6: Growing Diversity of Plants
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When plants developed basic tissues and organs and thus became mature enough to survive on land, they started to increase in their diversity. All plants studied in this and following chapters belong to plants\(_2\), or kingdom Vegetabilia which is split into three phyla (Figure 6.1.1): Bryophyta (mosses and relatives), Pteridophyta (ferns and allies), and Spermatophyta (seed plants). The most striking differences between these phyla lay in the organization of their life cycles.
Land plants have a sporic life-cycle (Figure 4.4.7) that begins with a diplont (sporophyte); the mother cell of spores goes through the meiosis and produces haploid spores. These spores develop into haplont which produces female and male gametangia (gamete “homes”). Female is called archegonium, the male—antheridium; the archegonium produces oocyte which is fertilized by the antheridium’s spermatozoon in the process of oogamy. When this fertilization happens, it forms a diploid zygote which then matures into a young sporophyte growing on a gametophyte. This kind of same species parasitism is almost unique in the living world. Only viviparous animals (like mammals with their pregnancy) could be compared with land plants.
- 6.1: Bryophyta - the Mosses
- Bryophyta has approximately 20,000 species. They do not have roots, but have long dead cells capable of water absorbency via apoplastic transport, these cells are called rhizoid cells. Their sporophyte is reduced to sporogon, which is simply a sporangium with setamosses: stalk of the sporogon (see) (stalk), and is usually parasitic. Gametophyte of bryophytes starts its development from a protonema, thread of cells.
- 6.2: Pteridophyta - the Ferns
- Pteridophyta, ferns and allies, have approximately 12,000 species and six classes. They have a sporic life cycle with sporophyte predominance whereas their gametophytes are often reduced to prothallium, small hornwort-like plant. Another frequent variant is the underground, mycoparasitic gametophyte. Pteridophyta (with one exception) have true roots. Most of them have vascular tissues and are homoiohydric. This is why seed plants together with ferns have a name vascular plants.