Made up of large cellswith thick cell walls.
column of cells acting as pipes.
transport water and inorganic ions.
waterproofed by lignin. (cells become lignified)
Lignin makes the plant stiffer (increases tensile strength.)
Xylem vessels die once lignified leaving hollow tubes.
tonoplast breaks down and autolysis of cell contents.
Xylem vessels and inorganic ions.
Mass flow system for transport of inorganic ions.
Absorbed into roots.
Lack of these ions....
Magnesium deficiency = older leaves become yellow.
Nitrogen dificiency = yellow leaves.
Calcium deficiency = stunted growth.
How water is transported through the xylem vessels
1) water diffuses out through stonma down concentration gradient.
2) water evaporates from th surface of cells lining the substomatal cavity.
3) water moveds across the leaf down diffusin gradient.
4) water leaving the xylem reduces the hydrostatic pressure
5) water moves up xylem due to the difference in hydrostatic pressure. (low at top high at root.)
attraction between unlike molecules..
Water adheres to walls of vessel.
high adhesive forces due to large surface area to volume ration within tubes.
Cohesion Tension Theory.
Attraction between like molecules.
i.e. water to water.
ensures breakages along water route.
Cohesive forces ensure that whole column of water moves upwards.
Transpiration rate will vary...
Faster in warmer dryer conditions, (increased evaporation from leaf surface.)
slower in dark or short of water. (when stomata on leaf surface are closed.)
Sclerenchyma also become lignified.
Associated with vascular bundels in the stem.
Fibres die once lignified leaving hollow fibres.
Strength varies in different species. depends on length of fibres and degree of lignification.
Importance of water.
Why is water polar...
Oxygen carries a lone pair of electron and donates one two each hydrogen in the water molecule.
Covalent bond formed
molecule is a v shape.
Oxygen is highly electro-negative.
hydrogen bonding present because of the difference in electro-negativity.
importance of water 2
Many chemicals dissolve easily in water.
Allows vital biochemical reactions to take place.
Dissolved substances can be transported around organisms.
Embroyonic development in plants.
Three distict parts.
Radicle (Young Root)
Plumule (young shoot)
cotyledons. (seed leaves)
Seeds which do not germinate even in ideal conditions are said to be dormant.
Dormancy can be broken through...
1) Exposure to extended period of chilling.
2) Intense heat
3) Microbial degradation of seed coat.
4) a minimum period of light.
5) Chemical action in animals gut.
1) Seed takes in water through micropyle.
2) cells in the embrypo expand as the seed coat ruptures.
3) metaboolic changes triggered.
4) Plant growth substances secrete enzymes which mobilise food reserves.
5) Maltas and amylase breakdown starch to glucose which is converted to sucrose for transport to radicle.
6) Proteases breakdown protiens to give amino acids.