This lesson traces the early evolution of land plants and explains how their reproductive life cycles changed. These changes set the stage for seed plants to reproduce in environments where free water was not always available.
The earliest land plants were likely related to the ancestors of modern bryophytes, including mosses and liverworts. They appeared roughly 500 million years ago. Primitive vascular plants followed; their descendants include modern ferns and other pterophytes.
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Alternation of generations
A life cycle alternating between a haploid gametophyte that produces gametes and a diploid sporophyte that produces spores.
Pollen
A male gametophyte that transports sperm-producing cells and helps reduce dependence on free water for fertilization.
Heterospory
Production of two spore types: megaspores, which develop into female gametophytes, and microspores, which develop into male gametophytes.
Progymnosperms
Transitional plants that produced wood through secondary growth but reproduced by releasing spores rather than seeds.
Fruit
A structure commonly formed from ovary tissues that can protect seeds and aid their dispersal.
Quiz questions
How does pollen help seed plants reproduce without sperm swimming across an exposed film of water?Answer: It transports the male gametophyte to a protected fertilization site.
How can seed dormancy help a plant's offspring?Answer: It allows seeds to wait until conditions become favorable for growth.
What is coevolution between flowers and pollinators?Answer: Reciprocal evolutionary influence between flowering plants and their pollinators.
What does a node in a phylogenetic tree represent?Answer: A branching point representing a common ancestor of descendant lineages.
Why can comparisons of conserved DNA regions help estimate evolutionary relationships?Answer: Sequence similarities and differences can provide evidence about shared ancestry.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.