Secondary Growth: Quick Revision

A brief note on Secondary Growth. Read it before a test. Try each question at the end before you open its answer.

Growing thicker

• Secondary growth makes dicot stems and roots thicker. Lateral meristems do it. Monocots have closed bundles and no cambium, so they show none.
• Vascular cambium: fascicular cambium in the bundles plus interfascicular cambium from the medullary rays. Together they form a ring.
• It cuts secondary xylem inward and secondary phloem outward. It makes much more xylem. Wood is secondary xylem.
• Ray initials make secondary medullary rays.

The outer cover

• Cork cambium (phellogen) arises in the cortex. It cuts cork (phellem) outward and phelloderm inward. The three make the periderm.
• Cork is dead, with suberin in its walls. It is waterproof and airtight.
• Lenticels are patches of loose cells in the cork. They let gases pass.
• Bark means all the tissues outside the vascular cambium.

Rings and wood

• Spring wood: wide vessels, light, low density. Autumn wood: narrow vessels, dark, high density. One of each makes an annual ring.
• The number of rings gives the age. Wide rings mean good years.
• Heartwood: dead, dark with tannins and resins, hard, does not conduct. Sapwood: outer, light, carries water and minerals.

A worked example

A tree is felled and its stump shows a pale, soft outer ring of wood and a dark, hard inner core. A hollow in the core has no effect on the leaves above.

1. A dark, hard core. It is heartwood, the oldest wood. It is dead.
2. A pale outer ring. It is sapwood, which carries the water.
3. Leaves unaffected by the hollow. Water still moves up the sapwood, so the tree lives.

Practice with new numbers

A program to try

The program lists the layers of a woody stem from the outside inwards, with the job of each.

Mistakes to avoid

• Giving monocots secondary growth. Their bundles are closed and have no cambium.
• Saying the cambium cuts xylem outward. Xylem is cut inward and phloem is cut outward.
• Saying heartwood carries water. It is dead. Sapwood carries water.
• Counting two rings for each year. Spring wood and autumn wood together make one ring.