SUPPORT AND MOVEMENT IN PLANTS
Necessity for support and movement
- Movement is a characteristic of all living organisms.
- It enables animals and plants to adjust to their environment.
- Most animals move from place to place but some are sessile (i.e. fixed to the substratum).
- Majority of plants move only certain parts.
- However, though not easily observed all living protoplasm shows movement of one type or another.
Necessity for support and movement in plants
- They enable plants to be held upright to trap maximum light for photosynthesis and gaseous exchange.
- To hold flowers and fruits in appropriate position for pollination and dispersal respectively.
- To enable plants to grow to great heights and withstand forces of environment e.g. strong winds.
- Movement of male gametes to effect fertilization and ensure perpetuation of a species.
- Plant parts move in response to certain stimuli in the environment of tropisms.
Tissue distribution in Monocotyledonous and Dicotyledonous plants
- Vascular bundles are the main support tissues in plants.
- In monocotyledonous stem they are scattered all over the stem.
- While in dicotyledonous stem they are found in a ring or rings.
- In monocots the xylem and phloem alternate around with pith in the centre.
- In dicots of the xylem forms a star in the centre – there is no pith.
- Phloem is found in between the arms of xylem.
- Dicotyledonous plants have cambium which brings about secondary growth resulting in thickening of the stem and root hence providing support.
- Secondary xylem becomes wood, providing more support to the plant.
Role of support tissues in young and old plant
Plants are held upright by strengthening tissues;
- xylem tissue.
Parenchyma and collenchyma are the main support tissues in young plants.
- They are found below the epidermis.
- They form the bulk of packing tissue within the plant between other tissues .
- They are tightly packed and turgid they provide support.
- Their cell walls have additional cellulose deposited in the corners.
- This provides them with extra mechanical strength.
- Their cells are dead due to large deposits of lignin on the primary cell wall.
- The lignified wall is thick and inner lumen is small, hence provide support.
- Sclerenchyma fibres are arranged in elongated and in longitudinal sheets giving extra support.
- They are found in mature plants.
- Has two types of specialised cells.
- Vessels and tracheids.
- Vessels are thick-walled tubes with lignin deposited in them.
- They give support and strength to the plant.
- Tracheids are spindle-shaped cells arranged with ends overlapping.
- Their walls are lignified.
- They help to support and strengthen the plant.
Plants with weak stems obtain their support in the following ways.
- Some use thorn or spines to adhere to other plants or objects.
- Some have twinning stems which grow around objects which they come into contact with.
- Others use tendrils for support.
- Tendrils are parts of a stem or leaf that have become modified for twinning around objects when they gain support.
- In passion fruit and pumpkin, parts of lateral branches are modified to form tendrils.
- In the morning glory, the leaf is modified into a tendril.