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, fusion of two pairs of stamens has resulted in some genera (e.g. Cucumis) having one small unilocular and two large bilocular stamens. The three stamens are usually attached to each other to some degree by their anthers, as in squash; filaments are short and often united also.
In the Cucurbitaceae, the subfamily Zanonioideae has relatively homogeneous pollen morphology. In the subfamily Cucurbitoideae, pollen is quite variable, but consistent within genera. For example: in squash, pollen grains are very large, spherical and spiny, but in cucumber the pollen grains are more globular and smooth.
Pistillate flowers have an inferior ovary below the hypanthium. The pistil, which often has a fused style but separate stigma lobes, generally has three or five carpels. The fleshy placentae bear numerous ovules in most species, but only one in chayote.
Floral nectaries attract pollinating insects. These structures are borne inside and at the base of the hypanthium in both staminate and pistillate flowers. The nectary forms a continuous ring surrounding the base of the style(s) in the female flower, whereas the nectary and its associated pistil rudiment form a button-shaped mound at the centre of the male flower.
Fruit
Fruit of the Cucurbitaceae are extremely diverse in many characteristics, including size, shape, colour and ornamentation. Those of bryony are small (ca 5 mm), spherical, and green, red or black in rind colour. Angled luffas are club-shaped, about 60 cm long, and prominently ribbed. Some of the many shapes of bottle gourds are described in Chapter 4. The striped, mottled, bicoloured or solid-coloured fruit of squash are smooth, wrinkled, warted, furrowed or ridged. Sicyos has stiff, dry spines, whereas those on the teasel gourd are soft and fleshy.
Cucurbita maxima is well named, for it is this species that is the giant of the plant kingdom. Every year, there is a contest to grow the world’s largest pumpkin. In this contest any squash fruit with an orange skin is considered a pumpkin, and the winners are invariably C. maxima. Weights crossed the 500 kg threshold in 1999 and the 1000 kg threshold in 2014. The winning fruit in 2016 weighed an astounding 1193 kg. Giant watermelon contests are also run, with the winning fruit in 2015 weighing 137 kg.
Cucurbit fruit are generally indehiscent ‘pepos’, usually with one or three ovary sections or locules (Fig. 1.2). A pepo is a fleshy fruit with a leathery, non-septate rind derived from an inferior ovary. However, fruit of some cucurbit genera, e.g. Momordica and Cyclanthera, split at maturity. Fruit of squirting cucumber forcefully eject their seeds through a blossom-end pore. Fruit may be dry when mature, as in luffa, where seeds fall out through a hole at the bottom of the pendulous fruit. Mature fruit of many cucurbits have a hard, lignified rind, but various squash cultivars have been bred to have a tender rind.
Fig. 1.2. Frontal and cross section of female flower from Cucurbita pepo (spaghetti squash).
Cucurbit fruit flesh is generally white to pale yellow and moist, but cultivated cucurbits such as melon, squash and watermelon have been bred to have a range of flesh colours. Melon and squash have also been bred to include green or orange flesh, and watermelon flesh can be white, salmon yellow, canary yellow, orange, coral red or scarlet red. In melon, watermelon and squash, the flesh is derived from the fruit wall. In other species, including cucumber, the edible flesh may be mostly placental in origin.
Many cultivated cucurbits produce a fleshy fruit at maturity. Others, such as gourd and luffa, dry at maturation. When a luffa fruit matures and dries, what is left is a papery outer skin and a fibrous mass surrounding the seeds. This tangled mass of modified vascular bundles, consisting mostly of fibre cells, makes up a luffa sponge.
Seeds
There
