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-celled pollen grain. Review Figure 27.2
   
• 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.