Читать книгу Cucurbits - James R. Myers - Страница 16

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.