm in diameter, capped by the persistent stylar region, lacking trichosclereids, or if trichosclereids present confined to the walls, very rarely present in the stylar region; the berries free from one another.
Monstera obliqua has been divided into a number of putative species, principally on the basis of differences in leaf shape, but these variations are of little taxonomic significance. For example, about 80% of the specimens of this species that I have examined have entire leaves; the rest have leaves which are slightly to profusely perforated. The material with perforated leaves has been separated as M. expilata, but this variation shows no ecological or geographic pattern except that the perforated forms are absent from Panama. Furthermore, some individuals have mostly entire but occasionally perforated leaves.
There is also great variation in the relative width of the leaves, which is correlated with the shape of the leaf base. The shape ranges from narrow, lanceolate leaves with acute bases to broadly ovate leaves with truncate or subcordate bases. The narrowest have been separated as Monstera falcifolia and the widest as M. sagotiana, but again this variation shows no ecological or geographic correlation, and it is impossible to draw a line separating the plants into two or more meaningful groups on this basis.
Jonker-Verhoef and Jonker, in the Flora of Surinam (1952), separated Monstera sagotiana and M. expilata from M. obliqua on the basis of leaf shape. They also described differences in the fruits, M. sagotiana supposedly having a white to yellowish-green spadix and M. obliqua white berries with orange styles. These apparently represent differences in maturity rather than taxonomic differences. In M. obliqua the spathe and spadix are green initially, becoming greenish-white and finally bright yellow. After anthesis the color of the spadix changes from yellow to green, in some cases to olive-green, later becoming pale and finally turning a deep orange at maturity.
In terms of its growth habit and floral and fruit morphology the species is a very coherent one, despite the variability in leaf shape. The deep yellow spathes, the flowering spadices and the orange fruits are equalled in the genus only by Monstera xanthospatha of the Colombian Andes. The fruits of M. obliqua are unique in several respects. The berries are globose and free from one another rather than prismatic and closely pressed together as in the other species. This is probably, in part, a consequence of the small size of the spadix and in particular its very narrow axis. With the exception of a few Panamanian specimens, trichosclereids are absent from the stylar region, though they may be present in the ovary walls. This is clearly related to the failure of the fruits of M. obliqua to break in two at maturity and the consequent loss of the ability to get rid of stylar trichosclereids which protect the developing seeds but would deter a dispersing agent when the seeds are mature. In this respect M. obliqua seems to have lost an adaptation shared by other Monstera species.
Another feature unique to Monstera obliqua is that the inflorescences are usually produced sympodially in groups of six to eight, rather than in groups of one to four.
Some of the larger, perforated individuals of Monstera obliqua approach M. gracilis vegetatively, but the latter species is separated by its lighter coloured spathes and spadices and its prismatic berries with trichosclereids in the stylar region.
The small size of Monstera obliqua enables it to exploit substrates unavailable to other monsteras. It can grow to maturity on saplings, in shrubs and on small twigs of trees; it may even be epiphytic on other aroids. These habitats are more ephemeral than the trunks of large trees, and it seems likely that M. obliqua may grow faster and mature more rapidly than other species. Plants of M. oblique tagged in the wild showed an annual growth rate of 30 – 70 new leaves vs. 5 – 12 new leaves in large-leaved species.