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Inside the cambium region is the xylem or wood. Xylem includes three principal types of cells. There are numerous tubular conductive cells running parallel to the trunk or branch that they form, and adapted to carry water and minerals upward from the roots. There are also sheets of ray cells -- tubes running from the inner bark inward toward the center of the tree. The third type is parenchyma (pa-REN-ki-ma); these cells, while they are still alive, warehouse starch crystals made by the action of enzymes on sugar. This starch, reconverted to sugar by enzymes, is the principal raw material for the natural fungicides made by trees in response to injury, and the principal source of energy for growth after injury. Parenchyma can also become meristematic under some conditions, taking on a role similar to the vascular cambium when a tree has 
been injured.

Various species of trees also have other types of cells, such as resin ducts, fibers, and tracheids. However, conductive cells, ray cells, and parenchyma, make up the bulk of wood, and perform most of its functions. 

In heartwood-forming species such as red cedar, walnut, and oak, there may be a distinction between the newest, living layers of sapwood and heartwood. True strong heartwood is the result of aging in normal compartments of a tree. As sapwood becomes less involved in transporting and storing energy for the use of the tree, its cells become "toxic waste dumps." This gives the wood a distinctive color and sometimes gives it natural resistance to decay. Unlike sapwood, heartwood can not respond to injury by forming anti-microbial 
substances; however, it can discolor. 

"False Heartwood" is a central column of discoloration that occurs as a young tree matures, when many branches die and are shed even though there may be no decay. Discoloration that takes place in the early stages of decay after a tree is injured is not true heartwood, but it can keep heartwood from forming.

Inside the earliest wood in a trunk or branch is a column of spongy, styrofoam-like material called pith. Eventually covered over by layers of wood, pith is the remains of a primary tissue formed as a twig elongates. In some species it disintegrates or is crushed; however it remains in other species, such as black walnut, whose twigs can be easily recognized by their chambered pith.

When a tree is young, its goal is to get its head above the competition and catch as much sunlight as possible. In the forest where trees compete for light and space, the most efficient way to do this is with an "excurrent" growth habit -- that is, a single, undivided stem and lateral branches. As it reaches its mature height, the branching habit becomes "codominant" -- that is, its stem and branches often subdivide with forks instead of true lateral branches.

But when we domesticate trees, we encourage them to make this transition much earlier in life... and closer to the ground. In fact, a standard nursery practice has been to force trees into a codominant branching habit. A fork near the tip of a branch has little effect on the tree's strength; but the lower the fork occurs, the worse the problem if it fails.

At the base of this twig is a swollen area called the branch collar. In this area the wood fibers of the trunk (or parent branch) veer around the twig on each side and continue toward the trunk or the base of the tree; the "plumbing system" in the branch also turns groundward -- none turns upward or goes around the trunk or parent branch. Since growth occurs at different times in various parts of the tree, the twig and branch fibers tend to form interwoven layers, a little like the laminations in plywood. Together, they create the extra wood thickness of the branch collar, which continues to grow as the twig matures. If the fibers in the crotch at the base of the twig knit well with those of the trunk or parent branch, a bark ridge emerges to some extent across the crotch. 

Also, natural fungicidal materials saturate the fibers in the base of the growing twig, forming a protection zone; this does not happen in the fibers of the trunk or parent branch. This has important implications for pruning: if you cut only the protected branch fibers outside the collar, you protect the tree from decay.

First, there is no bond or strength between the two bark faces. Second, as the two sides of the fork grow, the pressure between them tends to  spread the fork, increasing the splitting force on its the base Third, this pressure also crushes the living tissues under the bark, starving  this area and destroying its defenses. Finally, rainwater, fungus spores and other materials seep down into the fork,  rotting bark and wood so that the weaker side is likely to split from  its own weight or under wind stress

Here is a common problem to look out for in trees that have not been maintained carefully -- a weak fork with what is known as "included bark. "This makes a tree very likely to split during high wind, ice load, etc. Many people like the graceful appearance of a forked or multi-stemmed tree. It seems insignificant when a tree is young; but this is when a little preventive pruning can make a lot of difference. If you ignore it, you may find out too late that this is one of the most serious structural problems a tree can have.

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People want their trees to "look like trees" -- that is, to have a large, shady crown; and we think of multiple stems as "graceful," not understanding that this beauty often comes at the expense of strength and safety. At first blush, this would seem to force us into a choice between beauty and strength. But the more we understand about trees, the more we appreciate healthy structures. It is difficult to remain enchanted by something that is likely to split open and crush anything in its path. 

Myth:	The root system is more or less a mirror image of the top of the tree.
Myth:	Most tree species have deep taproots; if a tree's taproot is cut, the tree dies.
Myth:	A tree's roots extend to the tips of the branches.
Myth:	Tree species are "deep-rooted" or "shallow-rooted."
Myth:	Roots seek water.

Their principal jobs are:   uptake of water oxygen and minerals transporting water and minerals inward toward the trunk support, and growth.

Absorbing Roots feed into long, thin Conducting Roots, which carry water and minerals back toward the trunk. In undisturbed forest soils the conducting roots may extend outward as much as 2 or more times the height of the tree, mostly in the top 1-2 feet of soil. These conducting roots gradually converge into thick lateral Brace Roots, which provide most of the tree's support. So roots extend in an ever-widening disk, wherever they find all the things they need -- water, minerals, and oxygen. There is little oxygen deeper than about 18" in clay soils; in sandy soils the oxygen may go deeper, but the water or minerals may be in short supply; a high water table may also limit oxygen penetration; or bedrock may prevent roots from penetrating deeper.

To envision a tree's root system, imagine a wine glass sitting on a tray -- the root system is the tray, and its thickness is determined by some limiting factor. Tree species are not necessarily "deep-rooted" or "shallow-rooted" -- certain species are more or less sensitive to various levels of moisture, oxygen, and minerals. Often roots will "bounce"  off a soil hardpan or a high  water table, forming surface roots where other species may not be able to live at all.  In undisturbed forest soils, roots may extend well beyond the tips of the branches, but a heavy clay soil will greatly slow down their outward growth. Roots are opportunistic: they do not "seek" either water or minerals, but when they find them, they prosper.

In deep sandy soils with wide seasonal variation in rainfall, trees have been known to form "striker roots" -- taproot-like connections between two sets of conducting and absorbing roots, one just above each seasonal water table. However, this is fairly unusual. 

There are more similarities than differences between roots and structures in the crown. The connections between roots and parent roots are similar to branch structures, as are mechanisms for dealing with injury and decay -- work that roots are extremely effective in handling. And roots suffer the same loss of stored energy when they must react to injury and contain decay.

Anatomical differences include: (1) Roots do not have pith. (2) Roots usually have more parenchyma (living food-storage cells) and fewer fibers. (3) In roots there is less distinction between growth rings than in trunks and branches. (4) Roots do not produce normal heartwood.

Roots have important functional differences, as well. They are adapted for uptake of water and minerals, so (unlike trunks and branches) their bark withstands moisture and low light conditions. 

They also require considerable oxygen, which they must extract from small spaces between soil particles.

Perhaps the most important distinction is that we rarely see roots, so we know little about them, and we tend to ignore them. This is unfortunate, since most problems with trees reflect mismanagement of roots.