Plant Biology

Plants, characteristics:

  • Eukaryotic organisms
  • Pluricellular (with the cellulose cell wall and starch reserve).
  • Photosynthesizing autotrophs (with chlorophyll a and b).
  • The embryo is protected by a layer of cells -in the early stage of development.

Bryophytes: mosses, liverworts and anthocerans.

Characteristics

  • The first group to conquer the terrestrial environment.
  • They are common in humid and shaded places.
  • The plant is the gametophyte, with gametangia (antheridia and archegonia).
  • Gametes depend on water for transport.

Pteridophytes: ferns and avocets.

Characteristics

  • They are the first tracheophytes.
  • The plant is the sporophyte, with sporangia grouped in sori.
  • Underground stem type in most species (rhizome).
  • Spores are carried by the wind and originate from the prothallium (gametophyte), Gametes depend on water to be transported.

Gymnosperms: pine trees.

Characteristics

  • They are the first spermatophytes -with leaves specialized in producing spores (sporophylls, gathered in strobiles).
  • The plant is the sporophyte, and the strobiles produce microspores (forms the pollen grain) or megaspores (forms the embryo sac).
  • There is no longer dependence on water for fertilization (presence of the pollen tube).
  • After fertilization, the ovule becomes a seed (embryo + nutritive tissue + tegument).

Angiosperms: orange, apple, etc.

Characteristics

  • It is the group with the largest percentage of known plant species.

  • The plant is the sporophyte, and the gametophyte is reduced.

  • In the stamens, there is the pollen sac, where the spores are formed by meiosis, generating pollen grains. In the pistils, meiosis forms the megaspore -this will give rise to the embryo sac.

  • Pollination: transport of the pollen grain from one plant to the pistil of another.

  • Double fertilization occurs (forming the embryo and the endosperm).

  • The endosperm (3n) is the nutritional reserve of the embryo.

  • After fertilization, the ovary becomes a fruit, and the ovules become seeds.

Plant Physiology

Transport of raw sap

  • Salts and water are absorbed by the roots and transported into the plant by the xylem.
  • Root pressure pushes the sap upward.
  • The main factor for the ascent of sap is leaf transpiration and the cohesion between water molecules (cohesion-tension theory).

Leaf Transpiration

  • Transpiration occurs in any part of the plant that is above ground. However, most transpiration occurs in the leaves.

  • The biggest loss of water occurs through the stomas.

  • The stomas open when there is light and water availability.

  • And they close with excessive loss of water or the lack of light.

  • The low concentration of carbon dioxide also stimulates the stomas to open.

Transport of organic sap

  • Organic matter produced in the leaves must be transported to the consuming sources -via the phloem.
  • Sap moves from where hydrostatic pressure is highest (producing organ) to where it is lowest (mass flow theory).

Ring of Malpighi:

  • The removal of a ring -in the bark of the tree- interrupts the flow of the phloem and causes the death of the cells due to lack of nutrients.

  • The phloem, like the xylem, is an important plant conducting tissue. This tissue is concerned with the transport of important substances. Some examples are sucrose, amino acids, hormones, and vitamins.

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