History: Cross-Section of Tree Trunk

Cross-Section of Tree Trunk "American Woods" by Schoonover, pg. 8

A. - CAMBIUM. A very thin layer of living cells lying between the inner bark and sapwood, where the new wood and bark cells develop each year.

B. - INNER BARK. Serves to carry the food manufactured by the leaves to the cambium layer.

C. - OUTER BARK. The inner bark gradually transforms into the outer bark, a corky layer of dry dead cells forming a protective covering.

D. - SAPWOOD. The lighter colored band of wood between the inner bard and darker heartwood. It carries the sap from the roots to the leaves.

E. - HEARTWOOD. Formed by a gradual change in the sapwood by which it becomes darker, heavier and often more durable. Most trees form heartwood. It is inactive but gives strength to the trunk.

F. - PITH. The soft spongy tissue in the center of the tree trunk and all branches. The pith in some trees is so small it is not discernible without greatly magnifying it.

G. - MEDULLARY RAYS. Wood rays or medullary rays are very small, flat, vertical bands or ribbon-shaped sheets of tissue (ducts) connecting the pith with the various layers of wood and the inner bark, and by which food and water are stored and transported horizontally in the tree.

H. - ANNUAL RING. Each year a tree adds an additional layer of wood to is circumference, this layer forming immediately beneath the bark and is called "annual ring" or "growth ring". In the spring the newly formed cells are thin-walled and spongy, called "springwood," while in midsummer and fall the walls of the cells become thicker and more dense, called "summerwood."