The vascular cambium

Two kinds of growth

A young plant grows longer at its tips. This is primary growth, and the apical meristems make it. A plant that lives for many years also grows thicker. This is secondary growth.

Secondary growth is the work of two lateral meristems. They lie along the sides of the stem and root, parallel to the surface.

• The vascular cambium makes new xylem and new phloem.
• The cork cambium makes a new outer covering. You meet it in the next lesson.

Dicot stems and dicot roots show secondary growth. Most monocots do not.

Why a monocot stem stays thin

A vascular bundle of a dicot stem has a strip of cambium between the xylem and the phloem. The bundle is open, because the cambium can still divide. A vascular bundle of a monocot has no cambium. It is closed.

A palm trunk, a bamboo and a sugarcane stem become taller each year. They do not add new rings of wood. Their width is fixed early in life.

The cambium becomes a ring

The cambium forms a full ring in two steps.

1. In each open bundle, the strip of cambium between the xylem and the phloem is the fascicular cambium. It is part of the bundle.
2. Between the bundles lie the medullary rays, rows of parenchyma cells. Cells of the rays that touch the bundles start to divide again. They form the interfascicular cambium.

The two kinds join, and a continuous ring of vascular cambium surrounds the pith.

How the cambium divides

The cambium is a thin sheet of living cells. Most of its cells divide parallel to the surface of the stem. Each division gives two cells.

• One cell stays in the cambium and keeps dividing.
• The other cell is cut off. It grows and changes into a cell of a permanent tissue.

A cell cut off on the inner side becomes secondary xylem. A cell cut off on the outer side becomes secondary phloem.

Some cambium cells divide in the other direction, across the sheet. They make the ring longer, so that it keeps pace with the wider stem. Other cambium cells are ray initials. They make secondary medullary rays, thin radial strips of parenchyma. The rays carry water and food sideways.

More xylem than phloem

The cambium is more active on its inner side. It cuts off far more xylem than phloem. Two things follow.

• The secondary xylem builds up into a thick, compact mass. This mass is the wood of a tree.
• As the xylem grows, it pushes the cambium outward. The primary xylem stays near the pith. The primary phloem and the oldest secondary phloem are pushed out and crushed.

Only a thin band of living phloem is left next to the cambium. It carries the food.

Raise the ratio. The xylem grows much thicker than the phloem, while the cambium moves out.

Roots thicken in the same way

A dicot root has its xylem and phloem on different radii at first. There is no cambium yet.

• Cambium starts as strips of cells just below the phloem. These cells are called conjunctive tissue.
• Cells of the pericycle that lie opposite the xylem also start to divide and join the strips.
• The strips join into a wavy ring. The ring soon becomes round as the cambium cuts off more xylem than phloem.

From then on the root behaves like a stem. It forms secondary xylem inside and secondary phloem outside. The primary xylem stays in the middle.

Try it in Python

The program counts the cells a cambium makes over five years.

Practise with new numbers

Common mistakes

• Saying that secondary growth makes the stem longer. Secondary growth makes it thicker. Primary growth makes it longer.
• Giving monocots a cambium. Their bundles are closed, and they make no secondary xylem.
• Saying the cambium cuts off xylem outward. Xylem goes to the inside, and phloem goes to the outside.
• Thinking all the cambium lies inside the bundles. The interfascicular part lies between the bundles, in the medullary rays.

Remember these

• Secondary growth makes a stem or a root thicker. Lateral meristems do it: the vascular cambium and the cork cambium.
• Dicots have open bundles with cambium. Monocots have closed bundles with no cambium.
• Fascicular cambium is in the bundle. Interfascicular cambium forms from cells of the medullary rays. Together they make a ring.
• The cambium cuts off secondary xylem inward and secondary phloem outward. It makes much more xylem than phloem.
• Ray initials make secondary medullary rays.
• In a dicot root, the cambium arises from conjunctive tissue and the pericycle.

Check yourself

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

1. What is the difference between primary growth and secondary growth?

2. Where does the interfascicular cambium come from?

3. A cell is cut off by the cambium towards the pith. What does it become?

4. Why does a palm trunk not become thicker with rings of wood?

5. Why is there more xylem than phloem in an old dicot stem?