Embryophyte

Embryophytes are plants that live on land. They are complex, multicellular organisms that have special parts for making new plants. These plants can reproduce in two ways, using tiny cells called spores or seeds. The name "embryophyte" comes from how they care for their young embryos inside the parent plant.

There are two main groups of embryophytes: non-vascular plants, like mosses and liverworts, and vascular plants, which have stems, roots, and leaves. Vascular plants include ferns, gymnosperms like conifers, and angiosperms, or flowering plants. Flowering plants have become the most common group of land plants since the time of the dinosaurs.

Embryophytes are mostly found on land, unlike their close relatives, the green algae, which live in water. They have special structures that help them survive without being constantly wet. These plants are very important because they produce food through photosynthesis, using sunlight to make carbohydrates and release oxygen into the air. This process provides energy for almost all life on land and helps shape the Earth's climate and cycles of water and carbon.

Description

Embryophytes, also known as land plants, are a group of plants that separated from green algae a very long time ago, between half a billion to almost a billion years back. They likely came from freshwater green algae called charophytes. Moving onto land gave these plants more sunlight and carbon dioxide, which helped them grow and change Earth's atmosphere, leading to cooler global temperatures.

These plants are mostly found on land, though some live in water. Their cells have walls made of cellulose and contain structures called chloroplasts that help them make food through photosynthesis. They have a special life cycle where they alternate between two forms: one that makes eggs and sperm, and another that grows from those eggs and sperm. What makes embryophytes unique is that their eggs and sperm are made inside special structures, and the early development of the new plant happens inside the parent, protected and nourished. This protected early growth is why they are called "embryophytes."

Evolution

The green algae and land plants form a clade, the Viridiplantae. Scientists think that these plants and algae split apart about 1,200 million years ago into two groups: chlorophytes and streptophytes. The chlorophytes were mostly sea algae, but some moved to fresh water. The streptophytes also live in fresh water and later moved onto land.

During a time called the Ordovician, some streptophytes moved from water to land. This was the beginning of land plants. Scientists think this happened because living in fresh water helped these plants get used to different land conditions, like rain and changing temperatures.

Diversity

Further information: Alternation of generations

Non-vascular land plants include mosses (Bryophyta), hornworts (Anthocerotophyta), and liverworts (Marchantiophyta). These plants are usually small and need moist environments because they rely on water to help their gametes move. Many grow in tropical areas, but some live in cold Arctic regions. They often cover the ground in tundra and Arctic-alpine areas or grow on trees in rainforests.

These plants share similar life cycles. Their haploid gametophyte stage is the main part of their life, while the diploid sporophyte stage stays small and depends on the gametophyte. Unlike other land plants, their sporophyte stage is short and relies on the gametophyte. Some mosses and liverworts have special water-conducting cells, but these are different from the vascular tissues in other plants.

During the Silurian and Devonian periods (around 440 to 360 million years ago), plants developed true vascular tissues with lignin-strengthened cells (tracheids). Some early plants were between non-vascular and true vascular plants. For example, Horneophyton had water-conducting tissue like mosses but a more developed sporophyte stage. Rhynia had simple tracheids, making it a kind of vascular plant. Over time, vascular plants spread and diversified, developing cuticles to prevent drying out and dominant sporophyte stages with leaves, stems, and roots.

Further information: Polysporangiophyte, Horneophytopsida, and Rhyniopsida

All vascular plants that spread through spores were once thought to be related. Recent research shows that leaves evolved separately in two groups. Lycophytes or lycopodiophytes, including clubmosses, spikemosses, and quillworts, make up less than 1% of living vascular plants. They have small leaves called 'microphylls' that grow from the base of the stems. During the Carboniferous period, tree-like lycophytes like Lepidodendron formed huge forests.

The euphyllophytes, which are over 99% of living vascular plants, have large true leaves (megaphylls) that grow from the sides or tips of stems. One idea is that megaphylls developed from three-dimensional branching systems that flattened and then filled in with tissue.

Main article: Lycopodiophyta

The ferns and horsetails (Polypodiophyta) use spores to spread. Whisk ferns and horsetails differ from true ferns because they lack large megaphylls, likely due to reduction over time. Ferns are a large group with about 12,000 species. They usually have broad, divided leaves that unroll as they grow.

Main article: Fern

Seed plants first appeared near the end of the Paleozoic era and reproduce using seeds that can survive drying out. Seeds develop from spores through complex changes. The sporophyte produces two types of sporangia: megasporangium, which creates a single large spore (megaspore) surrounded by layers forming the seed coat, and microsporangium, which produces microspores that develop into pollen grains. Pollen can be carried by wind or animals to reach an ovule, where fertilization occurs. Seed plants include gymnosperms and angiosperms (flowering plants). In gymnosperms, seeds are not enclosed, while in angiosperms, they are inside a carpel, often with petals forming a flower.

Meiosis in land plants helps repair DNA in reproductive tissues. Sexual reproduction seems important for keeping genomes stable over long periods.

Main article: Spermatophyte