There are four major evolutionary groups of land plants: Bryophytes, Seedless Vascular Plants (SVPs), Gymnosperms, and Angiosperms. These groupings represent major changes in plant structure and life history characteristics over the course of time that coincide with major changes in the evolution of the Earth, as a whole. Early Earth would have looked quite different than the planet you know. When plants first ventured out of the water, there were no soils. The terrestrial landscape would have been rocky, potentially slick with microbial slime. The lack of surrounding water would mean tissues could dry out, as well as increased exposure to oxygen and damaging wavelengths of sunlight. They would also need more structural support, without water to float in, and changes in temperature would be far more extreme and rapid than their former aquatic habitat. However, they would have abundant CO2 and increased access to sunlight for photosynthesis.
- 2.5.1: Introduction to Early Land Plants
- Bryophytes were the first group of plants to evolve on land, followed by the seedless vascular plants. These early plants, accompanied by their fungal mutualists and other microbes, transformed the rocky terrestrial landscape into an ecosystem with stratified soils and complex biotic communities. Synapomorphies of bryophytes derive from the challenges of life on land, while those of seedless vascular plants relate to increases in height and opportunities for meiosis (i.e. competition).
- 2.5.2: Bryophytes
- There are approximately 23,000 species of bryophytes in three distinct lineages: Anthocerotophyta, Marchantiophyta, and Bryophyta. Lacking vascular tissue, these early plants generally have a prostrate form and grow closely appressed to the substrate. They lack true roots but have anchoring cells called rhizoids that extend from the gametophyte. Bryophytes have a gametophyte dominant life cycle and the sporophytes grow from the megagametophyte.
- 2.5.3: Seedless Vascular Plants
- Seedless vascular plants have lignified vascular tissue that allows them to transport water through woody xylem cells up from true roots, through the stems, up to their leaves. Photosynthetic tissues can distribute sugars through living phloem cells throughout the plant. SVPs are sporophyte dominant with reduced, thalloid gametophytes. Sporophytes are branched with many sites for spore production. They can be divided into two lineages: Lycopodiopsida and Polypodiopsida.
- 2.5.4: Chapter Summary
- A brief summary of the chapter concepts.
Maria Morrow (CC-BY-NC)