All bryophytes are relatively small and are usually confined to moist environments, relying on water to disperse their spores. Other plants better adapted to terrestrial conditions appeared during the Silurian, and during the Devonian they diversified and spread to many different land environments. These are called vascular plants or tracheophytes. They have vascular tissues or tracheids, which transport water throughout the body, and an outer layer or cuticle that resists desiccation. In most the sporophyte is the dominant individual, and develops true leaves, stems, and roots, while the gametophyte remains very small.
Many vascular plants still reproduce using spores, including the following extant groups:
* Lycopodiophyta (clubmosses)
* Equisetophyta (horsetails)
* Psilotophyta (whisk ferns)
* Ophioglossophyta (adders'-tongues and grape-ferns)
* Pteridophyta (ferns)
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.
source : hydroponicsearch
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