Cucurbits. James R. Myers
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but later erect. As it emerges, the hypocotyl straightens and the cotyledons ascend as the seed coat is dislodged by the peg, an outgrowth on one side of the hypocotyl. The function of the peg is to open the seed coat and permit the cotyledons to emerge. The photosynthetic cotyledons of most cucurbit seedlings are more or less oblong in shape. Between them lies the inconspicuous developing epicotyl.
Roots
Cucurbits generally have a strong taproot, which may penetrate the soil to a depth of more than 2 m, as in the case of squash. Even in cucumber, the tap root can extend 1 m into the soil. Cucurbits also have many secondary roots occurring near the soil surface. In fact, most roots are in the upper 60 cm of the soil. Lateral roots extend out as far as, or farther than, the above-ground stems. The cortex of the primary root is apparently involved in the development of secondary roots in cucurbits. Adventitious roots may arise from stem nodes in squash, luffa, bitter gourd and other cucurbits, sometimes without the stem having contact with the soil or other substrate.
Some xerophytic species have massive storage roots that enable the plant to survive severe drought. Those of Acanthosicyos can reach up to 15 m in length. The central taproot on one buffalo gourd plant weighed 72 kg (Dittmer and Talley, 1964). The above-ground parts of this species may die from lack of water or in response to freezing temperatures, but the plant regenerates from surviving stem tissue at the root–shoot transition area when favourable conditions resume.
Older vessels in the secondary xylem are often plugged with tyloses (extensions of the parenchyma cells), especially in watermelon, which can contribute to drought resistance. The sieve tubes in the secondary phloem are among the largest found in angiosperm plants.
Stems
The herbaceous or sometimes slightly woody stems are typically prostrate, trailing, or climbing, angled in cross-section, centrally hollow, sap-filled and branched. Primary and secondary branches can reach 15 m in length. Bush forms of cucurbits have much shorter internodes as well as total stem lengths than vining cultivars.
Many of the xerophytic cucurbits are true caudiciforms; that is, the lower part of the perennial stem, which is usually subterranean or at ground level, is thickened, succulent and drought resistant. In Marah, the large underground tubers originate from the hypocotyl and stem base (Stocking, 1955). The succulent stems of Ibervillea sonora (S. Wats.) Greene can continue to sprout new growth annually during periods of drought lasting 8 years or more (Macdougal and Spalding, 1910).
The vascular bundles of cucurbit stems are bicollateral (phloem to the inside and outside of the xylem), discrete, usually ten, and arranged in two rings around the pith cavity. The relatively large sieve tubes are also scattered in the cortex in some cucurbits (e.g. squash), serving to join all phloem elements together. The anomalous stem anatomy of cucurbits and other vines may serve to increase stem flexibility, to facilitate nutrient transport, to promote healing of injuries, or to provide protection against stem destruction via redundancy (Fisher and Ewers, 1991).
Many cucurbits have soft to rough hairs (trichomes) on their stems and foliage, whereas chayote, smooth luffa, stuffing cucumber and some other cucurbits are glabrous or nearly so. Trichome morphology is quite variable: hairs are glandular or eglandular, unicellular to multicellular, and simple or branched.
Leaves
Cucurbit leaves are usually simple (i.e. not divided into leaflets), palmately veined and shallowly to deeply three- to seven-lobed. There is usually one leaf per stem node. Along the stem, leaves are helically arranged with a phyllotaxy of 2/5; in other words, there are two twists of the stem, which segment contains five leaves, before one leaf is directly above another. This means that the angle of divergence between neighbouring leaves is 144° (2/5 of 360°).
Leaf stomata are mostly anomocytic, lacking subsidiary cells. The petiole in cross-section often has a crescent or ring of unequal vascular bundles, the larger ones bicollateral. Stipules at the base of the petiole are typically absent, but have been transformed into photosynthetic thorns in Acanthosicyos. Extrafloral nectaries, which frequently attract ants, occur on some cucurbit leaves (e.g. ivy gourd).
Succulent leaves are rare, even among the xerophytic cucurbits. Those of Xerosicyos have large water-storage cells in the inner mesophyll and perform crassulacean acid metabolism (CAM). However, the deciduous leaves of Seyrigia only perform C3 photosynthesis even though the succulent stem performs CAM. The large ephemeral leaves of most cucurbits adapted to an arid environment avoid heat damage by maintaining high levels of transpirational cooling (Rundel and Franklin, 1991).
Tendrils
Most cucurbits have solitary tendrils at their leaf axils. Tendrils are unbranched in species such as cucumber and branched in luffa and other taxa. They are often coiled, helping plants to cling to trellises and other supports. Terminal adhesive pads develop on the tendrils of several species, allowing attachment to tree trunks and other large textured objects. Some cucurbits lack tendrils, e.g. squirting cucumber and bush cultivars of summer squash, while other cucurbits may have more than one tendril per node.
Tendrils in most of the cucurbit crops are interpreted as modified shoots. However, in luffa and other species, they are considered a stipule–stem complex. There are still other interpretations concerning the anatomical origins of cucurbit tendrils and research is ongoing. In cucumber, a single nucleotide polymorphism (SNP) resulted in the transition from tendrils to tendril-less, removing the plant’s ability to climb (Wang et al., 2015).
Flowers
Many cucurbits have large showy flowers that attract pollinating insects, but Echinocystis, Sechium and some other genera have small, rather inconspicuous flowers. The typically unisexual flowers occur in leaf axils, either alone or in inflorescences. They are often white or yellow, but may be red (e.g. Gurania) or other colours. The hypanthium is cup- or bell-shaped. The sepals or sepal lobes, typically numbering five, and the corolla, which is usually five-lobed and more or less fused, extend beyond the hypanthium. Flowers have radially symmetrical, bell-shaped corollas that may differ between male (staminate) and female (pistillate) flowers.
Staminate and pistillate flowers on monoecious cucumber and squash plants are originally bisexual, with both stamen and pistil primordia initiated. During ontogeny, depending on the hormonal status of the tissue near the floral bud, development of the anthers may be arrested and a pistillate flower develops, or development of the pistil is retarded and a staminate flower is produced. Undeveloped stamens (staminodia) can be seen in mature pistillate flowers, and there is a rudimentary pistil (pistillodium) in staminate flowers (Fig. 1.1).
Fig. 1.1. Comparison of female and male cucumber flowers. (A) Female flower at anthesis, longitudinal section. (B) Tricarpellate ovary, transection at anthesis. (C) Male flower at anthesis, longitudinal section. (D) Simple stamen (1, 2) and two compound stamens (3–5). C, anther connective; CT, corolla tube; Fi, filament; H, hypanthium; Mi, microsporangium; N, nectary; O, ovary; Ov, ovule; OW, ovary wall; P, petal; Pi, pistillodium; S, sepal; Sg, stigma; St, stamen or stanimodium; Sy, style; T, theca. (Goffinet, 1990. Reprinted courtesy of Cornell University Press.)
Stamens are attached to the hypanthium and alternate with corolla lobes. The basic number of stamens in the Cucurbitaceae is five. Some cucurbits (e.g. Fevillea) have five free stamens, whereas all five stamens are fused together in Cyclanthera. During evolution,