Organismal Biology #18
VASCULAR PLANTS WITHOUT SEEDS

Vascular plants (tracheophytes):  Vascular tissue provides support that holds plants erect and allows them to grow much taller, also allowing transport of materials from one part to another.
Leaves are the principal organs of photosynthesis in most plants.
The light reactions of photosynthesis take place most efficiently in the palisade layer, and the light-independent (or "dark") reactions in the spongy mesophyll.
Vascular tissues (xylem, phloem) help transport materials within the plant.
Early vascular plants had dichotomous branching. Stems were photosynthetic, with no true leaves.
Lepidophytes and arthrophytes developed simple leaves (microphylls) with a single vein or vascular bundle.   Pterophytes developed larger leaves (megaphylls) with branching veins.

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.

NUTRITION and TRANSPORT in vascular plants:
  • Leaves are made of:
    • Upper epidermis, coated with a waxy cuticle.
    • Palisade layer, containing the highest density of chloroplasts.   The "light" (light-dependent) reactions of photosynthesis are most efficient here.
    • Spongy mesophyll:   The "dark" (light-independent) photosynthesis reactions are most efficient here because air spaces facilitate gas exchange.
    • Veins (extensions of xylem + phloem) run through the spongy mesophyll layer.
    • Lower epidermis, coated with a waxy cuticle, contains pores for gas exchange called stomates or stomata.
  • Phloem transport:   The phloem tissue transports photosynthetic products from the leaves to other parts of the plant, principally downward through the stem. The principal transport cells in phloem are seive tube cells.
  • Xylem and transpiration:
    • Water and dissolved minerals (ions, including K+, Ca2+, Mg2+, NO3-, PO43-) ascend from roots through stems to upper parts of the plant, traveling through tube-like tracheids of the xylem.
    • Loss of water from leaves is called transpiration.
    • The ascent of sap seems to be governed largely by transpiration pull, or reduced pressure from above, a process requiring long, unbroken chains of fluid with no air bubbles. Root pressure also helps a bit.
      Vascular plant 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.
Plants B

Life cycle comparison


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