Support in plants

Plants do not have a skeleton. They are supported in several ways that work together.

(a) Turgor pressure (water pressure inside cells).

• In young herbaceous plants, the cells are full of water.
• Water enters by osmosis and pushes the cell membrane against the cell wall; the stretched wall pushes back, and the cell becomes firm (turgid).
• Thousands of turgid cells press against one another and keep the soft stem and leaves upright, like air in a tyre.
• If water is lost faster than it is absorbed, the cells lose turgor (they become flaccid) and the plant wilts.
• This is why a plant that is not watered droops, and recovers when watered.

(b) Strengthening of the cell walls. The walls of plant cells can be made stronger or less permeable by the deposition of substances on or inside them:

• Cutin is a waxy, waterproof substance deposited on the outer walls of the epidermal cells (as the cuticle). The outer cells of a plant cannot otherwise prevent water escaping from the inner cells, so the epidermal walls become thick and impermeable. It protects and reduces water loss.
• Suberin is deposited in the walls of the cork cells. A plant may surround itself with an impermeable cork layer (as in the bark of older stems and in the skin of potato), which protects the plant and prevents water loss.
• Cellulose is a strong polysaccharide that forms the basic wall of every plant cell. Extra cellulose is deposited at the corners of the cells in some tissues.
• Lignin is a hard, rigid, waterproof polymer deposited in some walls. It makes the cell stiff and strong and it kills the protoplasm of the cells in which it is laid down heavily. Lignified walls are found in the xylem vessels and in sclerenchyma.

(c) Special supporting tissues. Two tissues specialised for mechanical support:

• The place where these cells lie and the way they are arranged support the plant.
• For example, collenchyma under the epidermis helps a young herbaceous stem to bend in wind without breaking, while sclerenchyma fibres around the vascular bundles act as reinforcing rods.

(d) Vascular tissue.

• The lignified xylem vessels and tracheids in woody plants are the main supporting tissue of trees.
• Their thick lignified walls form the "wood", which carries the great weight of a trunk and branches.
• Growth in thickness (secondary growth) adds more and more xylem each year.

(e) Special supporting organs and habits. Some plants cannot stand on their own and use other organs:

• Climbing stems and tendrils (pea, cucumber, grape): thread-like organs that twine round a support (see pulling movement in Lesson 3).
• Aerial/prop roots and buttress roots (maize, banyan) that hold large plants upright.
• Rhizomes, corms and bulbs stay anchored at a suitable depth in the soil by contractile (pulling) roots.
• Aquatic plants are supported by the buoyancy of water, so they have little mechanical tissue and long flexible stems; air spaces in their tissues help them float.