The Tree Tissue Issue
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June 02, 2005

The Tree Tissue Issue
Tree tissues are made up mostly of three basic groups: dermal tissue (bark), ground tissue (roots) and vascular tissue (under bark). To understand a tree, you must its tissue.

Bark or Dermal Tissue
Dermal tissues generally occupy the "skin" layer of all plant organs including trees. The tissues are responsible for controlling environmental conditions that continually interact and effect the tree's outside. The dermal tissues influence light passage, regulate gas exchange, recognize and defend against pathogens, control tree temperature and many other things.

Roots or Ground Tissue
Actually, this tissue occupies the space between the dermal tissues and the vascular tissues. These cells are much more than just filler, though. In roots the ground tissue may store sugars or starches to fuel the spring sap flow. In leaves, the ground tissue is that layer doing photosythesis, the mesophyll.

Vascular Tissues
The vascular tissues of higher plants (Kingdom Plantae) are divided into two sections: xylem and phloem. Xylem Xylem is basically a sclerenchyma tissue. Sure it contains some parenchyma, but the rest is sclerenchyma. Some of that sclerenchyma is fiber which does not conduct. The remainder, however, is all conductive. There are two fundamental cell types: tracheids and vessels. Tracheids are thick-walled, have no cytoplasm, and conduct from cell-to-cell through perforation plates in the end walls or through pits in the side walls. The earliest matured tracheids have annular, spiral, or reticulate wall thickenings. Later-maturing tracheids have pitted walls. Vessels are also thick-walled, dead, hollow cells, but lack end walls. They are typically much larger in diameter and are therefore major pipes in the plumbing for water movement. The wall thickenings in vessels are pitted. Xylem functions in the transport of minerals from the soil up the plant. In the spring sap flow of woody trees, however, the xylem briefly carries lots of sugars and other nutrients from root storage to "jump start" the production of leaves.

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