Embryophyte

Embryophytes are plants that grow on land. They are complex, multicellular organisms with special parts for creating new plants. These plants can make new ones in two ways, using tiny cells called spores or seeds. The name "embryophyte" comes from how they care for their young 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 are now the most common land plants.

Embryophytes live mostly on land, unlike their close relatives, the green algae, which live in water. They have special parts that help them live without being wet all the time. These plants are very important because they make food through photosynthesis, using sunlight to create food and release oxygen into the air. This process gives energy to most life on land and helps shape the Earth's climate and water cycles.

Description

Embryophytes, also called land plants, are a group of plants that split from green algae a very long time ago, between half a billion to almost a billion years ago. 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.

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

Evolution

The green algae and land plants form a clade, the Viridiplantae. Scientists think 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 start of land plants. Scientists think this happened because living in fresh water helped these plants get used to 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 small and need moist places because they need water to help their cells move. Many grow in tropical areas, but some live in cold Arctic regions. They often cover the ground in tundra or grow on trees in rainforests.

These plants share similar life cycles. Their haploid gametophyte stage is the main part, while the diploid sporophyte stage is 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 cells to carry water, but these are different from the tubes in other plants.

During the Silurian and Devonian periods (around 440 to 360 million years ago), plants developed tubes with strong cells (tracheids). Some early plants were between non-vascular and true vascular plants. For example, Horneophyton had water-carrying tissue like mosses but a bigger sporophyte stage. Rhynia had simple tracheids, making it a kind of vascular plant. Over time, vascular plants spread and changed, developing coverings to stay wet and bigger 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 branching systems that flattened and 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 changes 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 changes. The sporophyte produces two types of structures: 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 cells that make new plants. Sexual reproduction seems important for keeping plant genomes stable over long periods.

Main article: Spermatophyte