Summary

This chapter has traced the complete arc of sexual reproduction in flowering plants, beginning with

the basic whorled structure of the flower itself -- calyx, corolla, androecium and gynoecium,

arranged on a condensed, determinate floral axis -- since every subsequent event described in the

chapter takes place within one of these four whorls.

On the male side, the anther's microsporangia were shown to house microsporogenesis: the meiotic

division of diploid microspore mother cells into haploid microspore tetrads, followed by the

maturation of individual microspores into pollen grains, protected by a remarkably resistant,

sporopollenin-based exine and equipped, ultimately, with two non-motile male gametes. On the female

side, the ovule was shown to house the parallel process of megasporogenesis, in which a single

functional megaspore, surviving from a meiotic tetrad, develops through three free-nuclear

divisions into the mature, seven-celled, eight-nucleate embryo sac, comprising the egg apparatus,

the antipodal cells, and the central cell with its two polar nuclei.

Pollination -- the transfer of pollen from anther to stigma, whether by autogamy, geitonogamy or

xenogamy, and whether carried by wind, water, insects, birds or bats -- was shown to be reinforced

by a whole suite of outbreeding devices (dichogamy, herkogamy, self-incompatibility, dioecy) that

bias flowering plants toward the genetically advantageous outcome of cross-pollination. Once

compatible pollen germinates, pollen-pistil interaction governs the guided growth of the pollen

tube through the style and into the ovule, typically via the micropyle, delivering two male gametes

into a synergid of the embryo sac.

There, the unique angiosperm event of double fertilisation takes place: syngamy, forming the

diploid zygote, and triple fusion, forming the triploid primary endosperm nucleus, occurring

together within one embryo sac. The post-fertilisation events that follow were traced in detail:

the three patterns of endosperm development (nuclear, cellular and helobial); the staged

development of the dicot embryo from zygote through globular and heart-shaped stages to a mature

embryo with plumule, epicotyl, hypocotyl and radicle; the transformation of the ovule into the

mature seed, distinguishing albuminous from non-albuminous seeds; and the parallel transformation

of the ovary into the fruit, distinguishing true fruits from false fruits. …