The configuration of shoot vasculature of Philodendron has been extensively studied by French and Tomlinson (1980, 1981, 1984). They report the genus to be one of the most diverse in the family from an anatomical point of view. The axis of the vascular system of Philodendron is continuous throughout the stem because the renewal shoots develop precociously and because the morphologically terminal parts of the stem soon become branches (French & Tomlinson, loc. cit.). All species of Philodendron examined by French and Tomlinson have an "independent cortical vascular system of traces that pass from the leaf into the cortex, but without entering the central cylinder". Generally the cortex is wide with 3-5 or more indistinct series of bundles. Cortical bundles are collateral, typically with a fibrous sheath next to the phloem. Major leaf traces enter the cortex at an acute angle and promptly enter the central cylinder while smaller traces enter the cortex at a less acute angle and may enter the central cylinder well below where they entered the cortex. No endodermis was observed by French & Tomlinson for any of the species observed.
Philodendron subg. Philodendron has secretory resin canals occurring in the cortex. These consist of a schizogenous space lined with 2-3 layers of epithelial cells and contain a Sudan IV-staining resin (French & Tomlinson, 1984). The central cylinder is separated from the cortex by an arrangement of vascular bundles into which the leaf traces merge. Bud traces are equal on both sides and form an arc near the periphery of the central cylinder before joining with the axial bundles.
In their survey of 3-dimensional arrangement of vascular bundles leaf traces and axial bundles were distinct with the leaf traces being consistently collateral with protoxylem and usually with a prominent sheath of sclerenchma next to the phloem.
The axial bundles were divided roughly into five groups, four of which pertain to P. subg. Philodendron.
In the P. hederaceum pattern (French & Tomlinson pattern 1) there are simple collateral bundles with some bipolar bundles in the central cylinder of the internodes with thin-walled and non-lignified ground tissue.
Another relatively rare pattern involving P. sect. Baursia (but not a Central American representative of the section) and P. jacquinii (Pattern 3 of French & Tomlinson) has compound bundles throughout the central cylinder with individual compound bundles consisting of strands of xylem and phloem separated from each other by sclerenchyma "in the form of a partial or complete sheath" (French & Tomlinson, loc. cit.). In P. jacquinii leaf traces enter the central cylinder but make a variety of configurations, sometimes including pairing before joining the compound bundles. Compound bundles do not make a particularly straight course, sometime bundling and pairing within and between compound bundles.
A pattern exhibited by P. fragrantissimum and P. roseospathum (French & Tomlinson pattern #4) is similar to pattern #3 except that "the pattern of the vascular components with compound bundles is less clear because components are not separated by sclerenchyma" (French & Tomlinson, loc. cit). The latter occupies the central core but does not isolate individual bundle components.
The pattern for P. mexicanum and P. sagittifolium (French & Tomlinson pattern #5), described as the most common in the genus, has axial bundles strictly amphivasal with a central core of phloem and a peripheral region of xylem. The tracheary elements may form a more or less continuous cylinder or may be arranged in clusters around the phloem. Leaf traces penetrate the central bundle and fuse with the central cylinder while the sclerenchyma sheath of the trace migrates to the center of the axial bundle.