HABIT AND GROWTH PATTERNS
In terms of growth habit Philodendron is clearly one of the most variable genus in the Araceae (Blanc, 1977, 1977a, 1978, 1980; French & Tomlinson, 1981). Habit ranges from terrestrial to epiphytic or hemiepiphytic and the latter may be primary or secondary hemiepiphytes. Secondary hemieiphytes may be vines or appressed-climbers.
Relatively few Philodendron species are terrestrial, though a few species are consistently terrestrial. These include P. glanduliferum, P. grandipes, and P. malesevichiae, perhaps also P. hammelii (known only from a single collection). The stem of P. glanduliferum (Fig. 198) is usually repent Other species are mostly terrestrial, but sometimes hemiepiphytic. Such species include P. basii, P. roseospathum var. roseospathum, and P. warszewiczii. Philodendron knappiae is about equally terrestrial or hemiepiphytic, depending on the situation. Label data regarding habit on herbarium collections is often suspect with many herbarium labels using the term "epiphyte" when in fact the collections were probably rooted in the soil and were therefore technically hemiepiphytic. In addition, although many species have collections reporting them to be terrestrial the majority are probably collections found on the ground as a result of accidentally falling from trees. Many collections are naturally made in virgin forest areas where man-made disturbances such as road building give access to the forest. In such areas, and especially in the regrowth along road cuts aroids are very common. Many Philodendron species, for example P. lentii (Fig. 261) and P. squamicaule (Fig. 382), persist in excellent condition on steep roadbanks because the steep clay slopes allow adequate drainage for the plant to survive. Still, these are somewhat less than normal situations. On the other hand stream banks often serve the same purpose and the situation there regarding aroids is very similar.
Most members of P. subg. Philodendron are hemiepiphytic, meaning that they are growing on trees as appressed climbers or as vines, while being rooted in the soil. There are two types of hemiepiphytes (Putz & Holbrook, 1986). Primary hemiepiphytes, the first type, begin their life when seeds germinate on the host tree, in the same way as true epiphytes, then go on to develop slowly until they reach sufficient age and size to begin developing long aerial roots which eventually reach the ground. This growth strategy enables the species to quickly attain a height where light is adequate but alternatively nutrients are more difficult to obtain. Not surprisingly, one of the most successful species with this life form is P. megalophyllum Schott which often lives on ant nests from which it obtains extra nutrients. No Central American species are known to live exclusively with associated ants though occasional ant nests occur among the cluster of roots of many species. The roots serve to hold the otherwise fragile ant nest to prevent it from being washed away in the rain. Examples of this type of hemiepiphytes are P. goeldii G. Barroso, P. solimoesense A.C. Sm., P. megalophyllum (all from South America) and P. radiatum (Fig. 1), the only species known with this growth form in Central America. The second type start their lives on the ground or on tree trunks near the ground (where they send roots to the ground) and climb trees where they become adults and may lose their connection with the ground. These are referred to as secondary hemiepiphytes (Putz & Holbrook, 1986). Most Philodendron sect. Philodendron are members of this group.
Though the majority of Central American members of P. subg. Philodendron are secondary hemiepiphytes, there is a wide spectrum of growth patterns. Generally germination is in the soil and plants grow in a creeping fashion with long, slender internodes until a suitable tree is located. It may be presumed that, as with Monstera, growth is scototropic (Strong & Ray, 1975), i.e., seeking darkness, such that the plants at this stage of development grow not toward the brightest area on the horizon but rather toward the darker outline of trees. Once a plant reaches a tree or rock on which to climb growth proceeds upwards and a prompt physiological change takes place which causes the plant now to grow toward the more well illuminated regions higher up the trunk. As growth progresses and the plant gets higher and closer to the light filtering through the canopy, the internodes become shorter and thicker (Ray, 1981, 1987, 1987a). Ray (1987) reported that during the growth of hemiepiphytic climbers, the switching between the leafy and elongate phases of growth during the arboreal phase of growth is controlled by the substrate and the gain or loss of contact by the plant with the vertical support.
Most Philodendron have seed germination taking place on trees, followed by full development in situ. In such cases, though root development might be extensive, trapping debris containing nutrients, these roots do not extend to the ground.
As is the case with most Araceae, P. subg. Philodendron has homeophyllous monopodial growth (monophyllous sympodial of Ritterbusch, 1971) with each article bearing a cataphyll and a leaf and, when the plant is mature, each article is terminated by an inflorescence (Ray 1986, 1987b) or its aborted remains (Engler, 1877; Rittenbusch, 1971). The branching pattern for P. subg. Philodendron has the prophyll following each internode suppressed, with a single foliage leaf followed by the inflorescences and with the internode to the prophyll of the elongation shoot being suppressed. Growth patterns for P. subg. Philodendron were diagrammed schematically by Engler (1877) and by Ray (1988) and are discussed in detail by French & Tomlinson (1981). In contrast to P. subg. Philodendron, P. subg. Pteromischum has monopodial growth only up until the time of flowering when it is followed by anisophyllous sympodial growth, i.e., the stem articles bear a variable number of leaves before producing another inflorescence. Each new branch forms from a bud in the "axil of the penultimate leaf of the previous unit" (French & Tomlinson, 1981). In contrast, P. subg. Philodendron has development of the stem beyond each terminal inflorescence. Thus though P. subg. Philodendron appears to have an unbranched stem with an inflorescence in each leaf axil it is really producing a new branch after producing each leaf. Close examination will show that a bud for the renewal shoot occurs on each article just below the point of overlap of the sheath edges of the cataphyll (prophyll of Ray, 1987a). It is from this point that the new branch will form. There is also a second, supernumerary bud which lies below (proximal to) the bud for the renewal shoot. This acts as a reserve meristem, lying dormant unless the stem is severed just distally to it (French & Tomlinson, 1984).
Unlike P. subg. Philodendron, P. subg. Pteromischum typically does not branch after the production of each new leaf and therefore lacks comparable branch buds while the inflorescences are usually produced terminally on determinate lateral branches (Rittenbusch, 1971; Blanc, 1978; Madison, 1978; French & Tomlinson, 1981).
Internode length and width are altered markedly as the plant climbs, with the internodes getting ever shorter and thicker. Ray (1986) has shown a direct correlation between the length and width of an article (one segment of the stem) demonstrating that it is fixed and varies according to a set pattern). Though of no known significance (Ray, 1987b) categorized four different types of stem segments based on where the leaf was attached to the stem. In P. subg. Philodendron all species had the petiole attached to the lower end of the stem segments and these are referred to as "hypophyllous segments". Most species of P. subg. Pteromischum have "hyperphyllous segments" where the petiole is attached to the upper end of the stem segment. "Ambiphyllous segments", where the stem segment is so short that the petiole is attached across the entire segment is also known in P. subg. Pteromischum and is also the most common type elsewhere in the family. The fourth type of sympodial stem segment, referred to as an "peraphyllous segment" and presently known in the family only in P. subg. Pteromischum, has the stem segments elongated and extended below the point of attachment of the cataphyll.
Most members of P. subg. Philodendron have leaves which turn from juvenile to adult gradually as the plant climbs so that there is no marked transition where one can at once recognize the adult foliage.
Although not as pronounced as for those of Monstera or Syngonium, some species have dimorphic leaves. For example, in P. hederaceum var. hederaceum juvenile leaves have short petioles which are tightly appressed to the tree, more or less like the "shingle" leaves of Monstera. They are also dark blackish green and velvety above and often somewhat purplish violet beneath. Alternatively the adult plants have spreading leaves with longer petioles and smooth semiglossy blades. The juvenile blades seem to be associated only with the earliest growth. Once the plant turns into an adult, further growth, even when it represents a reversion to smaller leafed forms, results in the same smooth semiglossy texture as that of the adult.
Philodendron hederaceum and other scandent species tend to climb high up the trunk of the tree then spread into the canopy and finally often hang down from branches before they flower. Philodendron jacquinii often has a similar habit. Some scandent plants, such as P. brevispathum and P. sulcicaule typically sprawl over lower vegetation rather than high in the canopy.
The amount of internode elongation varies immensely with some vining species, such as P. immixtum, P. hederaceum, and P. sulcicaule, having internodes 10-20 cm long (even longer on plants that are juvenile or which have reverted to a "searching mode" as the result of dislodgement from the tree. Usually internode length varies considerably even with individual adult species depending on the light and nutrients available for growth. Even fully adult plants with very short internodes can be induced to produce longer internodes in cases where the plant is accidentally displaced from its growing situation or if it becomes heavily shaded.
Some species have evolved the ability to produce flagellate branching in an attempt to reposition themselves. These have long slender internodes and reduced leaves. At first the flageliform growth spreads laterally but if no other growth suport is in the immediate vicinity the branch inclines toward the ground where it creeps across the surface of the ground and ascends another tree. Blanc (1980) referred to these as "flagelles" or "stolons". Ray (1987b) indicated that these flagellate branches are usually developed when a plant overgrows its support or is accidentally dislodged, but in some species, e.g., P. fragrantissimum, the flagellate branches may develop from a normally growing plant (Croat, 1978). Philodendron linnaei Kunth, a South American species, has developed this method of locomotion to an extreme. That species produces a series of rosulate clusters of leaves all interconnected by more slender flagellate stems which ascend the tree trunk. After a period of rapid growth resulting in long, narrow internodes with the leaves reduced to small scales, the plant produces a series of short thick internodes, each of which is associated with an increasingly larger leaf, and leaves arranged in a tight rosette (Ray, 1987b).