20: Early Land Plants

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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.

20.1: 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).
20.2: 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.
20.3: Anthocerotophyta
The name Anthocerotophyta means 'horn flower plant'. These strange plants get their name from the horn-like sporophytes they produce. Hornworts have thallose gametophytes with monoplastidic cells, out of which grow their long photosynthetic sporophytes, composed of a sporangium that grows from a basal meristem.
20.4: Marchantiophyta
Liverworts can have either thallose or leafy gametophytes. Leafy gametophytes can be distinguished from mosses by the orientation of their leaves and the lack of a costa. Thalloid liverworts can be distinguished from hornworts by the multiple chloroplasts present in each cell. Some liverworts can reproduce asexually using gemmae.
20.5: Bryophyta
Most described bryophyte species diversity (around 13,000 species) belongs to the mosses. Unlike other bryophytes, mosses are exclusively leafy and have spirally arranged leaves. Sporophytes in most species form complex capsules, involving multiple layers of structures. Members of the mosses have defied many of the typical bryophyte descriptors.
20.6: 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.
20.7: Lycopodiopsida
Lycophytes are a group of SVPs that produce microphylls and can be homosporous (as in Lycopodium) or heterosporous (as in Selaginella). These plants produce spores within strobili. Extinct lycophytes formed large trees that dominated forests in the Carboniferous period. These plants formed the coal deposits we know today.
20.8: Polypodiopsida
Ferns and horsetails represent the second lineage of SVPs. These plants have megaphylls and produce homospores. Horsetails produce these spores within a strobilus, while ferns produce them within clusters of sporangia called sori. Whether the gametophytes produce archegonia, antheridia, or both is often dependent on environmental conditions.
20.9: Chapter Summary
A brief summary of the chapter concepts.

Attribution

Maria Morrow (CC-BY-NC)