• 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:

• 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:

✍️ Worked examples

• Worked example 1 (turgor).
• A fresh celery stalk is placed in strong salt solution.
• After an hour it is limp.
• Explanation the surrounding solution has a lower water potential than the cell sap, so water leaves the cells by osmosis; the cell contents shrink, turgor falls, and the support given by water pressure disappears.
• If the stalk is put back in pure water, water re-enters, turgor returns and the stalk becomes firm again.
• Worked example 2 (choosing tissue).
• Which tissue would you expect to find in the strings of a celery stalk, which are tough and fibrous, and which one in the young, flexible stalk surface?
• The tough strings are sclerenchyma fibres (lignified, rigid); the flexible ridged surface that bends without snapping is collenchyma (cellulose, alive, elastic).

• It consists of 33 vertebrae in five groups that differ in shape according to the region:

• Functions of the vertebral column:

• it is the main support of the body; it protects the spinal cord; it carries the skull and allows movement of the head and the upper body; it gives attachment to ribs and to many muscles.
• Cartilaginous discs between the vertebrae (cartilaginous joints) act as shock-absorbers and allow slight bending.
• Structure of a typical vertebra. A bony vertebra has:

• an anterior, thick, solid part called the centrum (body);
• two transverse processes projecting laterally from it (for muscle and rib attachment);
• a posterior bony ring, the neural arch, which carries a bony process directed backwards and downwards, the neural spine;
• the neural arch and the back of the centrum enclose the neural canal through which the spinal cord passes and is protected;
• articular surfaces (facets) by which neighbouring vertebrae articulate.
• Why are lumbar vertebrae the biggest? They carry the weight of the whole upper body, and so have the largest centra.

• The pelvic girdle consists of two equal halves joined in front (ventrally) at the pubic symphysis. Each half has:

• the ilium (dorsal), attached in front and ventrally to the pubis, and behind and ventrally to the ischium.
• where the three bones meet there is a deep cup, the acetabulum, into which the head of the thigh bone (femur) fits to form the hip joint.
• Each lower limb has:

• Why do limbs have girdles?

• The girdles connect the limbs to the axial skeleton, give a firm surface for the large muscles of the limbs, form the sockets (glenoid and acetabulum) of the ball-and-socket joints, and transfer the body weight (the pelvic girdle transfers weight from the vertebral column to the legs).
• They also protect organs (the pelvis protects the bladder and, in a woman, the uterus and ovaries).
• Worked example 3 (counting). How many bones form the skull? 8 cranial + 14 facial = 22. How many thoracic cage bones? 12 pairs of ribs = 24 + 1 sternum = 25 (the book describes the structure; the number is not required but shows the logic). How many phalanges in one hand? Four fingers × 3 + thumb 2 = 14.

• Tendons are strong, inelastic (non-stretching) cords of connective tissue that link muscles to bones.
• When a muscle contracts it pulls through its tendon on the bone and moves it at the joint.
• Example the Achilles tendon connects the calf muscle (gastrocnemius) to the heel bone.
• A tendon can tear because of violent effort, sudden contraction, or loss of elasticity in the muscles.
• Symptoms of Achilles tendon tear inability to walk normally, swelling at the site of the tear and acute pain.
• Treatment anti-inflammatory and pain-relieving drugs and a medical splint; surgery is needed only if the tendon is completely torn.

✍️ Worked examples

• Worked example 4 (joint choice).
• A footballer shows a wide swinging movement of the leg in many directions at the hip, but only forward-and-back bending at the knee.
• Explain the hip is a ball-and-socket synovial joint (head of the femur in the acetabulum); the knee is a hinge joint, and its ligaments (including the cruciate ligaments) allow movement in only one plane.