Plant growth, repair and cloning

Mitosis: one cell becomes two

Plants survive being grazed, trampled, burnt and chewed by insects. They can do this because their cells can divide to make new ones. Cell division for growth and repair happens by mitosis.

The instructions for building every part of a cell are stored in DNA inside the nucleus. Before a cell divides, it copies all of its DNA. The DNA then coils up tightly into chromosomes, which become visible under a microscope. The copies are pulled to opposite ends of the cell, and the cell splits in two.

The two daughter cells are genetically identical to the parent cell. After dividing, new cells usually become specialised for one job: a xylem cell, a guard cell, a root hair. Many specialised cells lose the ability to divide. Cells that keep the ability to divide and can become different types are called stem cells.

Where plants grow

An animal grows all over, but a plant only grows in particular places. Clusters of dividing cells at the tips of every root and shoot are called apical meristems. That's where a plant gets longer.

• The root cap protects the tip as it pushes through soil, wearing away and being replaced constantly.
• In the zone of cell division (the meristem), small cells with large nuclei divide by mitosis.
• In the zone of elongation, the new cells take in water and stretch, which pushes the tip forward through the soil.
• In the zone of maturation, the cells become specialised: some grow root hairs, others become xylem and phloem.

Woody plants also grow wider. A thin ring of dividing cells just under the bark adds a new layer of xylem (wood) on the inside every year. In a wet year with plenty of growth, the new layer is thick; in a drought it's thin. These layers are the growth rings you see in a cut stump.

Responding to the environment

Plants can't walk towards light or water, but they can grow towards them. A growth response to a direction is called a tropism.

• Phototropism: shoots grow towards light. A plant hormone called auxin collects on the shaded side of the stem and makes those cells stretch more, so the stem bends towards the light. That's why houseplants lean towards a window.
• Gravitropism (geotropism): roots grow down with gravity and shoots grow up against it, so a seed knows which way to grow no matter how it lands.
• Thigmotropism: a response to touch, such as a pea tendril curling around a stick.

Plants also respond to seasons. Lengthening days in spring trigger cell division for new leaves and flower buds, and many Australian plants respond to smoke or heat by germinating after a fire.

Repair and galls

When a branch breaks or bark is scraped off, cells around the wound divide to seal it over, a bit like a scab. After a bushfire, epicormic buds under the bark and lignotubers at the base of many eucalypts burst into growth, rebuilding the canopy from dormant cells (you saw these in Chapter 1).

Sometimes cell division is triggered by an attacker. Some wasps, insects, fungi and bacteria cause plant cells to divide out of control, forming lumps called galls. A gall walls off the intruder, though in many cases it also gives the insect larva a safe home to grow in.

Vegetative reproduction

Because mitosis makes identical cells, a piece of a plant can grow into a whole new plant that's a clone of its parent. This kind of asexual reproduction in plants is called vegetative reproduction. No flowers or seeds are involved.

Vegetative reproduction is fast and needs no partner or pollinator. That's why weeds like blackberry and mother of millions spread so aggressively. The downside is that every plant is identical, so a disease or a change in the environment that harms one will harm them all.

Australia's Wollemi pine shows the risk of relying on cloning. Fewer than 100 adult trees survive in the wild, in one hidden gorge in the Blue Mountains, discovered by a national parks officer exploring a canyon in 1994. They often reproduce by sprouting new trunks from their base, and the wild trees are almost genetically identical, so a single disease could kill them all. During the 2019–20 bushfires, firefighters were flown in to set up sprinklers and save them.

Cloning plants on purpose

People have been cloning plants for thousands of years to keep the good ones exactly as they are:

• Cuttings: a piece of stem is planted in moist soil or water and grows roots.
• Grafting and budding: a shoot or bud of a good variety is joined onto the roots (rootstock) of a hardy plant. Almost every apple, orange and avocado tree is grafted. One tree can even carry several varieties.
• Layering: a branch is bent to the ground or wrapped in moist moss until it grows roots, then cut free.
• Tissue culture: a few cells are grown on sterile jelly with nutrients and hormones, producing thousands of identical plantlets. It's used to multiply rare plants, including the Wollemi pine, a "living fossil" found in 1994 with fewer than 100 trees left in the wild.