The black walnut, Juglans nigra, is a tree native to eastern North America that is valued for both its nuts and its high-quality lumber. It grows up to 150 feet tall and has rough, grayish bark. The tree can be used as an ornamental specimen on larger properties, but the fallen nuts can be a nuisance, and the leaves and roots give off a chemical called juglone that is toxic to other plants, humans and horses.
The Trunk as Stabilizer
The stem or trunk of the black walnut tree gives strength and durability to a very large plant. Because of its tough, fibrous texture, the tree can preserve an extensive framework of branches for many years. These branches are supported high above smaller trees and shrubs, lifting the leaves and flowers into the sunlight for greater success at photosynthesis, pollination and nut production.
The Trunk as Intermediary
Carbohydrates and proteins are produced in the leaves using the energy of the sun and then travel through the branches and the trunk to the rest of the tree, including the roots. The roots absorb water and minerals that then travel up through the trunk to the branches and the leaves, becoming the building blocks for more food production.
The Role of the Cambium
The cambium is the thin green layer of cells beneath the bark that give rise to more bark on the exterior and more woody tissue on the interior. If a small part of the cambium is damaged, it can regenerate itself and heal the wound. However, if the cambium is removed or damaged around the entire trunk, which is a process known as girdling, the tree will eventually die.
The Role of the Wood
Inside the stem, the cambium produces two kinds of tissue -- straw-like tubes and fibers that strengthen the trunk. The tubes, called xylem, transport water from the roots up to the leaves. New wood, called sapwood, is laid down each year in annual rings. Older wood not needed for fluid transport dies and becomes known as heartwood. The heartwood of black walnut is dense and dark in color, with excellent resistance to decay. The sapwood, however, may be susceptible to beetle attack.
The Role of the Bark
The bark is a thick, somewhat spongy layer of tissue that protects the cambium from damage. New bark is laid down annually by the cambium. The newest tissue, a spongy inner bark called phloem, contains tubes that transport sugars and other foods from the leaves to the roots and other parts of the tree.