Title: Evolution of Seed Plants – Biology 2e
Headings: 132
Evolution of Seed Plants

132 Evolution of Seed Plants
Learning Objectives
Evolution of Gymnosperms
Conifers. This boreal forest (taiga) has low-lying plants and conifer trees. (credit: L.B. Brubaker, NOAA)
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Evolution of Angiosperms
Flowers and Fruits as an Evolutionary Adaptation
Evolution Connection
Section Summary
Critical Thinking Questions
Glossary

Content: Evolution of Seed Plants – Biology 2e
Seed Plants
132 Evolution of Seed Plants
Learning Objectives
By the end of this section, you will be able to do the following: Describe the two major innovations that allowed seed plants to reproduce in the absence of water
Explain when seed plants first appeared and when gymnosperms became the dominant plant group
Discuss the purpose of pollen grains and seeds
Describe the significance of angiosperms bearing both flowers and fruit
The first plants to colonize land were most likely related to the ancestors of modern day mosses (bryophytes), which are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants—the pterophytes—from which modern ferns are descended. The life cycle of bryophytes and pterophytes is characterized by the alternation of generations, which is also exhibited in the gymnosperms and angiosperms. However, what sets bryophytes and pterophytes apart from gymnosperms and angiosperms is their reproductive requirement for water. The completion of the bryophyte and pterophyte life cycle requires water because the male gametophyte releases flagellated sperm, which must swim to reach and fertilize the female gamete or egg. After fertilization, the zygote undergoes cellular division and grows into a diploid sporophyte, which in turn will form sporangia or “spore vessels.” In the sporangia, mother cells undergo meiosis and produce the haploid spores. Release of spores in a suitable environment will lead to germination and a new generation of gametophytes. In seed plants, the evolutionary trend led to a dominant sporophyte generation accompanied by a corresponding reduction in the size of the gametophyte from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte.