Petioles of P. subg. Philodendron are usually elongated, sheathed only at the base in adults and rarely with a conspicuous geniculum. Most species have blades as long as or longer than petioles but 63 species have blades which are at least sometimes shorter than the petioles. Twenty-one species have petioles which are reported as being as long as the blades while 93 taxa have blades that are at least sometimes longer than the petioles. The average longest blade for all species measures 1.5 times longer than the petioles, ranging from .75 to 5 times longer than the petioles. The average smallest blade is 1.014 times as long as the petiole. Petioles range in size from as small as 4 cm long for the lower end of the range in P. bakeri, or comparably 6 cm long in P. hederaceum, 7 cm long in P. ligulatum and P. correae and 7.4 cm in P. chirripoense or to 7.5 cm long for the longest petioles on P. immixtum. Alternatively on the higher end they range to 111 cm long on P. pterotum, 119 cm long on P. findens and 137 cm long on P. gigas. Two additional species, P. copense, and P. radiatum var. radiatum, have petioles to 108 cm long and still two more species, P. chiriquense and P. tenue, have petioles more than one meter long. Additionally 12 species have the longest petioles between 80 and 100 cm in length. These are P. antonioanum, P. davidsonii, P. dodsonii, P. grayumii, P. jodavisianum, P. pirrense, P. sagittifolium, P. schottianum, P. strictum, P. thalassicum, P. tysonii, and P. verrucosum. The average petiole length for the longest petioles is 56.1 cm long. The average petiole length for the shortest petioles is 27.7 cm long.
Fifty-seven species of P. subg. Philodendron in Central America have petioles which may be more than 50 cm long. Twenty-two species have petioles which are always less than 30 cm long, 11 less than 20 cm long and only four species, namely P. bakeri, P. brewsteriense, P. chirripoense, and P. ubigantupense, have petioles less than 15 cm long. Petioles range from less than 5 mm in diameter on fresh petioles of P. brewsteriense to ca. 7.5 cm diam. on P. wendlandii and on the basal portion of P. radiatum.
Usually the petioles are erect-spreading from the stem with the blades either extending initially in the same plane or, more frequently, they are somewhat pendent from the end of the petiole.
Petiole cross-section is not so variable in shape as in Anthurium but the variation is very important taxonomically. All too often herbarium collections make no mention of this often critical diagnostic feature.
Typically petioles are obtusely somewhat flattened or sometimes broadly and obtusely sulcate at the base (Fig. 212). In either event they typically become more terete toward the middle and then obtusely somewhat flattened toward the apex (Fig. 18). Frequently also the petiole has an obtuse medial rib toward the apex where it becomes somewhat flattened.
The majority of Philodendron species have terete or subterete petioles. Sometimes petioles are C-shaped, that is, subterete yet also sulcate. Forty-six taxa are described as having petioles subterete in cross-section. In addition to having subterete petioles some species have the adaxial surface also narrowly and obtusely sulcate as in P. cretosum (Fig. 126), P. jacquinii and P. roseospathum or sometimes in P. sagittifolium.
Twenty one Central American species of P. subg. Philodendron are described as having petioles variously D-shaped in cross section. Examples of species with D-shaped petioles are P. copense, P. findens, P. fortunense, P. fragrantissimum (Fig. 187), P. grandipes, P. jodavisianum, P. knappiae, P. lentii (Fig. 264), P. ligulatum var. heraclioanum (Figs. 275, 276), P. thalassicum and P. verapazense. Philodendron advena, P. pterotum, P. immixtum, and P. ligulatum var. ovatum are also sometimes sharply flattened adaxially. Considerable variation exists in petioles which are described as D-shaped in cross-section. A few species have petioles which are broadly and sharply sulcate as in P. bakeri, P. davidsonii, P. heleniae, P. lentii (Fig. 264), P. ligulatum, P. jodavisianum, and P. scalarinerve. Rarely the adaxial surface is sharply and broadly concave and with the petiole often much broader than thick, as in P. wendlandii (Fig. 453). In the case of the latter, the lateral margins may be very acute and directed outward. D-shaped petioles generally have the lateral margins weakly to prominently raised with the marginal rib either acute or obtuse. Species with acute margins include P. davidsonii, P. ligulatum var. heraclioanum, and P. wendlandii. They are sometimes also acute on P. chiriquense, P. llanoense, and P. warszewiczii. Sometimes, as in P. findens, P. fortunense and P. pterotum, the margins are prominently winged. The wing is erect-spreading and may be markedly undulate in the area of the geniculum as in P. fortunense (Fig. 183) and P. findens (Fig. 171). Sometimes the petioles of P. ligulatum var. heraclioanum also may be weakly winged (Figs. 275, 276). Even petioles which are not D-shaped often have a slightly thickened, slightly raised lateral margin on the adaxial surface. Examples include P. annulatum, P. cretosum, P. microstictum, P. thalassicum, and P. wilburii.
Sometimes in addition to the raised margins of the petioles, there may also be a prominent medial rib, such as in P. jodavisianum and sometimes in P. grandipes, giving the petiole a three-ribbed appearance adaxially. More commonly the medial rib is broad and obtuse, such as in P. angustilobum, P. annulatum, P. copense, P. crassispathum, P. ferrugineum, P. ligulatum, P. llanoense, P. schottianum, P. smithii, P. tenue, P. thalassicum and P. tripartitum.
Rarely the petioles may be markedly flattened dorso-ventrally, as in P. platypetiolatum. In this case the petioles are much broader than thick with the lateral margins nearly acute.
A few species have petioles which are markedly thicker than broad and which are U-shaped in cross-section. Examples include P. roseospathum, P. jodavisianum and sometimes also P. davidsonii. The latter species is more commonly obtusely V-shaped in cross-section.
Petioles of P. subg. Philodendron typically have very short sheaths when the inflorescence is aborted and a much longer sheath (to accommodate the emerging inflorescences) when subtending an inflorecence (Figs. 38, 67, 93, 150). Generally the sheath is markedly closed with one margin overlapping the other, making it inconspicuous (Figs. 83, 160). Occasionally the petiole sheath may be conspicuous, such as in P. correae (Fig. 119) some members of P. subsect. Belocardium.
A geniculum is typically not apparent, but the genicular area serves the same purpose as it does in Anthurium. It is involved with inclining or twisting the plane of the blade to obtain the best exposure to sunlight. Usually the genicular area is somewhat firmer than the remainder of the petiole and sometimes it is differently colored but it is usually neither swollen nor so conspicuously distinct as in Anthurium. Typically the genicular area is the same shape as the remainder of the petiole, but sometimes the cross-sectional shape is different, being more frequently bluntly ribbed and often rather deeply cracked or scurfy around the circumference (perhaps owing to the bending in relation to changes in light). The geniculum, when apparent is sometimes thicker than the remainder of the petiole as in P. brunneicaule, P. ferrugineum, P. heleniae, P. tenue. It may be darker than the petiole as sometimes in P. bakeri or P. glanduliferum or slightly paler as in P. ferrugineum or P. scalarinerve.
Just as house plants eventually will orient their blades toward a window in low light conditions, a plant attached to a tree adjusts the orientation of its leaf blades to select the most favorable light. Plants growing in low light areas position the blade perpendicular to the source of light. In excessively illuminated areas the plant will position the blades so that lesser amounts of light strike the surface of the blade directly. Plants which find themselves in an exposed area owing to deforestation will try to compensate by orienting the leaves more nearly in the direction of the sun to cut back on light striking the blade surface. In addition they begin to lose chlorophyll and blades become more yellowish.
Most species of Philodendron like Araceae in general have glabrous petioles but there are conspicuous exceptions. The petioles of P. brevispathum are setose-pubescent while several species have petioles which are conspicuously scaly. Those species with scaly petioles include P. brevispathum, P. hammelii, P. glanduliferum, P. malesevichiae (Fig. 283), P. squamipetiolatum (Fig. 387), P. squamicaule (Fig. 382), and P. verrucosum. The type of scales present is also variable. In some species, such as P. malesevichiae, P. glanduliferum and P. squamipetiolatum, the scales are acicular and more or less terete. The scales of P. glanduliferum and P. malesevichiae are spreading while those of P. squamipetiolatum are sometimes retrorse near the apex of the petiole. In addition they are densely granular-scurfy on the surface. In P. hammelii the scales are short and broadened laterally, similar to a fish scale and less than 3 times longer than broad. In P. verrucosum and P. squamicaule the scales are of two distinct types. In P. verrucosum there are short, broad and often lacerate scales interspersed with long-acicular scales. In P. squamicaule one type of scale is short, purplish, deltoid, and broader than high (about 2 mm high). Interspersed among these are acicular, greenish scales that are much longer, 3-5 mm long.
Petiolar scales are not uniformly distributed throughout the petioles. In P. squamipetiolatum, P. squamicaule and P. verrucosum the scales are usually scattered throughout the petioles. In P. malesevichiae they are sparsely arranged in the upper 1/2 to 1/3 of the petioles, increasingly denser toward the apex. In P. glanduliferum they are similar but more closely aggregated toward the apex. In P. hammelii the scales are restricted to a small area near the apex. Philodendron ornatum in South America is the most extreme example of this reduction. In that species the scales are reduced to stubby protuberances restricted to the apex of the petiole.
Surface features of both fresh and dried petioles are often conspicuous and frequently diagnostic. Though usually solid green petioles may be irregularly purplish-spotted as in P. edenudatum (Fig. 163) and P. sagittifolium. This purplish spotting may continue onto the lower midrib, a feature common to other types of markings as well which continue onto at least the proximal portions of the midrib. The surfaces of petioles may be minutely speckled but more commonly bear a light pattern of short dashes or streaks that are usually uniformly distributed throughout the petiole. They may be are densely to sparsely marked with short dashes which in turn may be either darker than the surface or lighter than the surface. I have referred to this feature in the descriptions as "short-lineate" or "lineate". Though the lineations are usually short, usually less than 3 mm long, they may sometimes be longer and sometimes are variable in length with both short and long lines interspersed (Fig. 19). Examples of species with short-lineate petioles include P. annulatum, P. auriculatum, P. copense, P. cotonense, P. edenudatum, P. fortunense, P. immixtum, P. ligulatum, P. pirrense, P. purulhaense, P. smithii, P. subincisum, P. thalassicum, P. warszewiczii, and P. zhuanum. The petioles of P. brunneicaule are red short-lineate. The short lineations may be associated as well with much longer, continuous, and frequently weakly raised striations or ribs. Species described as short-lineate to striate include P. antonioanum, P. clewelii, P. gigas, P. glanduliferum, and P. panamense. Still other species have more or less continuous striations throughout. Such species include P. anisotomum, P. davidsonii, P. dressleri, P. findens, P. madronoense, P. rothschuhianum, P. straminicaule, P. tenue, and P. warszewiczii.
Fresh petioles of P. subsect. Belocardium usually have a distinct purplish (or rarely dark green) ring (Fig. 20) around the entire petiole just below its junction with the blade. The species which share this feature, mostly members of P. subsect. Belocardium, include P. annulatum (Fig. 59), P. bakeri, P. correae, and P. ligulatum (all three varieties; see Figs. 271, 275) with a purplish or purple-black ring, and P. auriculatum (Fig. 20), P. immixtum, P. dolichophyllum, P. pseudoauriculatum, and P. wendlandii with a dark green ring at apex of petioles. Philodendron annulare, not currently considered a member of P. subsect. Belocardium also has a purple ring at the apex of the petiole.
Other apparently unrelated species likewise have purple rings around the apex of the petioles. These include P. brenesii, P. davidsonii, P. dressleri, P. ferrugineum, P. microstictum, P. smithii, P. warszewiczii, and sometimes P. grayumii. In the case of P. subsect. Belocardium, the petioles are also quite spongy in living condition.
Petioles may produce a cluster of viscid droplets of a sweet, sugary solution on the abaxial surface at the apex of the youngest leaves. They have been observed on P. davidsonii var. bocatoranum (Fig. 19) and in P. megalophyllum a common South American species. These droplets would appear to have no function in pollination or in blade orientation and I speculate that the droplets act as a food source for ant guards which serve to prevent the young blades from predation by phytofagous insects. This seems all the more obvious because the droplets are associated with new leaves which are typically very tender and thus most easily damaged.
Fresh petioles which are severed usually form resin droplets in the same manner as do the stems, and may in time become completely covered with resin. Of rare occurance is the presence slender strands of latex on snapped and partially severed petioles as on P. malesevichiae (Fig. 21).
Dried petioles yield another suite of characteristics which are generally unrelated to the characters exhibited on fresh plants. Indeed, there seems to be little correlation between the appearance of any feature on a live plant and its corresponding dried condition. Aside from blade shape and the length and number of various parts, it is much like knowing two sets of species, one set of live plants and another set of dried plants. The dried petiole is generally diagnostic and in some cases, such as P. hebetatum and P. schottianum, petioles have a dried epidermis is so conspicuously yellow-brown as to be nearly unique. Though less conspicuous, the petioles of P. schottianum and P. thalassicum also dry quite yellowish-brown.
Petiole firmness is variable in P. subg. Philodendron but this character is difficult to quantify. Unfortunately Engler used this character as one of his major key characters to separate subsections of P. sect. Cardiobelium, in this case separating P. subsect. Macrobelium and P. subsect. Belocardium with very fleshy petioles from P. sect. Oligocarpidium, and P. sect. Doratophyllum (now P. sect. Bulaoanum) with firm petioles. Unfortunately this character is difficult to describe and appears to range from species which are quite firm to those which are quite spongy without any major discontinuations. Most species have petioles which are firm to weakly spongy when squeezed and generally they are quite flexible, capable of being bent to a great extent without breaking. However, spongy petioles, such as those in P. ligulatum which can be easily crushed by squeezing, are also more likely to be brittle.