Like everything else on earth, ancient trees sprung from the sea and are dependent on water. A tree's root system comprises the important water-collecting mechanism that makes life possible for trees and ultimately for everything on the planet that depends on trees.
A Tree's Roots
An important biologic functionary of the tree root system is the tiny, nearly invisible root "hair". Root hairs are located just behind the hard, earth-probing root tips that burrow, elongate and expand in search of moisture while at the same time building a tree's ground support. Millions of those delicate, microscopic root hairs wrap themselves around individual grains of soil and absorb moisture along with dissolved minerals.
A major soil benefit occurs when these root hairs grab soil particles. Gradually, the tiny roots reach out to so many particles of earth that the soil becomes firmly tied into place. The result is that soil is capable of resisting the erosion of wind and rain and becomes a firm platform for the tree itself.
Interestingly, root hairs have a very short life so the root system is always in expansion mode, growing to provide sustained maximum root hair production. To take full advantage of finding available moisture, tree roots run shallow with the exception of the anchoring tap root. The majority of roots are found in the top 18 inches of soil and over half are actually in the top six inches of soil. The root and drip zone of a tree is fragile and any significant soil disturbance close to the trunk can potentially harm a tree's health.
A Tree's Trunk
A tree's trunk is critical for limb support and root-to-leaf nutrient and moisture transport. The tree trunk has to lengthen and expand as the tree grows in its search for moisture and sunlight. A tree's diameter growth is done via cell divisions in the cambium layer of the bark. The cambium is comprised of growth tissue cells and found just under the bark.
Xylem and phloem cells are formed on both sides of the cambium and continually adding a new layer each year. These visible layers are called annual rings. Cells to the inside make up the xylem which conducts water and nutrients. In xylem cells the fibers provide strength in the form of wood; the vessels allow water and nutrient flow to the leaves. Cells to the outside make up the phloem, which transports sugars, amino acids, vitamins, hormones, and stored food.
I cannot overstate the importance of tree trunk bark and how it protects the trunk and tree. Trees ultimately deteriorate and die due to damaged bark from insects, pathogens and environmental damage. The condition of a tree's trunk bark is one of the most important factors effecting a tree's health.
A Tree's Leafy Crown
A tree crown is where most bud formation takes place. The tree bud is simply a small bundle of growing tissue which develops into embryonic leaves, flowers and shoots and is essential for primary tree crown and canopy growth. In addition to branch growth, buds are responsible for flower formation and leaf production. A tree's small budding structure is wrapped in a simple protecting leaf called the cataphylls. These protected buds allow all plants to continue to grow and produce tiny new leaves and flowers even when environmental conditions are adverse or limiting.
So, A tree's "crown" is that majestic system of leaves and branches which are formed by growing buds. Like roots and trunks, branches grow in length from growth cells that make up the meristimatic tissues which are contained in growing buds. This limb and branch bud growth determines a tree crown shape, size and height. The tree crown's central and terminal leader grows from a bud cell called the apical meristem which determines tree height.
Remember, not all buds contain tiny leaves. Some buds contain tiny preformed flowers, or both leaves and flowers. Buds may be terminal (on the end of the shoot) or lateral (on the side of the shoot, usually at the base of the leaves).