Philodendron roots have an anatomically distinct layer of exodermis beneath the epidermis, distinguished, among other things, by a long-cell-short-cell pattern (French, 1987a). "Thick-walled, pitted sclereids form a cylinder adjacent to the endodermis and similar sclereids also occur singly or in bands with suberized cork cells in the periderm of older roots" (French, loc. cit.). Like other members of tribe Philodendreae, Philodendron roots have sclerotic hypodermis in the roots. French reports it to be distinctive because of its position next to the exodermis and its occurrence in the primary axis. Another anatomical feature of the roots of Philodendron is resin canals with sclerotic sheaths (French, loc. cit.).
All species of Philodendron produce adventitious roots at some or all nodes. The number of roots developed seems to have more to do with the environment than with the species involved. Plants which are appressed-climbing and in close contact with the substrate generally produce the largest number of adventitious roots. Roots may be of two types which differ both morphologically and anatomically (Lierau, 1888; Porsch, 1911), either for anchoring the plant to the substrate or for feeding (Tieghem, 1867; Went, 1893). The anchor roots (Fig. 54) tend to be more numerous and shorter, often with a dense layer of root hairs (they are sometimes restricted to the side of the root which contacts the substrate). They also have a proportionately much smaller central cylinder and more mechanical tissue to give them strength (Engler & Krause, 1908) than those of feeder roots. They arise principally at the nodes but may arise all along the internodes. Anchoring roots may be spreading from the nodes as in P. auriculatum (Fig. 12) or they may be closely appressed to the surface as in P. gigas (Fig. 13). In contrast to the anchoring roots the feeder roots (Fig. 14) tend to be much thicker and longer and usually extend back to the ground. This behavior is to be expected since Goebel & Sandt (1930) reported that feeder roots of aroids are negatively heliotropic and positively hydrotropic. Feeder roots have a much broader central cylinder and have broader vessels and sieve tubes. Feeder roots arise only at the nodes (Grayum, 1990). Normally, the feeder roots are 2-4 times thicker than the anchor roots and in the case of P. gigas the feeder root may be somewhat woody and up to 3.5 cm diam.
Some species, such as P. auriculatum (Fig. 12) have spine-like branch buds sparcely scattered along their length, especially near the stem. Some hemiepiphytes, such as P. solimoesense in South America, have roots which may become markedly roughened with warty tubercles. In such cases the only portion of the stem that has root hairs is the apex where the roots branch as it nears the ground. It is unknown whether these roots are capable of absorbing atmospheric humidity as is true for some Anthurium species but certainly they must be able to take in the free water that runs down the root.
French (1987a) reports that in P. subg. Philodendron sclerotic hypodermis is entirely absent in subterranean roots but present in the aerial roots. Philodendron subg. Philodendron has elongated, infrequently anastomosing resin canals that extend lengthwise through the cortex (French, 1987c). They are lined with a layer of epithelial cells that consist of thin-walled, unlignified parenchyma. In P. subg. Philodendron and P. subg. Pteromischum the epithelium is surrounded by a sheath with lignified cell walls. In contrast, P. subg. Meconostigma has resin canal sheaths which lack sclerenchym and instead have 2-5 layers of unlignified collenchymatous cells which are easily distinguished from the ground tissue (French, loc. cit.).
While seldom used taxonomically, roots are variable to some extent from species to species. Fresh root coloration (ranging from whitish to green to brownish), length, diameter and texture (smooth, coarse or even warty) as well as the dried color and degree to which they are fissured or folded are all features which may be recorded. These features have not been used extensively since the roots are generally removed from the stems of herbarium specimens before the specimens are prepared.