Course 6.3Lesson 2 of 3

The cork cambium, bark and lenticels

As a stem thickens, the cork cambium makes cork outward and phelloderm inward. Together with the cork cambium these form the periderm. Lenticels are gaps in the cork that let air pass. Bark is everything outside the vascular cambium.

about 25 min1 simulation1 practice question1 Python program5 questions to check yourself

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A new cover for a thick stem

The epidermis cannot stretch for ever. As the wood inside grows, the stem becomes wider. The epidermis and the cortex outside it are stretched and then torn. The stem needs a new covering.

The cork cambium

The new cover comes from another lateral meristem, the cork cambium, or phellogen.

It starts in the cortex, from parenchyma cells that begin to divide again.
Its cells are thin-walled and narrow. In cross-section they look like short rectangles.
It cuts off cells on both sides, like the vascular cambium.

The cells cut off on the outer side form the cork, or phellem. The cells cut off on the inner side form the phelloderm, which is also called the secondary cortex.

Layers of the periderm drawn from the outside of the stem inwards. At the top, rows of flat, brick-like dead cork cells. In the middle of the cork is a lenticel, a gap filled with loose, rounded cells that lets air pass. Below the cork is a single row of cork cambium cells, then two rows of living phelloderm cells, then the secondary phloem and cortex. A bracket on the left marks the cork, cork cambium and phelloderm together as the periderm.

Cork and phelloderm

Cork (phellem). Its cells are packed tightly, with no gaps. A waxy substance, suberin, is laid down in their walls. The cells die and lose their protoplasm. They form a dead layer that is waterproof and airtight.
Phelloderm. Its cells are living parenchyma. They look like the cells of the cortex.

The cork cambium, the cork and the phelloderm together make the periderm.

Add rows of cork. The share of dead rows rises, and the living layers stay thin.

Why the old outer cells die

Cork is waterproof. The tissues outside it cannot get water or food from the cells inside. They die and peel off. This is why the bark of an old tree is rough and cracked.

The first cork cambium works for a time. Later a new cork cambium may form deeper in the stem. It makes a new periderm, and the older layers outside it are cut off from supply.

Lenticels

A tight cork layer would also stop the living cells beneath from getting oxygen. The stem solves this with lenticels.

A lenticel is a small, lens-shaped opening in the cork.
It forms where the cork cambium makes loosely arranged cells instead of tightly packed cork.
Air moves through the gaps between these loose cells, so gases are exchanged.
Lenticels are found in the bark of most woody trees.

A lenticel has no guard cells and cannot close. It lets air pass throughout the year.

What bark is

Bark is a general word. It means all the tissues outside the vascular cambium. This includes the periderm, the secondary phloem and what is left of the older tissues.

The bark made early in the season is soft. It is called early bark. The bark made late in the season is hard. It is called late bark.

Many useful things come from bark, such as cinnamon, which is the dried inner bark of a small tree.

Try it in Python

The program lists the layers of a periderm and finds the share of dead rows.

Practise with new numbers

Common mistakes

Saying cork is living tissue. Mature cork cells are dead, and their walls hold suberin.
Forgetting the phelloderm. The cork cambium cuts cells inward too, and those cells form living phelloderm.
Mixing up the two cambia. The vascular cambium makes xylem and phloem. The cork cambium makes cork and phelloderm.
Saying a lenticel is a pore with guard cells. A lenticel is a patch of loose cells in the cork.

Remember these

The cork cambium, or phellogen, arises in the cortex and cuts off cells on both sides.
Cork, or phellem, is made outward. It is dead and has suberin in its walls.
Phelloderm, or secondary cortex, is made inward. It is living parenchyma.
Periderm means the cork cambium, the cork and the phelloderm.
Lenticels are gaps of loose cells in the cork. They let gases pass.
Bark means all the tissues outside the vascular cambium. Early bark is soft and late bark is hard.