Simple permanent tissues

Permanent tissues

Cells cut off by a meristem grow and then differentiate. Some change so much that they never divide again. These cells make the permanent tissues.

A permanent tissue can be of two kinds.

• A simple tissue is made of only one kind of cell.
• A complex tissue is made of more than one kind of cell, and all of them work together.

This lesson is about the simple tissues. There are three: parenchyma, collenchyma and sclerenchyma. They differ in the cell wall, and that tells you what each one does.

Parenchyma

Parenchyma is the most common tissue in a plant. It forms the bulk of the soft parts: the cortex of a stem, the pith, the flesh of a fruit and the middle of a leaf.

• Its cells may be round, oval, many-sided or long.
• The wall is thin and made of cellulose.
• The cells may be packed tightly, or they may leave small spaces between them.
• The cells are living. They can still divide in some conditions, for example when a wound is healing.

Parenchyma does many jobs. It carries out photosynthesis, stores food, and makes some secretions.

The job decides the name.

• Chlorenchyma is parenchyma with chloroplasts. It makes food in leaves and green stems.
• Aerenchyma has large air spaces between its cells. It is found in water plants, where the air makes the plant float.
• Storage parenchyma holds starch, oil or water, as in a potato tuber.

Collenchyma

Collenchyma lies in layers under the skin of a young stem, and in the stalks of leaves. It is found in many herbs and in the stems of dicots. It is missing in monocots and in roots.

• The cells are living, and they are oval, round or many-sided.
• The wall is thickened at the corners. The thickening is cellulose, hemicellulose and pectin.
• There is little space between the cells.

Collenchyma gives mechanical support to the young parts of a plant. Its walls are not hard, so the tissue can bend and stretch while the stem is growing. When the cells have chloroplasts, they make food too.

Sclerenchyma

Sclerenchyma is made of long, narrow cells with thick walls. The wall is full of lignin, a hard substance. It is so thick that the cell has almost no space inside, and the cell is dead at maturity.

The walls have few or many thin spots called pits. Sclerenchyma gives hardness and strength. There are two kinds of cells.

• Fibres are long, thick-walled cells with pointed ends. They are usually found in groups. Fibres from plants such as jute and flax make ropes and cloth.
• Sclereids are short cells. They are round, oval or like short rods, and their walls are so thick that the space inside is a narrow line. They make the hard shell of a nut, the grit in the flesh of a pear or a guava, and the coat of a seed. They are also in the leaf of the tea plant.

A hard bite on the gritty part of a pear is a bite on sclereids.

The three tissues side by side

• Parenchyma: makes food and stores it.
• Collenchyma: gives flexible support.
• Sclerenchyma: gives hard support.

Remember that only sclerenchyma is dead in this list. When a wall becomes thick, a cell can no longer get what it needs through it. This is one reason why the cells of sclerenchyma die.

You should find that the wall of parenchyma is thin, the wall of collenchyma is thick at the corners, and the wall of sclerenchyma leaves very little space inside the cell.

Try it in Python

The program keeps the facts about the three tissues in a dictionary and picks out the one that is dead.

Practise with new numbers

Common mistakes

• Thinking all supporting tissue is dead. Collenchyma supports the plant and is living.
• Giving collenchyma a lignified wall. Its thickening is cellulose, hemicellulose and pectin. Lignin belongs to sclerenchyma.
• Placing collenchyma in roots. It is not found in roots and monocot stems.
• Saying that parenchyma is only for storage. It also makes food in photosynthesis and gives buoyancy as aerenchyma.

Remember these

• Simple tissue: one kind of cell. Parenchyma, collenchyma and sclerenchyma.
• Parenchyma: living, thin cellulose wall. Photosynthesis, storage, secretion.
• Collenchyma: living, corners thickened with cellulose, hemicellulose and pectin. Flexible support.
• Sclerenchyma: dead, thick lignified wall. Fibres and sclereids. Hard support.

Check yourself

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

1. Which simple tissue has the thickest walls and a very narrow space inside the cell?

2. What is chlorenchyma?

3. Name the substances that thicken the corners of collenchyma cells.

4. State one difference between fibres and sclereids.

5. Why is collenchyma a good support for a young growing stem?