Stems vary considerably in length depending on the nature of growth. Vines, such as P. hederaceum, may produce stems which are well over 30 m long whereas the more slow-growing appressed climbing hemiepiphytes rarely have stems more than about 2 m long. The youngest part of the stem bears most of the leaves and has the newest root growth including the most active feeder roots. Older portions of the stem have thick, strong roots which are usually tightly fastened to the tree, anchoring it in place. Some roots all along the stem extend downward along the stem and may lead all the way to the ground. The older portion of the stem is often devoid of any leaves and usually lacks even cataphyll fibers. The bare stem clearly shows the petiolar and cataphyll scars (Figs 9-12). See discussion below. Inspection usually shows that the older portions of the stem have been at some time attacked by root borers so there may be surface damage or more likely active root borers in the center of the stem. Eventually the lower part of the stem rots away as the stem climbs higher on the side of the tree.
Stems of Philodendron are typically unbranched, but may be branched naturally as in many species of P. subg. Pteromischum or through injury. Commonly internodes become increasingly shorter and thicker on appressed-climbing plants but some species, e.g., P. fragrantissimum, may from time to time begin to produce long slender internodes to enable the plant to enter a colonizing mode (See above section on "Habit and Growth Patterns").
Mayo (1991) described differences in the branching pattern of P. subg. Philodendron and P. subg. Meconostigma. While most P. subg. Meconostigma have very short internodes, some species, e.g., P. leal-costae Mayo & Barroso and P. corcovadense Kunth have elongated internodes. In these species the elongated internode of each article is the one between the cataphyll (prophyll of Ray, 1987a) and the succeeding foliage leaf while the internode preceding the cataphyll is abbreviated. In P. subg. Philodendron the elongated internode is the hypopodial internode (that proceeding the cataphyll, while the internode between the cataphyll and the foliage leaf is the abbreviated one.
Stems of Philodendron are typically rich in taxonomic characteristics and together with their associated cataphylls they yield some of the best key characters for identification. Normally these stem characteristics are useful only at the specific level but in P. subg. Pteromischum Grayum (1996) has found the dried stem color to be useful in separating the two sections of P. subg. Pteromischum. Unlike most Anthurium species which have internodes so short that the epidermal features are for the most part obscure or very uniform, Philodendron has features which are important both before and after drying. Internode length and width, though variable with age, is relatively fixed at the time of flowering, so that vines still have relatively long internodes even at flowering time (or at least a few nodes below the inflorescence have moderately elongate internodes). Appressed-climbing hemiepiphytes typically have short, thick internodes at the time of flowering. For most species of appressed-climbing hemiepiphytes the internodes are actually broader than long at maturity (Fig. 10, Fig. 394, 443) but internode length varies with some species such as P. advena (Fig. 38), P. platypetiolatum (Fig. 308), P. pseudoauriculatum, P. roseospathum and P. squamicaule, P. straminicaule Fig. 392) among others which have internodes typically longer than broad but at the same time they are short enough that the plant is not considered scandent (Fig. 38). Only nine species in Central America have internodes longer than 25 cm on adult or preadult plants. Only three additional species, P. clewelii, P. immixtum and P. sulcicaule have internodes which even reach 15 cm in length. Fourteen additional species have internodes which reach or exceed 10 cm long (but are shorter than 15 cm long). Alternatively many species have short internodes with 33 Central American species having internodes only to five cm long or less.
Internode width is somewhat less variable, ranging from less than 3 mm diam. in P. chirripoense to 10 cm diam. in P. gigas. Only nine additional species have stems which attain diameters greater than 7 cm. These are P. chiriquense, P. copense, P. dressleri, P. ferrugineum, P. fortunense, P. grayumii, P. pterotum, P. schottianum, and P. warszewiczii. An additional nine species have stems which attain up to 5 cm diam. or more. These are P. advena,
, P. basii, P. davidsonii, P. dominicalense, P. edenudatum, P. lazorii, P. llanoense, and P. pirrense. Alternatively most species have relatively narrow internodes with at least 49 taxa, involving 44 species, having internodes of no more than 3 cm diameter and 16 species have internodes of less than 1.5 cm diameter.
Fresh stems often have characteristic surface features. Surfaces are frequently speckled with a lighter or darker pattern of green. The surface may also be short-lineate (Fig. 2) or may represent a combination of short lines and speckles (Fig. 3). At other times the surface may be weakly to prominently striate as in P. anisotomum, P. hederaceum, and P. wilburii. The stems may be glaucous as sometimes in P. dodsonii, P. fortunense, P. immixtum, P. ligulatum, P. mexicanum, and P. wendlandii. Frequently the internodes are coarsely short-costate (Fig. 2). near the apex of the internode, as in P. chiriquense, P. grandipes, and P. schottianum. These apparently represent areas at which roots will emerge later.
Stems are commonly smooth but may be minutely wrinkled, as in P. verrucosum, or sparsely warty, as in P. brunneicaule, and usually glabrous but may be setose as in P. jacquinii or with often branched, trichome-like scales (Fig. 5) as in P. brevispathum. Other species with scaly stems include P. hammelii, P. malesevichiae, P. squamicaule, P. squamipetiolatum, and P. verrucosum. Even species having stems with smooth epidermis are often weakly fissured even in fresh condition (Fig. 2).
Coloration of fresh stems may also be characteristic, typically medium to dark green when fresh and turning gray-green to brownish, yellow-brown or reddish brown in age. Often the transition goes through more than one color before the final stem color is reached, e.g., medium green to gray-green, to yellowish green and finally yellowish or reddish brown. This final stem color is often the same as the color of the stem of artificially dried herbarium collections but at other times the dried stem color of the herbarium collection is not the same. Frequently it is darker than that which would occur if allowed to dry naturally. Thus it may be important to note the color of the fresh stem before drying takes place. Even before the specimen drying process takes the natural aging of the stems in live condition often causes a scurfy condition which results from numberous close cracks (Figs. 4, 5). Sometimes these minute fissures are restricted to a specific point of stress yet not visible elsewhere (Fig. 5).
Other important features brought out by the drying process is the extent to which the stem wrinkles or cracks or is otherwise distorted by the drying process. The results of the drying process is usually quite consistent from collection to collection such that the dried stem and the features it exhibits provide useful recognition characteristics.
Stems commonly dry with irregular (or sometimes regular) ribs and intervening sulcae simply to allow for shrinkage of the relatively indurate outer surface of the stem compared to the rather soft, somewhat aerenchymatous stem interior. In some cases the regularity and severity of this ribbing provides especially useful characteristics, e.g., P. sulcicaule which becomes prominently ribbed in the course of normal development and P. verapazense where the stems become regularly and conspicuously ribbed on herbarium collections.
In addition to the frequently present longitudinal ribs e.g., P. findens, P. fortunense, and P. heleniae, stems may be transversely fissured or checked with small to large fissures. These in turn may be quite regular or irregular in severity or spacing. While sometimes characteristic, these transverse fissures tend to be somewhat less diagnostic than are the longitudinal fissures. However for some species such as P. edenudatum, P. ferrugineum, P. findens, P. fortunense, P. ligulatum, P. malesevichiae, P. mexicanum, and P. wendlandii (among others) they are frequently transversely fissured. This feature is particularly noteworthy in an undescribed South American species, e.g., P. suberocaulum sp. nov. from the Pacific coast of Colombia.
Another feature exhibited by the dried stems is exfoliating epidermis. Examples of species exhibiting this feature are: P. angustifolium, P. cotonense, P. dodsonii, P. hederaceum, P. schottianum, P. smithii, P. straminicaule, P. subincisum, and P. tripartitum. It provides an additional recognition characteristic for those species exhibiting it. In some cases the epidermis not only cracks but may begin to loosen and fall off or protrude away from the stem (Fig. 8), such as in P. brevispathum, P. cotonense, P. dodsonii, P. ligulatum, P. purpureoviride. Occasionally this feature is exhibited on fresh stems as well. This is especially true if the stem is forced to bend by falling from its support such as in P. immixtum, P. sulcicaule. The epidermis appears to be hard and brittle while the underlying stem appears to be green and supple. In some cases the epidermis seems to be naturally shed and replaced by another epidermal layer on the fresh stem. This would appear to be a natural method of growth, something akin to a snake shedding its skin.