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.