By Len Phillips
Trees have two general systems that grow along an axis, the shoot system (with leaves and stems) above ground, and the root system below ground. Because root systems are unseen, many people don’t think about them unless the roots have grown so much that sidewalks get cracked and lifted or a drainage system needs cleaning because roots have invaded the drain tiles.
The roots of trees are required for anchorage, absorption of nutrients and water, and the storage of starch. The development of a root system is dependent upon the tree’s genetics, the soil, and the environment. A tree’s root system must balance its shoot system, but not by weight or dimensions. The root system must supply the shoot system with sufficient moisture and nutrients, and the shoots must manufacture enough food to support growth of the root system.
Trees require water, oxygen, carbon dioxide, light, nutrients, appropriate temperature, correct pH, physical space for growth processes, and open soil surface area for replenishment of essential resources. Roots utilize space in the soil. The more space that is infiltrated by roots, the more potential resources (water and nutrients) are available; and this is directly related to tree health.
Growth in trees represents an expansion of tissues into new spaces. For roots, the tips elongate and the tissues thicken in diameter. Roots develop internally rather than from buds as occurs on stems. To develop in this manner, a root has several parts.
The root cap is the part of the root is at its very tip. The cap protects the tip of the root as it is forced through the soil by the elongating tissue behind it. The root cap cells are coated with mucigel, a slime sheath that helps to lubricate the root as it penetrates through soil. These cells are replaced constantly.
The apical meristem generates the cells that form the root cap in front and the region of elongation behind this area.
Cells in their fixed positions elongate to reach mature dimensions in the region of elongation. The vacuole (a large sac of fluid within a cell) plays a major role in this process, using water pressure to push against the walls to stretch the cellulose fibers as the cells elongate. Virtually all increase in root length occurs in the region of elongation. This active cell division region forces the root through the soil against the mass of the tree. It is during this time of elongation that roots are also sensitive to gravity and respond with gravitropism.
In the region of root hairs or differentiation root cells develop a more mature form and are differentiated into the epidermis and cortex. Differentiation is when a cell changes its structure, such as when a parenchyma cell becomes enlarged to form a vessel; then it is no longer a parenchyma cell. The cell is differentiated and the process is the differentiation of cells.
The epidermis contains a single layer of flattened cells with very little cuticle or suberin. Root hairs are found in this region. A root hair is the extension of a single epidermal cell. As the epidermal cells mature, the root hairs atrophy and are replaced by root hairs on newer cells in the early stages of maturation. Root hairs absorb water and elements dissolved in the water. Root hairs are organs that grow within days when water, temperature, and soluble essential elements are at optimum levels. Root hairs die and are shed after a few weeks. As root hairs and mycorrhizae atrophy, they add organic material to the soil. Non-woody roots shed dying and dead root hairs and epidermal cells. Soil micro-organisms digest the shed cells and recycle elements essential for life. Root hairs do not become lateral roots. Root hairs and mycorrhizae are alive and well in midwinter in non-frozen soil below frozen soils.
Firs, redwoods, and Scots pine do not have root hairs. Instead, they absorb water and nutrients through the thin-walled epidermis. In contrast to this, some trees such as the redbud and honey-locust have root hairs that last for several years.
The soil area around the root tip and along the absorbing root-soil interface is called the rhizosphere. It is a zone about one millimeter in width surrounding the living root hairs and their mycorrhizae, as well as hyphae growing out from some mycorrhizae. Constantly changing mixes of organisms inhabit the rhizosphere and surrounding soil. Bacteria, soil viruses, actinomycetes, fungi, protozoans, slime molds, algae, nematodes, enchytraeid worms, earthworms, millipedes, centipedes, insects, mites, snails, and small animals compete constantly for water, food and space.
The root cortex is composed of loosely packed round tissue cells with large diameters. Absorbed water moves readily between cells through the porous cell walls of the cortex parenchyma. Cortex cells typically contain amyloplasts, plastids (storage cells) filled with starch granules and occur in seeds, roots and stems. The pathway of interconnected cell walls that facilitates water movement is generally regarded as the apoplast. An alternative water pathway, which carries water from cell to cell through plasmodesmata passing through the interiors of the cells, is called the symplast.
The inner layer of cortex is the endodermis. As roots mature, the endodermis becomes a barrier to further movement of water and minerals between cells.
Within the cortex are the xylem, phloem, and pericycle. The xylem consists of large-diameter vessels. Xylem is not wood; it is one of the transport tissues in vascular plants. It transports free water and substances dissolved in the water from absorbing non-woody roots to leaves. When xylem is lignified it is then correctly called wood. Lignified means that high amounts of the natural “cement” called lignin is deposited within the cellulose strands in the cell walls. This makes the cell walls very tough. Having tough, lignified cell walls is a unique feature of trees. The phloem is found in patches between the xylem arms and is comprised of sieve elements and companion cells. Phloem is another transport tissue. It transports energy-containing substances (carbohydrates) made in leaves to the roots. The pericycle consists of parenchyma cells just inside of the endodermis, and forming the rest of the stele other than the xylem and phloem.
Symplast is the network of highly ordered, connected, and living axial and radial parenchyma cells in sapwood and inner bark. The symplast stores energy reserves. The living protoplasm is contained in thin-walled cells called the parenchyma, which have small cell wall openings that act as tunnels where the protoplasm of one cell connects with the protoplasm of adjoining cells. The symplast stores energy reserves. The apoplast (dead fibers and tissues) stores bound water. As the symplast decreases, so does storage space and as storage of energy reserves decreases, so does the defense potential. Pathogens seem to know this very well.
The symplast is made up of radial and axial parenchyma cells. The radial cells run perpendicular in the trunk, and axial parenchyma cells run parallel to the trunk. The radials form the wood rays and phloem rays. Sapwood has an interconnected network of living axial and radial parenchyma.
In older parts of the root, another meristem forms between the xylem and phloem. This cambial zone is sometimes called the vascular cambium. It is rarely made up of a single layer of cells. Mitosis in the cambium produces new secondary xylem to the inside and secondary phloem to the outside. The cambium zone in roots everywhere is like an accordion — during the resting period it is closed and during the growing season it is open.
A cambium cylinder develops from parenchyma cells between xylem and phloem in the primary root stele. Once formed, the cambium produces xylem inward and phloem outward. Additional parenchyma cells form rays. Outer bark or periderm is mostly dead cells lined with a fatty substance called suberin or cork. The phellogen (bark cambium) is the outermost part of the symplast and the end of the phloem rays. The annual growth of “wood” and cork-like bark in secondary roots is very similar to stem secondary growth.
In the area behind the region of root hairs, lateral roots are formed by sending out a root cap, apical meristem tissue, etc. into a new area of soil. The ability of primary root tips to enter soil pores, further open soil pores, and elongate through soil pores is dependent upon the force generated by the root and the soil’s resistance to penetration. Cell division and subsequent osmotic enlargement of each new cell generate root growth forces. Adequate water is required, as well as oxygen for respiration, and tree roots can consume large amounts of oxygen during elongation.
Len Phillips, ASLA Emeritus, can be reached via e-mail at email@example.com
Note: The author referenced the materials listed below when compiling this overview.
Coder, Dr. Kim D. “Tree Root Growth Requirements,” City Trees, The Journal of The Society of Municipal Arborists, Vol. 38, Number 2.
Phillips, Leonard, “Root Physiology,” City Trees, The Journal of The Society of Municipal Arborists, Vol 35, Number 4.
Richardson, Rosemary, “Biology 203,” Bellevue Community College Science Division.
Rindels, Sherry, “Tree Root Systems,” ISU Extension, Prepared by Department of Horticulture Iowa State University, Ames, Iowa.