Читать книгу Cucurbits - James R. Myers - Страница 18

There may be only one seed in the fruit, as in chayote, or more typically, tens to hundreds of seeds. Cucurbit seeds, which are rarely winged, are usually flat. The seed coat encloses a collapsed perisperm, an oily embryo and little or no endosperm. The tiny endosperm is consumed during seed development. Two cotyledons make up much of the contents of the seed. Seeds of some cucurbits are enveloped in a false aril of placental origin; in bitter gourd this sarcotesta is red and fleshy, attracting birds as seed dispersal agents.

Seed size, shape and colour vary greatly among the cultivated cucurbits (Fig. 1.3). The largest unwinged seed is that of Hodgsonia, measuring about 7 cm long. The nearly spherical seed of Bryonia is sometimes less than 3 mm in diameter. Cultivars within a crop, including watermelon and squash, may differ considerably in their seed sizes and other seed characteristics. Depending on the cultivar, watermelon seeds are white, tan, brown, black, red, or green. They can also have patterns on them, referred to as dotted, rimmed (dark seed margin), tipped (dark seed tip) or clump (dark seed centre).

Fig. 1.3. Seeds of the Cucurbitaceae. The largest seed (Fevillea cordifolia) shown here measures ca 53 mm across. Also shown are seeds of Cucurbita, Siraitia, Marah, Momordica, Sicana, Trichosanthes, Luffa, Lagenaria, Echinocystis and Melothria (the smallest seeds).

The complex seed coat anatomy of cucurbits has been well studied (Singh and Dathan, 1990). The testa develops from the outer integument of the anatropous, bitegmic, crassinucellate ovule. The inner integument degenerates in fertilized ovules. The mature seed coat in the subfamily Cucurbitoideae consists of an epidermis, hypodermis, main sclerenchymatous zone, aerenchymatous zone and inner parenchymatous or chlorenchymatous zone. In the Zanonioideae, the sclerenchymatous layer is poorly or not differentiated from the hypodermis, and the well-developed aerenchyma has distinctive lignified thickenings. Within each subfamily, there is further anatomical diversity among genera.