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9.1 – Plant Structure and Growth

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

Stem Cross-Section of a Dicotyledonous Plant

  • Epidermis – Surface of the stem made of a number of layers with a waxy cuticle to reduce water loss
  • Cortex Tissue – Forming a cylinder of tissue around the outer edge of the stem. Often contain cells with secondary thickening in the cell walls, which provides additional support
  • Vascular Bundle – contains xylem, phloem and cambium tissue
  • Xylem – a longitudinal set of tubes that conduct water from the roots, upwards through the stem to the leaves
  • Phloem – Transports sap through the plant tissue in a number of possible directions
  • Vascular Cambium – a type of lateral meristem that forms a vertical cylinder in the stem. This produces the secondary xylem and phloem through cell division in the vertical plane
  • Pith – found in the centre of the stem. Composed of thin walled cells called parenchyma. This degenerates in some plants to leave a hollow stem

  • Cuticle – Waxy layer which reduces water loss through the upper epidermis
  • Upper epidermis – A flattened layer of cell that forms the surface of the leaf and makes up the cuticle
  • Palisade Layer – The main photosynthetic region of the leaf
  • Vascular bundle – Contains the transport system and vascular meristem tissue (xxylem and p-phloem)
  • Spongy Mesophyll – Contains spaces the allow the movement of gases and water through the leaf tissue
  • Lower Epidermis – Bottom surface layer of tissues which contains the guard cell that form each stoma

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

 

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

  • Phloem – transports the products of photosynthesis (sugars, amino acids) to the rest of the plant
  • Xylem – Transports water and minerals into the leaf tissue from the stem and roots
  • Epidermis – Produces a waxy cuticle for the conservation of water. The guard cells of the stomata regulate inward gas exchange. It is tough and transparent to allow light absorption.
  • Stomata – The site of inward diffusion of CO2
  • Palisade Layer – Main photosynthetic region
  • Spongy Layer – Creates the spaces and surfaces for the movement of water and gases
  • Lower Epidermis – Contains stomatal pores which allow gas exchange with the leaf, mainly carbon dioxide.
  • Vascular Bundles – Spread through the leaf like a network. No mesophyll cell is ever more than a few cells from a vascular bundle. They also support the leaf
  • The thin, flat structure of the leaf results in a large surface area to maximise light absorption in the chloroplasts of the mesophyll cells.
  • The air spaces between the mesophyll cell allow gas exchange and are the pathway for
    diffusion

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

Bulbs

  • Onions 

– have scaly outer leaves

– inner leaves filled with food reserves

– the heart contains the terminal bud, the lateral bud and the stem, which eventually                                  becomes a new plant by bulb division short lateral stem

– short lateral stem

Stem Tubers

  • Potatoes

– once the leaves have manufactured sugar from photosynthesis, it is converted into starch

– the potato is attached underground to the lateral stem

– the tuber [potato] is packed with starch and some protein

– this is then able to sprout into a new potato plant

  • Cacti

– leaves are spines to prevent water loss in transpiration

– stem is enlarged for water storage and carries out photosynthesis

  • Strawberries

– an example of runner stems spreading out from the main body of the plant o forms new                       roots where it touches the ground to independently establish small plantlets

– adapted to seek out water sources

Storage Roots

  • Carrots

– An example of a modified tap root

– the root has become swollen with food reserves

– also stores water

– serves to stabilise the plant in the soil

Tendrils

  • Stem tissue modified as a tendril

– these will grow around other stems or support structures

– develop to support the weak stems of climbing plants, such as grape plants

  • Leaf tissue modified as a tendril

– these also serve to support the plant

– an example is sweet peas

9.1.5 – State that dicotyledonous plants have apical and lateral meristems

  • Plants grow from the meristems
  • Meristematic cells divide by mitosis to allow growth of the plant
  • Lateral meristem

– forms from the cambium cells in the centre of the vascular bundles

– Causes secondary growth by adding vascular tissue

– Increases the girth of the stem

– Stem circumference and strength increase

  • Apical meristems

– Occur at the tips of the stem and root

– Case primary growth

– Cell division

– Cell enlargement

– Cell differentiation and specialisation

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

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

  • The concentration of auxin is greatest among cells undergoing cell division
  • Growth of the plant stem is inhibited by light
  • The growth response is regulated by auxin

– Auxin causes the cells to expand on the shaded side

  • This causes the shoot to grow towards the light source

Tropism – bending growth movement towards or away from a directional stimulus
Phototropism – bending growth towards the unilateral source of light
Auxins – type of plant growth hormones, or growth regulating factors

 

  • The cylindrical shoot is enclosed in a sheath of cells called the coleoptile
  • Darwin’s experiments showed that the tip is sensitive to light; the apical meristem
  • Chemicals are transmitted to the rest of the plant to affect growth; it is transported to the zone of cell growth
  • Greater concentration of auxin results in a larger degree of bending

  • Light stimulus leads to growth on the opposite side of the stem