CHAPTER 2 : LOCOMOTION & SUPPORT

LEARNING OUTCOMES• To explain the necessity for support in plants;

• To explain how support is achieved in aquatic plants;

• To explain how support in terrestrial plants are achieved through tissue modifications.

SUPPORT IN PLANTS

• Need to be an upright position to maximise the absoption of sunlight for photosynthesis.

• Flowers need to be lifted for pollination, fruit & seeds are held at high position for dispersion.

• Support provided by the turgidity of cells & the distribution of vascular bundles & cellulose in the cell wall.

AQUATIC PLANTS• Are supported by the upward thrust provided by the surrounding water.

• Have aerenchyma tissue & air sac in the roots, stems & leaves fro support & to enable them to float.

• Aerenchyma : loose parenchymal tissue with large air sacs in it.

• Aquatic plants have little xylem since they obtain water easily.

• The few lignified xylem vessels that are present are usually found in the centre of the stem resisting the pull of running water.

HYDRILLA SP.• Large air sac in the stem & leaves

• Fine & thin leaves bend & wave with the water current

• soft, thin stems bend with water current.

• Aerenchyma tissue with abundant air spaces support & allow gaseous exchange.

WATER HYACINTH• Large air sacs in the roots &

leaves to keep floating.

• Few vascular tissues found in the middle of the stem to bend without breaking.

• Xylem tissues are not lignified.

• Aerenchyma tissue with abundant air spaces support & allow gaseous exchange.

SUPPORT IN TERRESTRIAL PLANTS

• Have a fairly rigid structure. The weight of a plant has to be support on its base.

WOODY PLANTS• Young : the support is provided by the turgidity of the cells & the cellulose cell walls.

• Older : provided by lignified xylem vessels, tracheids & schlerenchyma tissues.

• The walls of tracheids & xylem vessels are thick & lignified harden them. Give strength & rigidity to the plant.

• As the plant grows older, secondary thickening in xylem cells deposit more wood or xylem tissues to provide greater support.

• Sclerenchyma tissue has thin lignified cellulose walls which are impermeable to water, giving strength to the stem.

• Two types of sclerenchyma :– Sclerenchyma fibres : long cells found outside the vascular bundles, providing support to the plant.

– Sclerids : small, short, irregularly shaped cells with lignified walls which harden the outer layer of the seeds.

HERBACEOUS PLANTS• Support is provided by the

turgidity of parenchyma & collenchyma cells & xylem tissues.

• Parenchyma cells are filled with water turgid

• Collenchyma cells have unevenly thickened cellulose walls which provide support in stem & the midrib of the leaves.

CREEPERS• Plants with modifications to

obtain support as they are not able to support themselves upright.

• Use tendrils to climb.

• Use other trees to support.

• Lianas, vines & buttress root are another adaptation to provide support.

EXERCISE 2.31. Explain how support is achieved in

terrestrial plants.(10 marks)

Hints :3 points (3P-3 marks), each P have 2 explanation (2E – 2 marks), 1 bonus mark (1B).

(1P+2E) x 3 + 1B = 10 marks