Leaves of P. subg. Philodendron, like those of P. sect. Meconostigma and P. sect. Pteromischum, have supervolute invernation (Cullen, 1978). Like most other aroids, Philodendron leaves undergo heteroblastic development, a gradual change in morphology from juvenile to adult forms (Grayum, 1990). However, development of leaves of P. subg. Philodendron is not so severe as in other genera, especially in those in subgenus Lasioideae and Monsteroideae. Typically, the juvenile blades are not dramatically different from those of the adults, commonly they are of similar shape as the adults, typically ovate to oblong initially and they virtually always lack posterior lobes. In rare cases, such as with P. hederaceum var. kirkbridei (Fig. 223), the juvenile blades are velvety, a feature which is caused by the markedly convex or even somewhat cone-shaped epidermal cells. Ecologically, this lack of glossiness has the effect of allowing nearly all light which falls on the leaf to be absorbed. In all but a few species, notably P. gigas, the velvety leaves are transformed to those with glossy to semiglossy leaves.
In terms of adult blade shape P. subg. Philodendron is exceedingly diverse, encompassing more morphological variation than is exhibited in P. subg. Meconostigma or P. subg. Pteromischum and indeed all the variation exhibited in the even much larger genus Anthurium. Leaves are typically clustered at the end of the stem and rarely more than ten are present at a time. Older leaves are eventually deciduous, falling free along with the petiole. In some rare cases the blades fall free first, followed by the somewhat more persistent petioles. The resulting petiolar scar (discussed above under stem) may not be apparent initially on species with persistent cataphylls but even in these cases they are typically apparent on the older stems. Still, they are never so conspicuous as the petiolar scars in P. subg. Meconostigma.
In his review of leaf morphology and function Ray (1987a) divided leaves into three main types, foliage leaves, reduced leaves (10-70% the size of the foliage leaves) and cataphylls (reduced to less than 10% of a foliage leaf). The first leaf on a vegetative axis he refers to as a prophyll (in Philodendron always a cataphyll). This structure is usually 2-ribbed and is in this regard similar to the bracts that subtend inflorescences. The latter are referred to as bracteoles by Ray (loc. cit.).
The leaf immediately following the cataphyll (prophyll of Ray, 1987a) is a fully developed leaf, referred to as a mesophyll [metaphyll of Grayum (1996)]. All three types of leaves mentioned above may be modified by the terms proleptic and sylleptic, depending on the type of growth involved, i.e., proleptic for monopodial growth and sylleptic for sympodial growth (the latter with petioles borne on the side of the stem while the former has conspicuously sheathed petioles which encircle the stem). Any leaf subtending an inflorescence or an aborted inflorescence primordia is termed a mesophll (Ray, 1987a). In P. subg. Philodendron all adult leaves are of this type. Juvenile leaves of P. subg. Philodendron, on the other hand, are all monopodial leaves (Grayum, 1990).