Origin of the flower
The flower of an angiosperm represents a further reduction and specialization of the heterosporous form discussed in relation to the other seed plants. The anther is simpler than that of the Bennettitales or Gnetales (Topics R1 and R3); the ‘fertile leaf’ is, in most, a simple stalk or filament. The male gametophyte is always reduced to a total of three cells . The origin of the carpel surrounding the ovules is obscure but it may be a modified leaf, or a cupule as in the Caytoniales . In nearly all angiosperms the ovary completely encloses the ovules. The carpel also has a stigma and usually a style joining this to the ovary. The carpel may have evolved in response to insect visitationof the flowers, perhaps as protection for the vulnerable and nutritious ovule. Insects could transfer pollen to other flowers and insect pollination is thought to be the primitive pollination type in angiosperms.
The female gametophyte that develops within the ovule, known as the embryo sac, is much reduced compared with other seed plants . No archegonia are produced and, in many, nuclei are formed without cell walls.
From pollination to fertilization in angiosperms takes a few hours and, in contrast to all other seed plants, the double fertilization leads to growth of the endosperm as the seed’s food store. This means that the food store only grows in fertilized seeds, potentially saving resources. In all other seed plants the female gametophyte provides the food store.
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