Trees Survive Wounds and Infection Because They Are Highly Compartmented Plants That Compartmentalize the Injured and Infected Tissues. |
Trees have evolved over a period of 200 to 400 million years while being under the constant stress of wounding. Even with this stress, they still have evolved to be the largest and longest lived organisms ever to inhabit the earth. And yet trees have
NO WOUND HEALING PROCESS-healing in a sense of REPLACING or REPAIRING injured tissues. HEAL means to restore to a previous healthy state. It is impossible to HEAL injured and infected xylem. Trees have evolved as highly ordered, COMPARTMENTED plants, that instead of healing, COMPARTMENTALIZE in an orderly way the injured and infected tissues. A coded MODEL SYSThM for explaining how a tree is compartmented and how it compartmentalizes infected and injured wood has been developed. It is called CODIT, an acronym for COMPARTMENTALIZATION OF DECAY IN TREES. Terms such as "walls" and "plugs" are used in the model only to help present a mental image of the compartments. These terms are not meant as technical terms. (fig. 14)
Wall 1. After being wounded, the tree responds in a dynamic way by plugging the vertical vascular system above and below the wound. The conducting elements-vessels in angiosperms and tracheids in gymnosperms are plugged in various ways: tyloses, gum deposits, pit asperations, etc. The plugged elements complete the transverse top and bottom walls of the compartments. Wall 1 is the weakest wall.
Wall 2. The last cells to form in each growth ring make up the tangential walls of the compartments. These walls are CONTINUOUS around each growth ring~xcept where sheets of ray cells pass through. Wall 2 is the second weakest wall.
Wall 3. Sheets of ray cells make up the radial walls. They are DISCONTINUOUS walls because they vary greatly in length, thickness,
and height. Walls 3 are the strongest walls in the tree at the time of wounding.
Wall 4. After a tree is wounded, the cambium begins to form a new protective wall. The wall is both an anatomical and a chemical wall. This wall separates the tissue present at the time of wounding from tissue that forms after. It is the strongest of the four walls. (fig. 15)
Figure 15 For additional information about CODIT see Agricultural
Information Bulletin Number 405.
A More Detailed Look Shows That, in a Diagrammatic Way, a Tree is made Up of Many Rooms or Compartments.(fig.16)
|In a sense, a tree is a multiple |
perennial plant. Every growth ring
can be thought of as an individual
tree. Every new "tree" envelopes all
the older trees. (fig.17)
Each growth ring is subdivided into compartments that have sheets of ray cells as radial walls (Walls 3) and the cells that are the last to form in each growth ring are the tangential walls (Walls 2). Within these walls there are fibers, vessels, and axial parenchyma in angiosperms, and longitudinal parenchyma and longitudinal tracheids in gymnosperms. The vessels and the pits between the longitudinal tracheids keep the tops and bottoms of the compartments partially open. This is essential to maintain the vertical flow of liquids.(fig. 18)
BUT AFTER WOUNDING, THE TREE REACTS.
The vertical conducting elements begin to plug in various ways. This completes the transverse walls (Walls 1).(fig. 19)
Another close look at Walls 2 and 3
gives the impression of subdivided
three dimensional wheels.(fig.20)
There is great variation
in the dimension of Walls 3 (fig.21)
To the invading microorganisms, Walls 3 present a maze of obstacles. Each sheet of ray cells in sapwood contains living parenchyma cells that present a chemical as well as an anatomical barrier. The chemicals in the living ray cells are altered after a tree is wounded. The altered chemicals may be poisonous to some microorganisms. (fig. 22)
Within the compartments are the elements-vessels,
tracheids-that conduct liquids vertically. (fig. 23)
In angiosperms, one typical arrangement for the vessels is a uniform distribution of similar-sized vessels throughout each compartment. This is typical for diffuse-porous trees such as maples and birches. (fig. 24)
In other angiosperms, most of the larger diameter vessels may be clustered more towards the beginning of each growth ring or compartment, while much smaller diameter ones are found at the end of each growth ring or compartment. This ring porous arrangement is typical for oak, cherry, and locust. Many trees have variations on these basic arrangements. (fig. 25)
In gymnosperms, the compartments are filled with longitudinal tracheids that are the vertical conducting elements. In conifers, there are also parenchyma cell arrangements that form resin ducts or canals in the rays. Their position is usually near the end of each growth ring or compartment. (Shown here in green.) There may also be radial resin ducts in the rays; however, none are shown in the drawing. (fig.26)
After wounding has occurred, the conducting elements may be plugged by a wide variety of materials that come from living cells that surround the elements-often contact parenchyma in angiosperms. Or the pits between the tracheids may close. (fig. 27)
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