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