No previous worker has presented a sectional classification for Syngonium. This may be because until recently, two of the four sections 1 am proposing had not yet been discovered. Syngonium crassifolium and S. schottianum have both juvenile and adult leaves distinctly different from any species Engler & Krause (1920) included in their revision of Syngonium, but these two species were placed in a distinct genus (albeit presumably based on other characters, see section discussing the genus Porphyrospatha). Nevertheless, Engler and Krause did include in their revision two species with adult leaves that were not the typical trisect or pedatisect leaves of section Syngonium. These were S. hastifolium, which in every other respect is like a juvenile plant of S. podophyllum, and S. reticulatum, now known to be a species of Xanthosoma.

Apparently the juvenile phases, which help to understand sectional differences, were not available to Engler. Nevertheless, the sectional classification 1 am creating is most readily recognized in the adult foliage, although there are corollary differences in the juvenile and pre-adult phases of growth.

I divide Syngonium into four sections based on the shape of the adult lamina.

Section Syngonium: Leaves trisect or 5-11 -pedatisect. This section is by far the largest and most variable. It is also the only section with any truly widespread species. The species exhibit marked heteroblastic development, with changes in leaf development in the juvenile, creeping, climbing, and adult phases. These are described in detail under the heading LEAVES.

Section Cordatum: Leaves entire and with conspicuous posterior lobes. This section is the second largest section in the genus. Some of its species are relatively widespread. All (except perhaps S. hastifolium) are thick-stemmed, slow-growing hemiepiphytes that have juvenile leaves similar to the adult forms, only smaller. The section is represented by five species, three of which are restricted to Central America. Juvenile leaf phases are ovate-cordate and intermediate stages begin to approach the adult shape with no major modifications. One species in section Cordatum, S. chiapense, occasionally has broad, rounded lobes separated somewhat from the posterior lobes. This tendency to produce auricles at the outer edge of the posterior lobes is characteristic of section Syngonium and reflects the relationships of these two sections. Nevertheless, S. chiapense has the entire, ovate-cordate leaf blades characteristic of section Cordatum.

Section Oblongatum: Leaves entire, elongate, the petiole broadly winged to the apex. This section differs not only in the shape of its leaves but in having a petiole that is broadly sheathed its full length with a free-ending ligule extending generally beyond the base of the blade. Juvenile leaves differ little from the adult except in size. The section is represented by two species, S. llanoense from the Isthmian region of Panama and S. armigerum from Costa Rica.

Section Pinnatilobum: Leaves deeply incised-lobate; petiole sheathed to about the middle; juvenile leaf blades entire, more or less ovate, cordate at the base; intermediate leaves increasingly incised. The section is represented by a single species, S. steyermarkii, from Mexico and Guatemala.