Chapter Summary
37.1 Most Angiosperms Reproduce Sexually
Sexual reproduction promotes genetic diversity in a population. The flower is an angiosperm’s structure for sexual reproduction.
Flowering plants have microscopic gametophytes. The megagametophyte is the embryo sac, which typically contains eight nuclei in a total of seven cells. The microgametophyte is the pollen grain, which usually contains two cells. Review Figure 37.2, Activity 37.1
Following pollination, the pollen grain delivers sperm cells to the embryo sac by means of a pollen tube.
Plants have both physical and genetic methods of preventing inbreeding. Physical separation of the gametophytes and genetic self-
incompatibility prevent self- pollination. Review Figure 37.4 Most angiosperms exhibit double fertilization: one sperm cell fertilizes the egg cell, forming a zygote, and the other sperm cell fertilizes the central cell, where its nucleus unites with the two polar nuclei to form a triploid endosperm. Review Figure 37.5, Animation 37.1
Ovules develop into seeds, and the ovary wall and the enclosed seeds develop into a fruit. Review Figure 37.6
The hormone abscisic acid (ABA) promotes seed development and dormancy.
37.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State
In annuals and biennials, flowering followed by seed formation usually leads to the death of the rest of the plant. Perennials live a long time and typically reproduce repeatedly.
For a vegetatively growing plant to flower, an apical meristem in the shoot system must become an inflorescence meristem, which in turn must give rise to one or more floral meristems. These events are under the influence of meristem identity genes and floral organ identity genes. Review Figure 37.7
Some plants flower in response to photoperiod. Short-
day plants (SDPs) flower when the nights are longer than a critical night length specific to each species; long- day plants (LDPs) flower when the nights are shorter than a critical night length. Review Figure 37.9 The mechanism of photoperiodic control involves phytochromes and a circadian rhythm. Review Investigating Life: The Flowering Signal, Animation 37.2, Activity 37.2
A flowering signal, called florigen, is formed in a photoperiodically induced leaf and is translocated to the sites where flowers will form. Review Figures 37.10, 37.11
In some angiosperm species, exposure to low temperatures—
vernalization—is required for flowering; in others, internal signals (one of which is gibberellin in some plants) induce flowering. Review Figures 37.12, 37.13
37.3 Angiosperms Can Reproduce Asexually
Asexual reproduction allows rapid multiplication of organisms that are well suited to their environment.
Vegetative reproduction involves the modification of a vegetative organ—
usually the stem— for reproduction. Review Figure 37.14 Horticulturists often graft different plants together to take advantage of favorable properties of both stock and scion. Review Figure 37.15
Some plant species produce seeds asexually by apomixis.
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