Syngonium

Seedling, climber and adult carry three different leaves on one stem, and species have been described twice because nobody had seen the same plant grow up.

Petiole and sheath

The petiole is always conspicuously sheathed, in most species from about its middle to near the top, the sheath broad, unequal-sided and often ending in a free tip that may be pointed, rounded or notched. In the oblong-leaved species formerly set apart as section Oblongata the sheath runs the whole length of the petiole and its free ligule reaches past the base of the blade. Petioles are soft — full of air spaces, easily crushed compared with an Anthurium — and the free part below the blade carries a rib on its upper face that ranges from nearly absent in Syngonium chiapense to sharply acute in Syngonium angustatum and Syngonium triphyllum, a small character that helps in the field.

Three leaves in a life

Every species begins with the same leaf: a simple ovate or elliptic blade, soon weakly lobed at the base. Once the shoot has found a trunk and begun to climb, the leaves become conspicuously arrow- or halberd-shaped — the familiar juvenile of the trade — and in the largest section they then change again. The posterior lobes narrow at their base, pinch off, and become free lateral leaflets; the outer half of each lateral is much the wider and grows an auricle at its base; and in the pedatisect species that auricle in turn pinches off to make another leaflet, and another, until Syngonium podophyllum may carry eleven. The higher the leaf on the stem, in general, the more divided it is, and older plants in any population are more divided than young ones. In section Cordata the sequence is shorter: the seedling leaf simply grows into the adult heart-shaped or oblong blade with no distinct intermediate form, and in Syngonium steyermarkii, alone in the genus, the margin becomes progressively incised until the adult leaf looks vaguely like a Philodendron.

Two consequences follow. Plants often flower before the leaf has finished changing, so a young adult may have fewer leaflets than an old one — and species have been described as new from adults that did not match the descriptions of their own juveniles. And the climbing stage is when the species are hardest to tell apart: nearly every member of section Syngonium passes through the same arrow-shaped leaf, though even here the angle and width of the posterior lobes begin to separate them.

Venation

The blade is net-veined in the manner of the whole tribe. The primary lateral veins leave the midrib at an acute angle near the base and a wider one toward the tip, run out toward the margin, and join into a collective vein that follows the edge to the apex; there are always three to five of these collective veins, the inner one largest and the outer ones so close to the margin that the fourth and fifth cannot be seen without a lens. Between them the tertiary veins, arising from midrib or laterals, merge into a reticulum of fourth- and fifth-order veins whose smallest areoles have three to five sides and no free-ending veinlet. A pair of laticifers runs beside every vein, sometimes down to the fifth order, so the leaf too bleeds white.

Fig · One plant, three leaves

A three-day flower that heats up on schedule, opens for beetles, and in one Costa Rican species has recruited the bugs that used to eat it.

Arrangement and the spathe

Inflorescences are produced in a series at the end of the shoot, separated by two-keeled bracts. Many species make only two, some one; Syngonium angustatum, Syngonium podophyllum and Syngonium macrophyllum make four to eleven, depending on the species, and fewer of them set fruit than reach flowering. The peduncle is short and bluntly three-sided at flowering, and lengthens afterward until the weight of the fruit bends it over. The spathe in bud is rolled tightly around the spadix and overtops it by a fifth to a third; a constriction about midway marks off the tube from the blade. At flowering the blade unfurls into a shallow cup behind the spadix, white or cream at least within, the tube often flushed red or violet inside, and in Syngonium neglectum the blade folds right back. The tube never fully opens, but it opens far enough to let a pollinator in to the female flowers. After two or three days the spathe closes again over the spadix and the blade withers.

The flowers

The spadix has three zones. The lowest is female and much the narrowest, green or occasionally pale orange, 7 to 48 mm long by species; above it a band of sterile male flowers, larger and more irregular than the fertile ones and sometimes fused in pairs, swells the spadix before the fertile male zone, which occupies the upper two-thirds or more. Each female flower is two fused carpels, rarely three, with one ovule in each cell, and the flowers are themselves fused to their neighbors — this is the character the genus is named for, and it means the female zone ripens as a single unit. The stigma comes in four forms, disc-shaped in Syngonium podophyllum and its allies, two-lipped, orbicular in Syngonium wendlandii, cup-shaped in Syngonium triphyllum. Each male flower is usually four nearly sessile anthers fused into a truncate synandrium with a rhombic to hexagonal outline, and the pollen falls into three types, spiny, smooth or knobby, that follow the same groups of species as the stigmas.

Heat, and who comes

Flowering is markedly protogynous. The female flowers are receptive one to two days before pollen is shed and already receptive when the spathe opens; by the second day the pollen is emerging in stringy masses from between the synandria and the stigmas have finished, while the spathe tube has contracted almost shut around whatever is inside. Two species have had their temperatures taken through the cycle. Both run three days — spathe opening and receptivity on the first, the female phase through the second, the male phase on the third — but they heat differently: the spadix of Syngonium schottianum warms at the start of the second and third nights, while Syngonium angustatum warms twice on the second day, once that night and once on the third day.

The syndrome presumed for the genus is the one shared with Philodendron, Dieffenbachia and Xanthosoma, whose flowers are visited by large dynastine and ruteline scarab beetles, and the one beetle report for the genus — Cyclocephala beetles at Syngonium triphyllum in Costa Rica, given in 1981 as a personal communication — has stood for the genus ever since. Then, in 2022, Syngonium hastiferum was shown to be pollinated by a plant bug, an undescribed species of Neella in the Miridae, drawn by a floral volatile not previously known from any plant. Plant bugs are florivores; the species has, in effect, turned the insects that ate its flowers into the insects that pollinate them, and its floral traits differ accordingly from those of its beetle-pollinated relatives. Whether the rest of the genus is beetle work or has other recruits of its own is not known.

Fig · The spadix, the tube and its visitors

The spathe tube shuts, colors, and opens again months later around a single fused fruit — a bite-sized meal for whatever finds it.

Once pollinated, the spathe tube closes tightly and the upper spathe rots away along a line of thickened tissue, taking the fertile male flowers with it. Part of the male spadix, usually all of the sterile flowers, stays sealed inside the tube and rots there, and it has been suggested that this decaying tissue is a hatching ground for the larvae of insects that lay eggs in the inflorescence. The tube itself, green at flowering, colors as the fruit ripens — red, orange or yellow outside, sometimes brightly colored within — and at maturity it usually splits or turns inside out to expose what it has been holding.

What it holds is one fruit. The female flowers were fused, and the berries ripen fused, into an ovoid syncarp whose smooth surface still carries the brown flecks of the original flower tops. The syncarp is pale in most species and, by Croat's count, white in three — Syngonium wendlandii, Syngonium mauroanum and Syngonium triphyllum — where a white body set against a red, everted spathe might be a fruit made for birds. The rest are duller and fragrant, which points instead at mammals, presumably monkeys, and ripe fruit is rarely found in the wild because whatever takes it takes it quickly. A grower who opened the bright red spathe tubes of an escaped plant in Florida found the whole female spadix fused into one mass, the thin brown skin tearing away to expose a white pulp around pea-sized seeds, slightly sweet and not unpleasant, and noted that once the tube is broken the entire infructescence is a single bite.

Each fruit body holds 50 to 100 or more seeds, ovoid to cylindrical, usually 5 to 10 mm long and 3 to 6 mm thick, rounded at both ends, with a smooth, thin, shiny black or dark brown coat, a white interior, a large embryo and no endosperm. They germinate promptly, always in the soil, and lose their viability promptly if they are allowed to dry.

Fig · One fused fruit inside the spathe tube