Florence Grovida Gardening

Floral Biology As Related To Controlled Pollination

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The flower (Fig. 2.1) differentiates by the successive development of four sepals, six stamens, two carpels and four petals. The carpels form a superior ovary with a 'false' septum and two rows of campylotropous ovules. The nucellar tissue is largely displaced by the embryo sac and, when the buds open, the ovules mainly consist of the two integuments and the ripe embryo sac. The buds open under pressure from the rapidly growing petals. Opening starts in the afternoon, and usually the flowers become fully expanding during the following morning. The bright yellow petals grow to 10-25 mm long and 6-10 mm wide. The sepals are erect. Pollination of the flowers is by insects, particularly bees, which collect pollen and nectar.

Fig. 2.1. Generalized structure of the flower and half-flower of Brassica (M.H. Dickson).

Nectar is secreted by four nectaries situated between bases of the short stamens and the ovary. The flowers are borne in racemes on the main stem and its axillary branches. The inflorescences may attain lengths of 1-2 m. The slender pedicels are 15-20 mm long. After fertilization, the endosperm develops rapidly, while embryo growth does not start for some days. The embryo is generally still small 2 weeks after pollination, and at this stage embryo rescue can be first attempted.

The embryo fills most of the seed coat after 3-5 weeks, by which time the endosperm has been almost completely absorbed. Nutrient reserves for germination are stored in the cotyledons, which are folded together with the embryo radicle lying between them. The fruits of cole crops are glabrous siliquae (pods) (Fig. 2.2), 4-5 mm wide and 40-100 mm long, with two rows of seeds lying along the edges of the replus. One silique contains 10-30 seeds.

Three to 4 weeks after the opening of a flower, the silique reaches its full length and diameter. When it is ripe, the two valves dehisce. Separation begins at the attached base and works towards the unattached distal end, leaving the seeds attached to the placentas. Physical force ultimately separates the seeds, usually by the pushing of the dehisced siliquae against other plant parts either by wind or in threshing operations.