BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.5. Sexual reproduction in flowering plants

21.5.1. Life cycle of flowering plants

The evolutionary success of flowering plants on land is largely due to adaptations associated with terrestrial sexual reproduction, as discussed in Section 2.7.7. The key adaptations include the following:

1) The production of seeds and fruits, which provide nourishment and protection for the embryo and facilitate plant dispersal;

2) the absence of free-swimming male Gametes. Male gametes are contained within pollen grains and are transferred to FEMALE REPRODUCTIVE Organs in a process called pollination. Once landed on the stigma, a pollen grain germinates to form a pollen tube through which the male gamete Nucleus reaches the female gamete;

3) extreme reduction of the gametophytic generation, which is poorly adapted to terrestrial life in more simply organized plants such as bryophytes (see below).

The life cycle of a flowering plant is illustrated in Fig. 21.15, A. When studying various plant groups, it is helpful to remember that their life cycle involves Morphology/12.html">ALTERNATION OF GENERATIONS, as simplified in Fig. 21.15, B. This is discussed in more detail in Section 2.7.1. In flowering plants, the gametophytic generation is virtually nonexistent as a free-living individual. It would be difficult to recognize that alternation of generations occurs at all were it not for comparisons between extant PLANTS AND THEIR more primitive ancestors.

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Fig. 21.15. A. Life Cycle of a flowering plant. B. Life cycle involving alternation of generations. The diploid sporophytic generation alternates with the haploid gametophytic generation. The sporophyte reproduces asexually via spores, whereas the gametophyte reproduces sexually via gametes. This type of life cycle is characteristic of all plants, although it is less obvious in flowering plants.