Meristematic tissues

Why we study the tissues of a plant

A plant is built from cells, but cells do not work alone. Look at a leaf, a stem or a root through a microscope and you will see cells in groups. Each group has a shape and a job.

A tissue is a group of cells that have a common origin and usually do the same work. The study of how tissues are arranged in an organ is called anatomy.

In a plant there are two big kinds of tissue.

• Meristematic tissues. Their cells can divide again and again.
• Permanent tissues. Their cells have stopped dividing and have taken up a fixed shape and job.

The permanent tissues are made from the meristematic ones. This course follows that story. This lesson is about the cells that divide. The next two are about the cells that do not.

A plant never stops growing

An animal grows all over its body, and it stops growing when it becomes an adult. A plant is different. It keeps adding new parts for as long as it lives. A tree puts out new leaves and new roots in a new season, year after year.

But not every cell in a plant divides. Division is kept to a few special places. These places hold the meristems. The word comes from a Greek word meaning to divide.

So the growth of a plant is localised. A new part appears only where there is a meristem. Everything else in the plant is made of cells that have settled down to a job.

The three kinds of meristem

Meristems are named from where they lie.

Apical meristems lie at the very tips of the roots and shoots. They are at the end of the main axis and of every branch. They make the plant longer, and they make the cells that become the leaves and the buds.

Intercalary meristems lie between mature tissues. You find them at the base of the leaves and at the nodes of grasses, and at the base of the internodes. They are the left-overs of an apical meristem. They are active for a short time only, and then they become permanent tissue. They also lengthen the plant.

Lateral meristems lie along the sides of an organ, in the mature parts of roots and shoots. They are seen in plants that make a woody axis, such as a tree. There are two.

• The vascular cambium lies between the xylem and the phloem. It makes new xylem and new phloem.
• The cork cambium lies under the bark. It makes cork on the outside of the stem.

The lateral meristems make the plant thicker. A tree gets wider because of them. The apical and intercalary meristems make it longer.

Primary and secondary meristems

The apical and the intercalary meristems are primary meristems. They are present in the young embryo and they make the first body of the plant. This growth is called primary growth.

The lateral meristems are secondary meristems. They appear later, in parts that are already mature. The growth they make is secondary growth, and it gives the woody trunk of a tree. A later course in this chapter takes that story up.

What a meristematic cell looks like

All the cells of a meristem look alike. The cells divide again and again, so they are made to divide well.

• They are small and round or many-sided, and they are packed tightly, with no gaps between them.
• The wall is thin and made of cellulose. It has many fine threads of cytoplasm, the plasmodesmata, passing through it.
• The nucleus is large and easy to see.
• The cytoplasm is dense. The vacuoles are small or absent.

Think of a cell that is ready to be a copy of itself. It has no use for a big store of water or a thick wall. It keeps its cytoplasm and its nucleus, and it divides.

From meristem to permanent tissue

The cells that a meristem cuts off do not all divide again. A cell moves away from the tip, grows larger, and changes. Its wall changes, its shape changes, and its job is fixed. This is differentiation.

After differentiation the cell is a part of a permanent tissue. It has lost the power to divide, at least for the present. The root cap, the skin of a leaf, the pipes inside a stem and the fibres of a rope are all made in this way.

Watch a meristem at work

A grass blade shows an intercalary meristem very well. Cows graze on grass and bite off the tips of the leaves, yet the grass grows back. The reason is the meristem at the base of the blade. It keeps adding cells below the cut.

You should find that the blade is as tall as the cut plus the new growth. If the growth in a day is doubled, the new growth doubles too.

Try it in Python

The program lists the five meristems, where each lies, and what it adds to the plant.

Practise with new numbers

Common mistakes

• Saying that all plant cells divide. Only the cells of the meristems divide. Most cells in a plant are permanent.
• Mixing up length and girth. Apical and intercalary meristems add length. Lateral meristems add girth.
• Calling the intercalary meristem a lateral meristem. It lies between mature tissues, and it adds length.
• Calling the lateral meristems primary. They appear later, so they are secondary meristems.

Remember these

• Meristematic tissue: cells that divide. Permanent tissue: cells that have stopped dividing.
• Apical meristems lie at the tips of roots and shoots. Intercalary meristems lie at the base of leaves and internodes.
• Lateral meristems are the vascular cambium and the cork cambium. They thicken the plant.
• Primary meristems: apical and intercalary. Secondary meristems: the lateral ones.
• A meristem cell has a thin cellulose wall, a large nucleus and dense cytoplasm, and it has no big vacuole.

Check yourself

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

1. Where do the apical meristems of a plant lie?

2. Why does grass grow back after it is grazed?

3. Name the two lateral meristems.

4. What is the difference between primary and secondary meristems?

5. Name two features of a meristematic cell.