Roots—where we come from; for plants and trees, that's out of the ground or, more specifically, out of the seed in the ground. A tree is never without roots, as the embryo in the seed forms root, stem, leaves. They are all there in miniature, waiting.
When conditions are right and germination begins, it's the radical, the original root, that pops out first. For without a root and the water and nutrients it supplies, nothing else can happen. This essential dependence upon roots never ends for trees, all through their lives. Annual new leafy growth seen in the branches is matched by the new growth of roots. Indeed, the leaves will not expand and break out of the buds without a readily available water source, which is always from the roots. The roots supply the water requirements of the whole tree.
The roots are not static, anything but; it's just that the most important tree organ is hidden, invisible underground. Do you want to see the roots? That’s easy, imagine turning the tree upside down. The endless branching into smaller and smaller twigs is a mirror of what happens with the roots. If we had an upside-down tree, structurally it wouldn't look much different from what we saw before flipping it, the main difference being that the branches form a tall crown, while the roots are relatively flat, that is, spread out below the ground. The branching of larger trunks into smaller and smaller branches, ending up with last year's shoots at the tips, is exactly the pattern used underground by the roots. When new leaves flush out from buds, new non-woody roots are already growing from their parent root that was grown last year.
Trees do not have internal water pumps or any mechanical aid; all the water needed by the thirsty leaves is drawn up the trunk after being absorbed by the roots. There is a balance, one root system equals one set of leaves, that's it with subtle differences. We have an equal amount of tissue below as above, the root mass equals the leaf mass. Although the count is different, every leaf has many small new non-woody root hairs for its water and nutrient supply and though they sometimes last for only a week, other growth continues. There is a balance here, one root mass equals one leaf mass.
Some will grow larger and in their second year become woody and be part of the permanent living greater root system. The roots hairs thrive only where conditions, especially water availability, are right. New root hairs trying to survive in conditions that are too difficult, dry soil, many times will die. Others finding what they need will thrive. Water availability and the roots’ search for it determine everything about how the tree will grow, lots of water, then lots of root growth, then lots of leaf and shoot growth. The opposite is more often the case; lack of water, especially in thin poor suburban soils, results in minimal upper growth, or even die-back. The tree always gives us a clear picture as to its water situation: full, healthy, thick with leaves and little to no deadwood means a good water supply. The opposite is also true: thin, patchy with little growth, lots of internal deadwood, disease and insects, almost always means low water availability.
Roots have four main functions: support, water and nutrient absorption, storage, and hormone generation. The large roots we see at the base of a tree are like fingers that claw a grip in the soil. It takes powerful winds, sometimes combined with drenched soil, to overpower that grip and wrench the tree from the ground. Maybe you have seen this, a whole tree trunk lying on the ground. At the root end, we see a large half circle, 180 degrees of the gripping fingers revealed. This is the buttress, or more accurately, the root plate. The root plate shows us how strong the main roots are close to the trunk. Even with the great force needed to topple the tree, these roots did not fail; the roots broke off several feet from the trunk. It usually requires more than one weather component to topple a mature healthy tree. High wind is always part of it, but lots of snow, rain or ice are the secondary factors needed. Trees, like all living things, are an aggregate of systems, and to thrive, all these systems need to be in communication. There is a steady communication between roots and shoots, an exchange of growth-regulating hormones that keeps things balanced. This exchange is very important to deal with herbivory and the growth and repair needed after an attack. Consider the large mass of the major roots and their great internal volume of healthy tissue. This is an optimal place to store lots of extra energy, usually as starch in the parenchyma cells called amyloplasts. When required, starch is readily broken back down into its constituent glucose units.
It took approximately 15,000 years since the last glaciers receded to manufacture our native topsoil. The annual growth, flowering, and decomposition of uncounted grassland plants slowly year by year adds up.
The removal of all the topsoil prior to suburban development is done because it is very valuable and can be sold afterwards, almost always in altered states, mixed with clay and other components, sometimes so much clay that it is more clay than soil. The problem here is that plants, your trees and lawn don’t really like clay soils. There are problems with clay from a plant’s perspective. First, clays by definition are composed of very small particles, which is why wet clay is so slippery. A downside to small particle size is that the soil pores, the natural spaces between particles, are very small. The smaller the pores, the less space for water and just as important, air, atmospheric gas containing oxygen. Without a steady and easily available supply of oxygen, roots cannot perform well and the whole plant suffers.
The “pump” for the water movement to the leaves is their steady water loss through evaporation that draws water upwards toward the leaves. The “pump” for the sugars travelling to the roots is osmotic pressure. The sugars are drawn downward toward tissue that has a lower concentration of sugars. The names of these systems can be useful. Upward movement of water and minerals is in tissue called xylem in the woody part of the trunk. Sugars are transported downward in a separate tissue called the phloem, which is in the bark. The size of baby hair, about 50 microns, is a good example of the size of these fluid conducting tissues, and think of it, they are mostly hollow.
Drip line is a common term used to describe the extent of a tree’s root system. The idea is that the circumference of the upper crown shows the extent of the root system below in the ground. Not a bad model, but what about a conifer? An evergreen, a spruce, most conifers have a more narrow canopy than a broadleaf deciduous tree. The root system of most conifers is much wider than their drip line, so you should water in a fashion similar to how you would water a broadleaf tree.
Water and ions (minerals) from the soil can move into and through the root system in two different ways. The two types of water movement are symplastic (through living cells) and apoplastic (around living cells). Symplastic movement is through tiny openings in cell walls called plasmodesmata. The fluid movement is through the root hairs into the cortex, through the endodermis and into the vascular tissue of the stele. From the stele, fluid moves upward to the leaves. With the apoplastic pathway, water and ions are drawn from the root hairs and through the intercellular spaces of the cortex, then through the endodermis and into the stele.
Roots are selective about what kind of molecules enter into the tree's system. In the symplastic model, it is ultimately the root hair that is allowed to make the “decision” about what enters. In the apoplastic model, a separate tissue, the casparian strip located in the cell wall spaces of the endodermis, makes the “decision.”
Most trees possess a tap root system or a modified tap root system in contrast to the fibrous root systems of grass-like plants. The tap root system is one where the primary root, the radical from the seed, grows downward and develops smaller lateral roots. Tap root systems work well in dry soils and grow towards sources of water and nitrogen and other ions in the soil.
Like the shoots above, roots experience primary growth (extensions) and the RAM, root apical meristem, produces three separate differentiating meristems that combined grow all the new tissues of the root. The xylem and phloem tissues in the stele are produced by the procambium. The rhizodermis (root skin) is produced by the protoderm. The tissues of the cortex are produced by the ground meristem. All three of the tissue-generating meristems are initially produced in the quiescent center, the RAM, located behind the root tip and its cap.
As the root grows, we differentiate three tissue zones from youngest to oldest: the zone of division, the zone of elongation, and the zone of maturation. In the division zone, all the new cells for the new root and its cap are produced. It is the daughter cells produced in the zone of division that form into the three other necessary meristematic tissues, the procambium, the ground meristem, and the protoderm. Further back toward the stem, and somewhat older, is the zone of elongation, where cells are growing into their mature size. These still-growing cells don't yet have any secondary walls and are not yet functioning in absorption. Further back and again somewhat older is the zone of maturation where tissue differentiation occurs. Here, the vascular tissues of the stele, the xylem and phloem, the cortex, and the rhizodermis are all achieving their mature form, placement, and function. The stele is formed once the xylem, phloem, and the endodermis (inner skin) tissues are mature.
As the new root hairs grow, they fulfill their function of uptake and transport of ions and water. Root hairs appear only after elongation is finished; if not, they would be broken off from the surface of the root as it pushes through the abrasive soil particles.
As the root tip continually moves forward through the soil, it is protected by its root cap. A coating of parenchyma cells, the root cap cells manufacture a slime called mucigel, which functions as a protective lubricant for the extending root, and its twisted path through hard soil and rocks.
A strong physical cue sensed by roots is gravity; gravitropism is the term used for plants sensing and responding to the earth's gravity. Positive gravitropism means growing downward with the force of gravity. The radicle, the root section of the embryo in the seed, will always grow down, no matter how the seed is positioned in the soil.
The root's rhizodermis is the equivalent of the epidermis of the young shoots and leaves, with several distinct differences. The rhizodermis has no stomata, cuticle, produces mucigel and is specialized in water and ion absorption. It is also covered by short-lived root hairs. The rhizodermis is not root bark; it is primary growth arising from the protoderm meristematic tissue. Later, in its second year's growth and ever after, the root produces a periderm from the phellogen, the cork cambium; this is root bark. A periderm is the name for complete bark tissue and includes cork to the outside, the phellogen in the middle and the phelloderm to the inside right beside the cortex. Secondary growth, expansion in the root's circumference, will strengthen the root and its ability to anchor the tree in the soil.
The zone of maturation, which generates the root hairs, is also the area where the newly formed stele is the most permeable. Endodermis, xylem, and phloem mature at the same rate, giving water and ions the least resistance to absorption by the xylem stream.
The ground meristem produces the cortex; its primary role is storage of water and starch. It's much larger in volume than the cortex in stems. The root's cortex has a rhizodermis as its outside boundary and has the endodermis, inside skin that wraps the stele, as its inside boundary. The endodermis is where the casparian strip is located, acting as a filter and helping to “choose” what enters the xylem stream.
The stele is the inner vascular cylinder containing the xylem, phloem, and the pericycle, bound by the endodermis to the outside. The stele contains a meristematic tissue called the pericycle, which is the origin of all the lateral roots. Lateral roots are initiated in the pericycle without any involvement from the RAM. The lateral roots' primary growth penetrates through the endodermis, the cortex, and the rhizodermis tissues in order to leave the pericycle in the stele and find its way out into the soil. The tissues first stretch and then break to allow the root hair passage outside. The new root cap develops at about halfway out through the cortex and is fully functional upon emergence. The damage from the exit hole is plugged by corky cells which surround the newly emerged lateral root.
Tissue inside the new lateral root develops into phloem and xylem tubes, which connect to the parent root's vascular tubes in the stele. It is life in the soil medium that has determined that the lateral roots do not originate from the RAM. If they did, they would all be torn off the sides as the root continues to elongate through the soil. Arising as described from the pericycle, the lateral roots emerge from the maturation zone, which has stopped elongating, and the new tender lateral roots can begin their absorption work.
In their first year's growth, non-woody roots and their uncountable short-lived root hairs do almost all of the absorption work for the tree. This is the same throughout the year and every year, as new primary growth is generated by the RAM. Root hairs flourish on new lateral root growth and sometimes only last for days. As the growing season heats up and water demands increase, so it is with root growth. The endless exploration of the soil by new roots for water and ions keeps pace with the growth and water demands of the leaves, above in the crown.
As we move into the second year of a root's life, the growing tips and the RAM continue primary growth, as described above. The increase in diameter in the second year is called secondary growth and generates the bark of the root, the sturdy, protective layer that will cover the root during its life. Secondary growth in the year-plus-old root involves the formation of two additional meristems, a vascular cambium (VC) and a phellogen or cork cambium (CC). The VC will manufacture the new secondary xylem and phloem tissues, and the phellogen provides the corky periderm, replacing the rhizodermis. These new meristematic tissues form perfect layers on the inside of the protophloem and are converted into cambial initials which produce xylem to the inside and phloem to the outside. Then pericycle cells outside of the protoxylem divide to grow the new cells of the VC. After some time, the two new groups of dividing cells grow together to form the circular meristem we see in year-plus aged root xylem tissue.
The VC will form from the inside cells of the new meristem, and the phellogen will form from the outside cells. A periderm with cork cells forms to the outside and the phellogen CC to the inside. Both the VC and the CC divide periclinally, adding new cells to the outside, and anticlinally, across or greater in circumference. Altogether this provides the thickening, the increase in diameter of the root as years go by. All of the initial tissues of primary growth outside the pericycle, the endodermis, the cortex, and the rhizodermis, die and are removed. The new phellogen produces the periderm for the rest of the root's life.
As a new growth year starts, and the VC is activated, new xylem and phloem tissues increase the diameter of the root. As years of xylem tissue accumulate, the root becomes larger and woody and contains growth rings similar to the trunk above.
Another interesting aspect of the life of roots is their symbiotic relationship with mycorrhizal fungus; over 5000 species of fungus form mutually beneficial relationships with trees worldwide. The microscopic fungal hyphae grow around and sometimes into the non-woody young roots. This greatly increases the roots' working surface area and penetration into the soil. The mycorrhizae greatly increase the roots' ability to take up several essential elements, and the tree is more than happy to exchange these ions for sugars produced in photosynthesis tissue elsewhere. In mutually beneficial groups of trees, these mycorrhizal relationships provide another form of connection to assist the whole grove. Mycorrhizae, or 'Mics' as they are known, are so complex that they will be covered in their own essay.
My interest in this comes from my decades of experience as a Calgary arborist and Calgary tree doctor. I do it for the health of our urban forest, a delicate created biome.
