Other groups, which first appeared towards the end of the Palaeozoic, reproduce using desiccation-resistant capsules called seeds. They are accordingly are called spermatophytes or seed plants. In these forms the gametophyte is completely reduced, taking the form of single-celled pollen and ova, and the sporophyte begins its life enclosed within the seed. Some seed plants may survive in extremely arid conditions. They include the following extant groups:
* Cycadophyta (Cycads)
* Ginkgophyta (Ginkgo)
* Pinophyta (Conifers)
* Gnetophyta (Gnetae)
* Magnoliophyta (Flowering plants)
The first four groups are referred to as gymnosperms, since the embryonic sporophyte is not enclosed until after pollination. In contrast, the flowering plants or angiosperms the pollen has to grow a tube to penetrate the seed coat. They were the last major group of plants to appear, developing from gymnosperms during the Jurassic and spreading rapidly during the Cretaceous. They are the predominant group of plants in most terrestrial biomes today.
Note the higher-level classification of plants varies considerably. Some authors have restricted the kingdom Plantae to include only embryophytes, others have given them various names and ranks. The groups listed here are often considered divisions or phyla, but have also been treated as classes, and they are occasionally compressed into as few as two divisions.
On a microscopic level, embryophyte cells remain very similar to those of green algae. They are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. These usually take the form of chloroplasts, which conduct photosynthesis and store food in the form of starch, and characteristically are pigmented with chlorophylls a and b, generally giving them a bright green color. Embryophytes also generally have an enlarged central vacuole or tonoplast, which maintains cell turgor and keeps the plant rigid. They lack flagella and centrioles except in certain gametes.
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