Growth and its phases

Watching a seed grow

Put a bean seed on wet cotton wool. In two days a root comes out. In a week there are small leaves. Weeks later there may be a flower. Nothing has been added from outside except water, air and light. The plant has built itself.

That building is what this chapter is about. We start with the simplest question: what is growth?

What is growth?

Growth is a permanent, irreversible increase in the size of an organ, of its parts, or even of a single cell. Growth also needs energy. It goes with metabolism, the chemical changes in a cell. The building up of molecules is called anabolic metabolism. The breaking down of molecules to release energy is called catabolic metabolism. Growth happens at the cost of this energy.

Two examples will make this clear.

• A young leaf grows larger. Its cells divide and enlarge, and the change stays. This is growth.
• A dry piece of wood is put in water and swells. The swelling goes away when the wood dries. It is reversible, and it does not use energy from metabolism. This is not growth.

So when you decide whether something is growth, ask two questions. Is the increase permanent? Is it made by the living cells using energy?

Plants grow all their life

An animal grows all over its body until it becomes an adult. A plant is different. Its growth is indeterminate, which means it has no fixed end. A tree puts out new leaves, new roots and new branches every season for as long as it lives.

The reason is the meristems, the groups of cells that keep dividing. They lie at the tips of roots and shoots, and in some plants along the sides of the stem. A plant keeps a supply of dividing cells for its whole life. Because new cells are always being added, we call this the open form of growth.

In the earlier chapter on anatomy you met these meristems.

• The apical meristems add length. This is primary growth.
• The lateral meristems add girth. This is secondary growth.

How do we measure growth?

You can measure growth in many ways, because growth shows itself in many ways. These are the usual measures.

• Increase in fresh weight. The plant is weighed as it is, with its water.
• Increase in dry weight. The plant is dried and then weighed. Only the solid matter is left.
• Increase in length. Used for a root, a stem or a pollen tube.
• Increase in area. Used for a flat leaf.
• Increase in volume. Used for a fruit or a tuber.
• Increase in cell number. Used where cells divide a great deal.

Different parts of a plant grow in different ways, so each is measured in the way that suits it.

• The root apical meristem of a maize plant makes more than 17,500 new cells in one hour. Here growth is best measured as cell number.
• Cells in a watermelon fruit may grow to as much as 350,000 times their first size. Here growth is a matter of cell size, or volume.
• A pollen tube grows only in length.
• A leaf with a flat blade grows in area.

Fresh weight or dry weight?

Fresh weight is easy to find, but it can mislead. A plant is mostly water, and the water content goes up and down with the weather and the time of day. A wilted plant weighs less. A plant in rain weighs more. Neither has really grown.

Dry weight is a better measure. It shows the real substance that the plant has made, which is the matter of its walls and cytoplasm. Try this with a number. A seedling has a fresh weight of $10\ \text{g}$, and $90\%$ of it is water. Its dry weight is only $1\ \text{g}$. If it takes up water and its fresh weight rises to $11\ \text{g}$, the dry weight may still be $1\ \text{g}$. The seedling is heavier, but it has not grown.

The three phases of growth

Growth in a root or a shoot happens in three steps, one after the other. Look at the tip of a root.

1. The meristematic phase. At the very tip are cells that divide again and again. They are small and packed tightly. Each has dense cytoplasm and a large, easily seen nucleus. The wall is thin, made of cellulose, and it is the primary wall. There are many fine strands of cytoplasm, the plasmodesmata, passing through it.

2. The elongation phase. The cells just above the dividing zone have stopped dividing. They now enlarge. The vacuoles grow bigger and run together, the cell takes in water and becomes longer, and new cell wall is laid down. This phase shows increased vacuolation, cell enlargement and new wall deposition.

3. The maturation phase. Still further from the tip, the cells reach their full size. The wall becomes thicker, and the cytoplasm changes to suit the job of the cell. They have now matured and become a part of a permanent tissue.

The same three phases are found in the shoot, below the shoot apex.

You should find that the cell is shortest at the tip, longest after the elongation zone, and that it stays at that length in the maturation zone.

Try it in Python

The program names the zone for a cell at a given distance from the tip.

Practise with new numbers

Common mistakes

• Calling every change in size growth. Wood that swells in water is not growing. Growth is permanent.
• Saying that growth ends when a plant is adult. Plant growth is open, and it goes on all through life.
• Using fresh weight as the best measure. The water content changes. Dry weight is the better one.
• Mixing up the phases. The cells that divide are at the tip. The cells that enlarge are above them. The cells of full size are the furthest from the tip.
• Saying that the elongation zone divides. Its cells enlarge. They have stopped dividing.

Remember these

• Growth: a permanent, irreversible increase in size, using energy from metabolism.
• Plant growth is indeterminate and open, because the meristems keep adding new cells.
• Growth is measured as fresh weight, dry weight, length, area, volume or cell number.
• Maize root apex: more than 17,500 cells an hour. Watermelon cells: up to 350,000 times in size. Pollen tube: length only.
• Three phases, from the tip: meristematic, elongation, maturation.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A dry piece of wood swells when it is soaked in water. Why is this not growth?

2. Name the three phases of growth in a root, from the tip upwards.

3. Which phase shows more vacuoles, larger cells and new wall material?

4. Why is dry weight a better measure of growth than fresh weight?

5. Name the best measure of growth for a pollen tube and for the apex of a maize root.