Cucurbitoideae

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Cucurbitoideae

Description

Flowers are solitary or (few-)fascicled in the leaf axils, or arranged in an axillary raceme (indeterminate). By contraction and reduction of foliage leaves to bracts the inflorescences may become (large) panicles. In dioecious species, male flowers are either solitary, or more usually arranged in a pedunculate, bracteate raceme, often with a single co-axillary male flower. Solitary female flowers develop at the nodes, or form (peduncled) clusters, e.g. in Pilogyne. In monoecious plants, solitary female flowers can be found at the nodes or (developing previously) beside the male raceme; which develops later (usually, female flowers develop after the male flowers). In some species of Trichosanthes the persistent pedicels of senescent or undeveloped solitary flowers appear as an elongated or ‘straw-like appendage’ beside the male raceme.
— The flowers are mostly unisexual, either white or yellow, and the perianths of male and female flowers are generally similar. In nocturnal species of Hodgsonia, Gymnopetalum, and Trichosanthes, the flowers are white, opening in the late afternoon or at night and close before sunrise, when the fallen corollas can be found on the ground. Other species e.g. T. cucumerina are largely diurnal.
— There are always 5 sepals, inserted on the margin of the receptacle (subfam. Fevilleoideae) or on the rim of the receptacle-tube (subfam. Cucurbitoideae).

Sepals are typically much smaller than the petals; (narrowly) triangular or linear, but are frequently enlarged, in Cucurbita moschata, or are larger than the petals in Anangia. Their shape in male flowers, whether entire or lobed, is taxonomically important in Trichosanthes.
Stamens 3 (or 5), inserted in the receptacle-tube, mostly free; Ovary constricted at apex; Fruit a juicy or dry berry (pepo-fruit), mostly indehiscent (in Luffa with an operculum). — These are very variable and distinctive for genera or species. Seeds of the capsular-fruited Fevilleoideae are usually winged (Gynostemma excepted). Seeds can be compressed, sculptured, and often possess a ‘margin’, all useful characters. Seeds of e.g. Diplocyclos and some Trichosanthes species seem 3-loculed with the embryo in the middle compartment. The two wart-like outer compartments can be hollow, or filled with large erect cells developed from the seed coat, either from the margin or the face. Seed coat anatomy is highly characteristic and has been used in taxonomy, notably for subdivision of the family into tribes (Sing & Dathan 1998; Jeffrey 2005). See also Corner (1976) who provided detailed notes on the seeds of several genera of Cucurbitaceae. Rugayah & De Wilde (1999: fig. 1), depict a choice of Trichosanthes seeds according to the sections.

The use of the words margin and edge in the descriptions — For leaves, bracts and sepals the word margin is used, which may be e.g. entire or dentate, etc. For seeds it is different: seeds are usually more or less, or strongly compressed (subglobose to almost flat), and seen at the faces there can be a distinct (or faint) narrow or broad bordering zone, called the margin, around the seed, differing from the rest of the seed-face in thickness, colour or surface (the seed-face can be smooth or sculptured). The very outer side of the seed showing the outline or profile is called the edge, described as e.g. straight (smooth), undulate, crenulate.

Distribution

Asia-Tropical:
Thirty-one genera in Malesia — Anangia, Baijiania, Benincasa, Borneosicyos, Cayaponia, Citrullus, Coccinia, Cucumis, Cucurbita, Cyclanthera, Diplocyclos, Gymnopetalum, Hodgsonia, Indomelothria, Kedrostis, Lagenaria, Luffa, Melothria, Momordica, Mukia, Muellerargia, Neoachmandra, Papuasicyos, Pilogyne, Scopellaria, Sechium, Siraitia, Solena, Thladiantha, Trichosanthes, and Urceodiscus.

Pollination

Flowers in Cucurbitaceae are unisexual, the plants either monoecious or dioecious. Hermaphroditic flowers are rare, occurring mostly in cultivated species and are typical in Neoachmandra hermaphrodita (endemic to Thailand). Sometimes they are parthenocarpic (e.g. cultivated Cucumis).

Cross-pollination by insects seems most likely throughout the family, but little is known for certain for most species. A variety of insects, including hoverflies (Syrphidae) have been seen visiting the flowers. Pollination by oil-collecting oil bees (genus Ctenoplectra) in the tribe Thladiantheae is remarkable (Vogel 1990; Schaefer & Renner 2008a; Renner & Schaefer 2010). Trichosanthes (most species) and some related genera (Gymnopetalum, Hodgsonia) flower at night. Their tubular, sweetly scented, flowers are possibly pollinated by sphingidae, or other nocturnal moths. We ourselves observed at night the very swift visits by hawk-moths to flowers in a large sprawling stand of Gymnopetalum scabrum.

The local endemic Borneosicyos has pollen in tetrads, presumably hinting at pollinator type.

In some groups, e.g., Trichosanthes and Pilogyne, the connective of the anthers is conspicuously set with comparatively stout, short hairs, which is also possibly related to pollination mechanics.

Dispersal

The conspicuously coloured, berry-like, pulpy fruits of most Cucurbitaceae suggest that seed dispersal is by animals eating the fruit. The pendulous fruit of Momordica, (notably M. charantia), splits open at the apex, revealing the seeds enveloped in bright red pulpy arils.

Gynostemma pentaphylla fruit is small, berry-like, green-yellow or purple, containing 1 or 2 seeds only. The single globose seeds of Neoachmandra sphaerosperma (Thailand), have been seen carried away by ants.

In South Africa the fruit of Cucumis humifructus is geocarpous and aardvarks play a role in fruit and seed dispersal.

In many Trichosanthes species the fruit pulp is green-black and bitter, in others it is yellowish, orange or whitish and not bitter; in T. villosa it is sweet. The fruit of Siraitia grosvenorii (cultivated) is known for its strong sweetness (Jeffrey 1979).

As noted above, species with capsular fruit, (mainly the subfamily Fevilleoideae, but also the fruits of Luffa aegyptiaca (synonym L. cylindrica)) open by valves or by an operculum at the apex, through which they shed their usually winged seed, although sometimes the wing can be narrow or absent.

Cultivated species or feral forms of these are generally dispersed by man. Their seeds often germinate from waste fruits, the plants either temporarily establishing or persisting for a longer time by roadsides, in waste places, abandoned fields, secondary scrub, or on or near rubbish dumps.

Morphology

Trichosanthes species are variously hairy: some are conspicuously villose or strigose, but most are subglabrous, or glabrescent when the hairs disappear with age. The hairs may be grey or brown. Most species have a whitish (or black or brown), chalky punctation (cystoliths; cf. Zimmermann 1922) originating from the hair scars or hair bases, especially on the upper leaf-surface, rendering the leaves scabrid. In several species white cystoliths can be found on the stem, petiole and nerves of the lower leaf-surface. The presence of hairs within flowers can be diagnostic (Zimmermann 1922). Siraitia is covered with blackish gland-hairs.

With experience, the drying colour of herbarium specimens is useful for identification: species of Neoachmandra generally dry greenish, those of Pilogyne brown or blackish. Erkens et al. (2008) made an assessment of age and greenness of herbarium specimens as predictors for successful extraction and amplification of DNA, including some Cucurbitaceae.

Cytology

There is a large diversity in chromosome number within Cucurbitaceae. Schaefer & Renner (for Kubitzki, in press) report numbers for all genera, where known. It is noteworthy that the cucumber (Cucumis sativus) and the much resembling melon (Cucumis melo), are in different subgenera, and the chromosome numbers are: x = 7 (2n = 14) and x = 12 (2n = 24) respectively (Kirkbride, 1993). Within the genus Cucumis the number varies, 2n = 14, or 24, or sometimes 20, 22, 48 or 72.

Uses

Jeffrey (2001) gives a detailed worldwide account of cultivated Cucurbitaceae, while in PROSEA 8 (1993) all useful Cucurbitaceae occurring in Malesia are treated in detail by several authors. Young sprouts and flowers of all cultivated Cucurbitaceae are edible. In addition to Gildemacher, Jansen & Chayamarit (1993) can be mentioned that of New Guinean Trichosanthes species with red fruit pulp the fruits and seeds are edible, especially when cooked in a young state. Edible Cucurbits of SE Asia have been enumerated by De Wilde & Duyfjes (2008 (‘2007’)).