What is locomotion and support
Importance of support in terrestrial plants
- Enables holding leaves to receive maximum sunlight for photosynthesis
- Enables exposing flowers in the most suitable position for pollination
- Allows holding fruits and seeds in the possible favourable position for dispersal
- Maintains plant shape.
Support Mechanisms In Dicotyledonous Plants
- Turgidity of cells
Turgor pressure: outward pressure from the inside of a fully turgid cell.
When fully turgid, the close packing of parenchyma cells in cortex and pith of the stem causes them to press against one another to keep herbaceous plants and young woody plants erect. Absence / insufficient water reduces turgor pressure causing loss of support due to wilts.
- Mechanical tissues
(a) Collenchyma cells have uneven thickened cellulose cell walls, and are alive.
- Collenchyma tissue provide flexible support (a mechanical function) to stems and leaves, enabling withstanding the lateral force of the wind.
- The walls of collenchyma cells can be deformed by pressure or tension and retain the new shape even if the pressure or tension ceases.
Location: in young plants, herbaceous plants and some organs such as leaves
(b) Sclerenchyma fibres and sclereids have lignified cell walls and are dead when mature.
- The tough and elastic cell wall of elongated fibres allow the cell to be deformed but can snap back to their original size and shape when the pressure or tension is released.
- Provides great tensile or compressional strength in plants parts, such as in the vascular tissues of stems and roots and the bundle sheath of leaves
- Support the tree while the elasticity allows the trunk and the branches to sway in the wind without breaking. Location: found in small groups in cortex, pith, phloem and shells of coconuts.
3. Distribution of vascular tissues (xylem vessels and tracheids)
The distribution is related to the resistance of the various forces acting upon them, e.g. in land plants the stem is mainly exposed to bending stresses due to the action of wind while roots experience pulling stress.
(i) Xylem vessels and tracheids are dead, the cell walls are lignified and thickened which provides great mechanical strength to resist bending in the stem, reinforce against pulling in the root and are the most important supporting cells in the veins of leaves.
- In leaves, vascular tissue is located at the upper side of midrib and lateral veins, and it extends throughout the leaf surface. This enables resisting tearing forces acting on the leaves blade by the wind.
- In woody stems, the lignified secondary xylem tissues (known as wood) occupy most part of the woody stem, which makes the stem very hard and rigid to avoid depending on cell turgidity for support
Support In Aquatic Plants (Hydrophytes)
Support from buoyancy is provided by:
- Surrounding water, whose density is much higher than that of air, hence providing a larger upthrust force.
- Presence of numerous large air spaces (intercellular spaces) in stems and leaves, which form air-filled cavities extending through the tissues, inside to give buoyancy.
Note: When removed from water, most hydrophytes collapse quickly because of having poorly developed (some lack) mechanical tissues (i.e. collenchyma and sclerenchyma) and xylem tissue is reduced, since it is unnecessary (no need to transport water within the body and buoyancy is provided by water for support).