Summary

Concept 27.1: 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 seven cells. The microgametophyte is the two- or three-celled pollen grain. Review Figure 27.2 and ACTIVITY 27.1
• Following pollination, the pollen grain delivers sperm cells to the embryo sac by means of a pollen tube.
• Angiosperms exhibit double fertilization, forming a diploid zygote that becomes the embryo and a triploid endosperm that stores reserves. Review Figure 27.4 and ANIMATED TUTORIAL 27.1

Concept 27.2: Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State

• In annuals and biennials, flowering and seed formation are followed by the death of the rest of the plant. Perennials live longer and reproduce repeatedly.
• For a vegetatively growing plant to flower, a shoot apical meristem must become an inflorescence meristem, which in turn must give rise to one or more floral meristems. These events are determined by specific genes. Review Figure 27.6
• Some plants flower in response to photoperiod. Short-day plants (SDPs) flower when nights are longer than a critical length specific to each species; long-day plants (LDPs) flower when nights are shorter than a critical length. Review Figures 27.7 and 27.8 and ANIMATED TUTORIAL 27.2
• The mechanism of photoperiodic control of flowering involves phytochromes and a diffusible protein signal, florigen (FT), which is formed in the leaf and is translocated to the shoot apical meristem. Review Figures 27.9 and 27.10
• In some angiosperms, exposure to cold—called vernalization—is required for flowering. In others, internal signals (such as gibberellin) induce flowering. All of these stimuli converge on the meristem identity genes.

Concept 27.3: 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 for reproduction. Review Figure 27.12
• Some plant species produce seeds asexually by apomixis. Review Figure 27.13
• Woody plants can be propagated asexually by grafting. Review Figure 27.14