MORPHOLOGY

STEM

Syngonium is characterized by polyphyllous sympodial growth with an elongate shoot and no branches. After the shoot terminates in an inflorescence or a series of inflorescences, a single branch arises from the lateral bud, two nodes back from the apex (Madison, 1978). The stem branching pattern as reported by Engler (1877) is as follows:

The pattern depicted here is indefinitely repeatable, limited only by change in the ecological conditions that allow flowering. (See also section on GROWTH HABITS.)

Stems may initially be only a few mm diameter in juvenile plants but up to 6 cm diameter in adult plants. Syngonium stems probably average 1 to 2 cm diameter. Stems are invariably green and presumably photosynthetic in juvenile plants, but the epidermis soon loses its color on adult plants (usually within 1 node below the inflorescence). The possibility of having photosynthetic tissue in the creeping phase of growth where internodes are elongate and leaves are small (see section on GROWTH HABITS) would be important ecologically.

In general the stems of Syngonium are unbranched though branching sometimes occurs, especially if the stem has been injured. Generally, climbing stems have elongated internodes. The internodes are often appreciably shorter on flowering portions than elsewhere. Some species, especially those species with thicker stems, such as S. macrophyllum, S. crassifolium, S. schottianum and S. sagittatum, may have the internodes quite short on flowering segments.

Stem segments are terete or more commonly oval in cross-section and have a shallow groove on one side above the axillary bud. This generally is diminished or absent distally. Birdsey (1955) reports a multiseriate epidermis in Syngonium with the developing stem near the apical meristem having only a single layer of cells and sections distal to the apical meristem having 3 or more layers of epidermal cells. Part of these layers may be suberized but at least the innermost is not suberized. Birdsey (1955) reports that it is not yet known whether this multilayered tissue arises from the epidermis or underlying collenchyma cells.

Stems are to some extent flexible but, if bent, the outer epidermis often cracks free. Even in undisturbed plants, the outer epidermis often becomes cracked and peels. In such cases the outer epidermis is generally brown or yellowish with the underlying epidermal cells green or olive green. The color of the epidermis is sufficiently variable to provide useful specific characteristics. For example, the outer epidermis of S. erythrophyllum always dries a characteristic brown only a short distance from the stem apex. Drying plant collections invariably causes shrinkage of the stem with the result that herbarium specimens have the outer epidermis cracked or fissured and often peeled free from the stem.

In the case of S. angustatum the epidermal cells are pushed out by underlying cells to provide rough spots visible to the naked eye (Birdsey, 1955), while the excrescences of S. podophyllum var. peliocladum are very large, providing the chief taxonomic characters for its separation from S. podophyllum var. podophyllum.

Another useful taxonomic character involving the stem is the presence or absence of a waxy layer, although the character does not necessarily maintain itself throughout the range of a species. For example, the stems of S. podophyllum in Mexico are glaucous and are useful there in separating it from S. angustatum, but in Panama and in South America the stems are never reported to be glaucous.

The glaucous condition is also present on other parts, especially the peduncle and the fruits, e.g., in S. chiapense and S. macrophyllum. The glaucous peduncles of S. angustatum are useful in separating it from S. podophyllum.

Stem anatomy was dealt with briefly by Van Tieghem (1867) and will be dealt with more completely in work underway by R. Keating of Southern Illinois University, Edwardsville. Birdsey (1955) also presents a thorough survey of stem anatomy.

Stem cross-sections of Syngonium show a well-defined layer of collenchyma (cortex) beneath the epidermal layers with a poorly defined boundary lacking between the cortex and the central vasculated region of the axis (Birdsey, 1955). Starch grains are randomly distributed throughout the parenchyma of both the central cylinder and the cortex, and idioblasts containing raphides as well as cells containing druses are present, though in lower frequency than in the petioles (Birdsey, 1955). The stem of Syngonium lacks a well-defined pith and has the scattered vascular bundles characteristic of monocots. Always present in the stems of Syngonium are articulated laticiferous cells, i.e., secretory cells, which are placed end-to-end, forming a continuous tube with numerous branches and anastomoses (Birdsey, 1955). The contents of the cells are rich in tannins as well as latex (Solereder & Meyer, 1928).

ROOTS

Adventitious roots of two types emerge just below each node of the stem. In addition to a number of skototropic clasping roots, whose main function is that of holding the plant to its support, there is a single feeding root that is positively geotropic and diverges from the stem at an angle that puts it into a position to absorb nutrients some distance from the plant (Birdsey, 1955). The anatomy of these roots differ at least quantitatively (Birdsey, 1955), with the feeding root having greater overall diameter, a proportionally larger central cylinder, and proportionally larger vessels.

LEAVES

Syngonium phyllotaxy is reported as 2 2/5 divergence (Engler, 1877), though the arrangement may be obscured by the slight twisting of the petioles (Birdsey, 1955).

The petiole of Syngonium is always conspicuously sheathed from about the middle to near the apex, generally between the middle and the upper 4/5 of the petiole. The sheath functions in protecting the newly emergent leaves and the inflorescences. The sheath is generally very broad, especially at the base, and the two sides are generally quite unequal in height. The apical portion of the sheath is often free-ending, and the free part varies from acute to rounded or emarginate at the apex. The petiole of section Oblongatum differs in being proportionately broader and extends often beyond the base of the blade. The petioles have a uniseriate epidermis, with numerous intercellular spaces (Birdsey, 1955). The vascular bundles are irregularly scattered throughout the fundamental tissues. Perhaps owing to the numerous intercellular lacunae, the petioles of Syngonium are soft and can be easily crushed in comparison to other aroid genera such as Anthurium. The cross-sectional shape of the free portion of the petiole beneath the blade often has taxonomic significance. Although all are characterized by being rounded on the abaxial side and most have a single more or less conspicuous adaxial rib, the rib varies from being almost absent in the case of S. chiapense, S. neglectum, and S. auritum to quite noticeable in S. podophyllum var. peliocladum, S. macrophyllum, and S. salvadorense, to noticeably acute in S. angustatum, S. triphyllum, S. wendlandii, and S. hoffmannii. In some species the lateral adaxial margin of the petiole is also ribbed. This is apparent in S. hoffmannii and S. wendlandii but especially in S. triphyllum.

The petioles of juvenile leaves often differ remarkably from that of the adult and have a greater tendency to be canaliculate rather than I-ribbed as in the adult leaves.

The leaf blade morphology of Syngonium, as in many other genera of Araceae, is quite variable. Species often begin flowering before the full course of maturation takes place in leaf development. For example, young adult plants may have fewer leaflets per leaf than older plants. This has created a situation where species have been described as new because the adult leaves were unlike plants which had previously been described. The leaves easily fall into four basic types, and it is along these lines that the sectional classification of the genus is arranged.

Section Syngonium.By far the most common leaf type in the genus is that of section Syngonium with trisect or pedatisect adult leaf blades. Leaf blades range from the strictly trisect blades of S. gentryanum to the more highly divided blades of S. podophyllum, which vary from trisect to 11 -pedatisect. Though the condition is not necessarily a strict matter of maturity, older, more mature plants in any population tend to be more highly divided. Any individual may have considerable variation in the degree of lobing of the leaf depending on where the leaf occurs on the stem. The higher up the stem, generally, the more highly lobed or segmented the leaf becomes. As mentioned in the section on STEMS, the seedling leaves of all species are entire. Generally the seedling leaves are ovate or elliptic, and frequently they are also weakly lobed at the base. The intermediate climbing leaves are always more conspicuously lobed at the base. The leaves of such intermediate, pre-adult climbing forms are generally conspicuously hastate or sagittate. It is during this pre-adult climbing stage that species are most difficult to distinguish from one another, since nearly all the species in section Syngonium have leaves that are remarkably similar. However, even at this stage, differences between species begin to emerge. Some species, such as S. triphyllum, have posterior lobes that are narrow and are directed almost perpendicular to the median lobe. The abruptness of change between an entire leaf with prominent basal lobes and a 3-lobed leaf varies considerably from species to species. In some species all intermediate stages are easily found, whereas in other species the plant seems to switch from an entire pre-adult leaf to a 3-lobed or 3-parted pre-adult leaf very quickly with no evidence of intermediate leaf forms. The general pattern of change extend from a more or less sagittate blade, to a hastate blade, to a 3-lobed blade with the posterior lobes very constricted near the base. The development proceeds to where the posterior lobes become lateral leaflets, i.e., they become completely free of the median lobe.

The lateral segments or leaflets are invariably inequilateral and in most cases the outer half of the segment or lobe is much wider and conspicuously auriculate at the base. The degree to which the auricle develops is very variable from plant to plant, even in the same species (particularly true of species like S. podophyllum), but it is often sufficiently consistent in a species to be of taxonomic value.

Leaf development of five or more pedatisect leaves invariably proceeds with the pinching off of an auricle to produce another segment or leaflet which in turn goes through the same process to produce still another pair of segments or leaflets.

The degree of separation of the segments or leaflets is an additional taxonomic character in some species (and not in others). Some species consistently have leaflets free from one another with exposed sections of the rachis dividing the leaflets. In morphologically variable species, such as S. podophyllum, the degree of separation of the leaflets is highly variable, even at the populational level with leaves varying from those with united segments to those with distinct leaflets.

Section Cordatum.In this section the leaves are entire with generally conspicuously developed posterior lobes. The overall leaf shape may be ovate or oblong-ovate or rarely hastate, and the anterior lobe is sometimes weakly constricted near its base. The section is the second most common and includes those species (among others) that Engler placed in his genus Porphyrospatha. Seedling leaves are similar to those of section Syngonium and soon become ovate-elliptic with small posterior lobes, but instead of becoming more 3-lobed, they begin to assume the shape of the adult blade and merely get larger. In section Cordatum there are no leaves with distinctly intermediate phases, which are so variable and interesting in section Syngonium.

Section Oblongatum.-The species in section Oblongatum differ markedly from all others in the genus because of the oblong to oblong-elliptic to ovateelliptic leaf blades and petioles, which are broadly sheathed to the apex with the emarginate free portion of the sheath often extending beyond the base of the blade. In this section the juvenile leaves are almost identical to the adult leaves, differing only in being much smaller. Intermediate stages are much like the adult blade.

Section Pinnatilobum. -This section is represented by S. steyermarkii, the only incised-lobate species in the genus. Juvenile leaves are ovate, soon becoming subcordate at the base and developing sinuate margins laterally. As the blade matures, it becomes increasingly more deeply lobed and more conspicuously lobed at the base until it assumes its adult form, which looks vaguely like the common Philodendron radiatum Schott with incised-lobate leaves.

Leaf blade venation of Syngonium easily characterizes it as a member of the Colocasioideae. The venation type is the brochidodromous type (Dilcher, 1974), and the primary lateral veins (i.e., secondary, see Croat & Bunting, 1979), after extending laterally from the midrib toward the margin, join into a collective vein which extends to the apex roughly parallel to the margin. In Syngonium there are always 3-5 separate collective veins. The primary (inner) one, being the largest, is initiated by one-of the lowermost primary lateral veins. Successively smaller collective veins, located at smaller distances from the blade margin, arise from progressively lower primary lateral veins or even weak secondary veins branching off the midrib. Frequently the fourth and especially the fith collective vein are so near the margin that they cannot be seen with the naked eye.

The origin of the primary lateral veins is usually acute near the base, often becoming more obtuse toward the apex and rarely arising at nearly right angles to the midrib.

One of the characteristics of Syngonium and other members of the Colocasioideae is the presence of conspicuous reticulate venation. Tertiary veins can arise from the primary lateral veins (i.e., secondaries) in which case they orient toward the margin and become parallel to the primary lateral veins, or they can arise directly from the midrib. The tertiary veins merge distally and give rise to quaternary veins. The quaternary veins are found irregularly between the tertiary veins and at oblique angles, randomly oriented. The quaternaries in turn give rise to the lowest order of veins, namely the quinary veins which form the areoles. The areoles are 3-, 4- or 5-sided and lack a free-ending veinlet. Both the 4th and 5th order veins are restricted to the adaxial plane of the blade (Birdsey, 1955). An unusual feature of the quaternary veins of Syngonium and other Araceae such as Xanthosoma is that they may cross over a series of tertiary veins before joining with another vein (Birdsey, 1955).

Laticiferous cells are often very prominent in the leaf tissue, with two series of laticiferous cells paralleling each vein, sometimes even up to the 5th order of veins (Birdsey, 1955).