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Lycophyta Sphenophyta |
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These three divisions along with the Pterophyta (ferns) are collectively know as the "seedless vascular plants". They have developed a vascular structure that permits the transport of water and nutrients but they do not reproduce by seeds Uptake and distribution of water became possible when plants developed roots and xylem. The movement of food and nutrients required the differentiation of phloem. In the more primitive plants the conducting tissues are arranged in a cylinder with phloem surrounding xylem. This is a protostele and is an arrangement that flowering plants have retained in their roots. It allows for vascular continuity to be maintained between the root and the shoot or the shoot and any structures arising from it. Psilophyta |
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Psilotum grows wild in Florida woodlands and all members of the division, Psilophyta, today are tropical plants. |
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With the development of proper vascular systems it became possible to supply water to larger leaves: megaphylls were developed, perhaps by the coalescence of smaller microphylls. So, contrary to expectation, compound leaves may have developed before simple leaves. The other thing that becomes apparent as you study the life cycle is that the diploid sporophyte became dominant. Most of the seedless vascular plants are homosporous, the spores grow into a gametophyte that is usually bisexual with both antheridia and archegonia. The development of anatomy, physiology and reproductive mechanisms adapted to life on the land permitted an explosion of three divisions of plant life which survive to this day although they have been overshadowed by the success of the flowering plants. It probably took the animals time to catch up with the opportunity provided by this new development and perhaps that is why the Upper Carboniferous or Pennsylvanian period was the highpoint of the earth's photosynthetic productivity. We are using the remains of that productivity today. The coal measure forests were dominated by tree ferns, horsetails and clubmosses. There are no truly woody members of these divisions alive today. (The stems of living "tree ferns" are supported by a dense mass of roots). Lycophyta |
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Selaginella species are mostly tropical; they are similar in structure to Lycopodium but rather more delicate and they usually grow in damp places. A big difference from Lycopodium is that they are heterosporous. The strobilus contains micro- and mega-sporophylls. They find some use as an indoor "ground-cover" plant in interiorscapes. Sphenophyta |
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Horsetails can be aggressive and troublesome weeds, although they are sold for water gardens and have some decorative value if they can be kept within bounds. |
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Copyright © Michael Knee, |