9.1 Plant structure and growth

9.1.1 Draw and label plan diagrams to show the distribution of tissues in the stem and leaf of a dicotyledonous plant.

Plant stem

Epidermis – Layer of cells normally covered with a waxy cuticle to prevent water loss

Cortex – Tissue on the outer edge of the stem which often contains cells with secondary thickening on the cell walls to provide extra support for the stem

Vascular bundle – Longitudinal set of tubes consisting of phloem and xylem tubes. They are responsible for the transportation of different substances around the plant (Contains xylem, phloem and cambium tissue)

Xylem – Longitudinal set of tubes which transport water from the roots upwards to the leaves.

Phloem – Responsible for transporting products of photosynthesis such as glucose around the plant.

Cambium – A type of lateral meristem found between the xylem and phloem tubes that forms a vertical cylinder in the stem. Produces the secondary phloem and xylem through cell division.

Pith tissue – Found in the centre of the stem

Leaf

Upper epidermis – Layer of cells found on top of the leaf covered in a waxy cuticle to prevent water loss.

Palisade mesophyll – Densely packed with chloroplasts, allowing photosynthesis to occur. Found on the upper portion of the leaf, where light is the most intense.

Spongy mesophyll – Cells here are loosely packed, with few chloroplasts. The cells being loosely packed allows for gas exchange to occur. Found on the lower portion of the leaf, close to the lower epidermis.

Stoma – Pore that allows CO2 for photosynthesis to diffuse in and O2 to diffuse out

Guard cells – Pair of cells found on the stoma which control when they close and open in order to control the amount of transpiration.

Lower epidermis – The bottom layer of cells on a leaf which contains the stoma and guard cells. Has a thinner wax cuticle, as it is in a cooler position.

Remember that the xylem is closer to upper epidermis, while the phloem closer to lower epidermis

 

9.1.2    Outline three differences between the structures of dicotyledonous and monocotyledonous plants.

Dicotyledonous plants

  • 2 cotyledons
  • Apical and lateral meristems – Grow both in width and in height
  • Net-like leaf veins

Monocotyledonous plants

  • 1 cotyledon
  • Only apical meristems
  • Parallel leaf veins

9.1.3    Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues.

Xylem vessels transport water and minerals from the stem and roots into the leaf.

Phloem tubes transports the products of photosynthesis (glucose) to tissue around the plant.

The epidermis and waxy cuticle serve to prevent water loss and regulate gas exchange.

Palisade cells contain chloroplasts, which are responsible for photosynthesis.

The spongy mesophyll layer and the air spaces within allow for gas exchange.

Stomata is where gas exchange occurs between the leaves and the external environment. Guard cells control the opening and closing of the stomata, in order to regulate gas exchange

9.1.4    Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and tendrils.

Roots

Tap root of a carrot

  • Swollen with water
  • Mass of root allows the plant to stabilise in loose soil

Stems

Stem tuber of a potato

  • Underground
  • Stores starch, produced by sugar from photosynthesis

Leaves

Bulb of an onion

  • Fleshy leaves which store nutrients

9.1.5    State that dicotyledonous plants have apical and lateral meristems.

Meristems are stem cells found on plants that grow indefinitely.

Apical meristems are found on the tips of stems and roots, and allow for the growth of a plant in an upwards fashion

Lateral meristems are found between the phloem tissue and xylem tissue in the cambium of a plant. This causes the plant to grow in a sideways fashion, an example being trees.

9.1.6    Compare growth due to apical and lateral meristems in dicotyledonous plants.

Apical meristems

  • Found on tips of roots and stems
  • Product of embryonic cells
  • Produces initial tissues of actively growing plant from the outset
  • Forms epidermis, ground tissues and primary phloem/xylem
  • Growth in length and height of plant

Lateral meristems

  • Found between phloem and xylem tissues
  • Originates from cambium
  • Functions in older stems (and  roots), and in woody stems from the outset
  • Forms secondary phloem and xylem tissue
  • Growth in girth of stem (width)

9.1.7    Explain the role of auxin in phototropism as an example of the control growth.

Tropism is the movement towards or away from a directional stimulus

A positive tropism means a movement towards the stimulus

A negative tropism means a movement away from the stimulus

Therefore, phototropism is the movement towards or away from the stimulus of light energy

Auxin is a growth hormone manufactured by cells undergoing repeated cell division, and thus can be found on stem and root tips.

When auxin is stimulated, it moves itself from within the stem/root tip. It moves toward the shaded area, in order to promote growth in that area. When the shaded side of the stem/root grows, it causes the stem/root to bend towards the light.

 

 

 

 

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