Structure of vascular tissue
- The vascular bundle, xylem and phloem, is arranged centrally.
- This is ideal to resist vertical stresses, and is helpful to anchorage.
- There are various vascular bundles arranged in a ring at the periphery.
- This gives flexible support, but also resistance to bending.
- The xylem is arranged in the midrib and the network of veins.
- Gives flexible strength and resistance to tearing.
The structure of the Xylem
Main up of two main cells; vessels and tracheids.
- They form a system of tubes which water travels in.
- As they develop and diffrentiate, material called lignin is incooperated in the cells.
- This means the living tissue inside the cells dies, and then the end of the cells, leaving a hollow tube.
- The lignin also strengthens the xylem and makes it impermeable to water (apart from bordered pits which are unlignified).
The xylem has two roles:
- transport and
- mechanical support.
Transport in the xylem
Water uptake by the roots
Water used by photosynthesis or lost by transpiration must be replaced. The area of greatest uptake is in the root hair zone, which has a great surface area.
The soil has a very dilute solution of mineral ions, and the plant has a very high concentration. So water moves into the celll by OSMOSIS.
Water can move through three pathways:
The apoplast- through the CELL WALL
The Symplast- through the cytoplasm and plasmodesmata
The vacuolar- through the vacuoles.
Apoplast probably carries the most water, as it is the quicklest route.
The Casparian *****
In the root, an endodermis surrounds the vasualr bundle in the middle of the root. In the endodermis cells, a layer of suberin in found, which makes up the Caparian strip. This is entirely waterproof, and prevents the use of the apoplast pathway. It diverts water into the cytoplasm of the cell, and it then moves by the symplast pathway.
The fact that mineral ions are moved into the xylem by active transport justifies the casparian strip. The mineral ions are pumped into the xylem, so as to lower the water potential and encourage the movement of water into the vessel, BY OSMOSIS. This water potential gradient is called root pressure.
Mineral ions are first uptaken by active transport from the soil, and then travel through the apoplast pathway. Then, on reaching the casparian strip, they have to be actively taken up into the symplast pathway. This allows the plant to be selective in which ions it uptakes.
Moving water from the roots to the leaves
Transpiration is a passive process.
- Water is lost from the leaves as water vapour through stomata.
- This loss of water draws waters from xylem and into the leaf.
- This occurs because of cohesive forces - the forces between water molecules,
- and adhesive forces- forces between water molecules and the hydrophilic lining of the vessels.
- This is known as cohesive-tension theory.
- Capillarity is another force that could contribute. This is the tendency of water to move up small tubes. This is more prevalent in saplings than trees.
Plants have to balance the water they lose with the uptake of water in the roots. This is a dilemma, because stomata need to be open to allow gas in, but also end up losing water. 5% of water is lost through the epidermis- though this is reduced by the waxy cuticle.
If a plant loses too much it wilts, and can die.
Factors that effect transpiration
Temperature increases kinetic energy of the water particles, and so increases the evaporation out of the leaves. The heat also reduces the water potential of the air outside, increasing the concentration gradient.
- Wind movement
On still days, a layer of water vapour can develop ontop of the leaves, and over stomata. This reduces the concentration gradient between the leaf and the atmopshere, and reduces transpiration rate. If there is wind, this water vapour layer is removed.
The UK rarely experience humidity over 70%, but a high humidity means there is a smaller concentration gradient between the leaf and the atmosphere, and reduces evaporation.
- Light intensity
Effects the stomata opening and closing, and so effects transpiration.
Actually measure the rate of water uptake, but since 99% of water is lost to the environment, there is little difference between that and transpiration.
It allows us to investigate various effects on transpiration. It is set up as follows:
- Cut a leafy shoot under water.
- Completely fill apparatus with water.
- Fit the leafy shoot into the potometer under water.
- Seal joints with jelly.
- Introduce air bubble into the capillary tube.
- Measure the distance the air bubble moves in a given time.
- Use the water resevoir to bring bubble back to the starting place.
CUT, FILL, FIT AND SEAL, BUBBLE, DISTANCE/TIME
These live in habitats with adequate water supply. This includes well drained soils, and moderately dry air. However, they have adaptions to survive winter months.
- Trees and shrubs sometimes shed leaves.
- The aerial- non-woody parts of plants die, but underground organs survive (e.g. bulbs and corms)
- Dormant seeds
Hydrophytes grow submerged or partially submerged in water.
- Stomata on upper surface.
- Undeveloped xylem, as surrounded by water (transport not needed.)
- No lignified tubes as supported by water.
- Little cuticle, as water loss is less of a problem.
- Large air spaces act as reserve of gases, but also keep the leaves buoyant.
These live in areas with very little water. Marram grass have to surviev through alck of soil, shade and in salt spray.
- Thick cuticle
This reduces the loss of water through the cuticle of the plant.
- Embedded stomata
These allow a layer of water vapour to be trapped outside the stomata, reducing the concentration gradient between the stomata and the atmosphere, and reducing transpiration.
- rolled leaves
These reduce the surface area that water can be lost through- stomata are found on the inside of the roll.
These also help trap water vapour between them, reducing water potential gradient.
The structure of the phloem
The phloem consists of two main cells: sieve tubes and companion cells.
Sieve tubes lack a nucleus and other organelles. Their end walls are perforated with pores, and are called sieve plates. Cytoplasmic filaments run the length of each sieve tube cell.
Each sieve tube is associated with atleast one companion cell. They have dense cytoplasm, a large nucelus and many mitochondria. They are attached by plasmodesmata.
The movement of glucose (as sucrose) and amino acids in the phloem is called translocation.
The leaves are a source of organic molecules, and growing tissues are the sink.
Theories of translocation
Early evidence about translocation of solutes was obtained from ringing experiments (cutting out pasrt of the stem and analysing above and below.)
The technique of radioactive tracing combined with using aphid mouthparts demonstrated that translocation is a rapid process. (Mouthpart creates a stylet, sap seeps out, and shows that translocation is too rapid to be diffusion.)
Radioisotope labelling using carbon dioxide combined with autoradiography shows that sucrose is transported bi-directionally to sinks. (radioactive isoptopes show up as 'fogging' and shows that sugar is in aerial and lower parts of plant.)
The mass flow hypothesis suggests that there is a passive flow of sucrose from source to sink. (no details required).
The mass flow hypothesis does not account for all observations such as movement in opposite directions at the same time and at different rates.
Other hypotheses have been proposed; including diffusion and cytoplasmic streaming. (no details required)
Mass flow theory
water moves into concentrated solution.
this increases hydrostatic pressure in the upper tube, pushing water out the diluted solution.
This water is drawn along into the concentrated solution, and the cycle continues.