|
Vascular plants are called tracheophytes. Vascular tissue
provides support that holds plants erect and allows them to grow much
taller; it also allows plants to transport materials from one part
to another. Vascular tissues (xylem, phloem): Plant tissues that conduct fluid through cells with stiff cell walls. The stiff cell walls (wood) allow plants to grow taller and erect. By conducting fluids, vascular tissues allow above-ground plant parts to receive water and nutrients absorbed by the roots, and they allow the parts that do not photosynthesize to receive sugars and other products from the green, photosynthesizing parts. Early vascular plants (Rhyniophyta and Psilophyta): The earliest vascular plants, like Rhynia and Asteroxylon, lived in moist, swampy places during the Silurian period. Dichotomous (two-fold) branching characterized all plant parts. Much of each plant grew out horizontally, but some parts turned upward and grew erect. No true leaves were present; stems were green and photosynthetic. Surviving relatives include only a few living genera (Psilotum, Tmesipterus) with spore-forming structures (sporangia) terminal in position. Stems are green and photosynthetic; no true leaves or roots are present. Stomates are distributed over the outside surface of the stems. Lepidophyta (lycopods): Club mosses and their relatives. True roots are present and dichotomously branched. Leaves (mostly small) are microphylls— each has a single vein in the middle, and the vascular bundle is not interrupted where this vein arises. Some leaves bear reproductive sporangia in the angle of attachment (axillary position). Modern lycopods are all small plants, but some Carboniferous lycopods grew to tree-like heights. Arthrophyta or Sphenopsida: Horsetails, with spore-forming parts grouped in a cone-like structure at the top, and spores hidden beneath scale-like sporophylls. Small leaves are arranged in tiers or whorls; true roots are present. Equisetum is the only living genus. Pterophyta: Ferns and fernlike plants, with true roots. Leaves are megaphylls— each has many branching veins, and the vascular bundle is interrupted by a leaf gap where the main vein arises. Leaves carry sporangia on their lower surfaces or their margins. Life cycle has a dominant sporophyte. |
| Prev | rev. July 2010 | Next |