name is often applied to a single species. For example, names for B. hispida include ash gourd, white pumpkin, wax gourd and winter melon. Sometimes different names refer to distinct crops within a species, such as pumpkin, zucchini and acorn squash in Cucurbita pepo. In other cases, the different common names for the same species are used interchangeably. Frequently used common names for cucurbit crops are given in the Appendix.
Popular terms for the cucurbits may be confused in other languages as well. Cucumber and melon are sometimes considered the same in India. Older terminology in Chinese included ‘guo-kua’, which is translated into English as ‘melon’, but it includes watermelon (C. lanatus) as well as melon (C. melo). Also, ‘tsaikua’, which usually translates as ‘gourd’, refers to B. hispida, C. sativus, M. charantia and species of Cucurbita and Luffa. This is no longer a problem in China, since the names in Mandarin are now precise.
The family Cucurbitaceae, which is not closely related to any other plant family, consists of two well-defined subfamilies, eight tribes (representing varying degrees of circumscriptive cohesiveness) and about 118 genera and 825 species (Jeffrey, 1990). The four major cucurbit crops (watermelon, cucumber, melon, squash) and five other important crops (luffa, bottle gourd, chayote, wax gourd, bitter gourd) in the family belong to the Cucurbitoideae subfamily. Four of these – watermelon, luffa, bottle gourd and wax gourd – belong to the tribe Benincaseae. The classification of these and other cultivated species is given in Table 1.1. Many more wild taxa have actual or potential economic value, making Cucurbitaceae one of the most important plant families for human exploitation.
Table 1.1. Taxonomy of cultivated cucurbit species.
Taxonomic studies of cucurbits at all hierarchical levels have been done. They include comparative analyses of morphology (including specialized studies on trichomes, stomata, palynology and seed coat anatomy), cytology, DNA, isozymes, flavonoids, cucurbitacins, amino acids and fatty acids in seeds, biogeography and coevolving insects. A monograph has been written on Cucumis (Kirkbride, 1993), and the taxonomic relationships within the Cucurbitaceae are being improved using molecular markers (Schaefer et al., 2009; Chomicki and Renner, 2014). These new studies have been useful to the areas of crop improvement and germplasm conservation.
Seedlings
Most cucurbit seedlings are epigean, germinating with the tips of the cotyledons initially inverted 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
