What’s in a root
If dirt is the ecstatic skin of the earth, then roots are its joyful frolicking companion. And yet, to us, they are reclusive, hiding beneath the soil, poking into our consciousness only when we trip on an exposed roots. Naturalist Ian Worley, once pointed out the obvious: roots only grow below ground. In other words, the only time we encounter roots is when something – like erosion or a tree falling over – has displaced the earth in which the roots developed. Yet despite their easy-to-forget nature, roots are essential to a tree’s vitality in a few key ways:
- Anchor a tree in place
- Absorb and transport water and nutrients
- Produce hormones to communicate to the crown about soil conditions
- Store energy
There are other more specialized functions that some roots perform, and there are also adventitious roots, structures produced by stems or leaves that develop into roots. We’ll cover root anatomy, some cool root adaptations that different species have, their role in reproduction, and what happens when roots don’t do job #1.
Structure of Roots
Roots develop from the RADICLE, the part of the embryo in a plant’s seed that emerges first and grows downward into the soil. The radicle ultimately develops and matures into a TAPROOT (though in many plants, the taproot dies rather quickly and a network of ADVENTITIOUS ROOTS sprout from the stem forming a FIBROUS ROOT system). As the roots expand into the soil, they begin to differentiate into 3 distinct zones:
- Apical meristem
- Zone of elongation
- Zone of maturation
The apical meristem is the site of active cell division. It is surrounded by a ROOT CAP, which protects the fragile new growth, communicates with mycorrhizal associates, and lubricates the root as it moves moves through the soil. Cells in the root cap are also responsible for sensing gravity (if the root cap is removed, the root will grow in random directions). Epidermal cells in the root cap secret MUCIGEL, a slippery lubricant that also, in part, nourishes the dense ecosystem of soil microorganisms that surround the root cap (this area around the root is called the RHIZOSPHERE).
ZONE OF ELONGATION
Vacuoles inside of each cell laid down by the apical meristem begin to fuse to form a large central vacuole. As water fills the vacuoles, the cell rapidly expands and stretches along the axis parallel to the direction of growth. The elongation effectively pushes cells in the apical meristem forward into the soil.
ZONE OF MATURATION
Epidermal cells in maturing roots sprout extensions that reach out into the surrounding rhizosphere. These extensions, called ROOT HAIRS, drastically increase a root’s surface area, increasing the rate of absorption of water and minerals.
Anatomy of a root
The inside of a root looks similar to other transverse sections of plant organs. The 3 main tissues are present: epidermis, ground tissue (cortex), and vascular tissue. The epidermis is a thin, semipermeable barrier on the outermost part of a plant’s root. In mature roots, cells are modified with tiny projections, called root hairs. The cortex lies just inside the epidermis and is largely responsible for storing starches. The purple grains in the transverse section below are amyloplasts filled with starch. Water and minerals move through the cortex via diffusion rather than through vascular tissue. Water and low-weight molecules can move freely through the SYMPLAST or APOPLAST. The apoplast terminates at the ENDODERMIS, which is a cylinder surrounding the pericycle and vascular tissue. A water-impermeable strip – formally, the CASPARIAN STRIP – within the cell walls of endodermis cells are impregnated with suberin, which prevents any further movement through the apoplast and directs materials into the symplast where materials can be checked before entering the vascular tissue.
A closer look at roots
Now that we’ve got a better sense of the terminology used to describe the anatomy of roots, lets take a closer look at some familiar roots. While roots are largely hidden from us in situ, we’re all familiar with roots in the kitchen. Taproots modified for starch storage have an exceptionally large cortex. Periderm initiates the growth of lateral roots, and occasionally these can be seen in transverse and longitudinal cuts the taproots. The gallery below has labeled images of some common culinary roots.
Some species have roots that go above and beyond the call of duty by undertaking tasks usually done by other plant organs or by pursuing other novel ventures of a root’s own design. While not all of these adaptations are relevant to Vermont trees, I’ll cover a few of the more common ones here:
Vegetative reproduction is common in many primary succession trees. Primary succession species rely on frequent disturbances across a landscape to maintain adequate conditions across time. Unfortunately these conditions don’t persist through time, so these species must continually move from one disturbed location to the next. By producing huge seed crops annually, which are typically dispersed far and wide by wind and/or water, they put their hopes in a seed finding that one-in-a-million spot. And then in the off chance a seed does make it to a site with the proper growing conditions, they’re not always guaranteed a potential mate right away. It’s helpful for a species to be able to reproduce asexually by sending up suckers and monopolize a patch. It also allows that species to regenerate if the disturbance is cyclical (e.g. rockslides, flooding, beavers).
- Staghorn sumac: DIOECIOUS, reproduces vegetatively to create umbrella-shaped clonal stands to monopolize a site.
- American beech: The stems and crowns of mature beech are killed by the non-native Nectria fungus. Fortunately, beech can sprout suckers from the roots, which are unaffected by the roots. This is saving it from succumbing entirely to the epidemic.
The ability to fuse roots with other individuals (called grafting) is certainly a boon for individuals who may have sprouted on nutrient poor soils. Sharing roots allows a population of trees to compete against other species in an area. Many species also have symbiotic relationships with fungi, exchange sugars for nutrients via a mycorrhizal network.
- Hemlock: Hemlocks can grow in solid stands, casting a thick shade into the understory. Their seedlings can wait patiently in the understory for more than a century for a break in the canopy to open up. Their roots graft to those of older trees in the canopy to “share” sugars and nutrients, which allows the seeds to cope with the rough conditions. Hemlocks lack the ability to stump sprout and so if a hemlock trunk is knocked off by wind or cut by logging, the stump will remain incapable of photosynthesizing. But with fused roots, the stump can slowly leach nutrients from its neighbors for decades.
A few of our native plants have lost their ability to photosynthesize entirely. Instead, they rely on modified roots to infiltrate host plants and steal nutrients.
- Beech drops: An annual plant that grows exclusively as a parasite on the fine roots of beech (link).
- Dodder: Small vining plant that produces adventitious roots (haustoria) that are inserted into the vascular tissue of the host plant.
Occasionally plants will sprout roots from organs other than the roots (though these adventitious roots are used for the same things roots are). As mentioned above, the fibrous root systems found in monocots and many other species are adventitious, developing from the stem. The tendrils of grape and Virginia creeper are adventitious roots, as are the haustoria in dodder, the roots from jade plant leaf cuttings, and the prop roots of corn.
Many of our trees live well over a hundred years. And over a century or more of living, a tree is subject to a torrent of potentially destructive forces. Trees are able to grow tall while staying strong by having pliable wood, which allows the trunk to sway against the forst of strong winds. The tree then relies on its roots to handle the force exerted by trunk, which acts like a lever. A deep ROOT PLATE is one safeguard against getting knocked over (one measurement of root plate tilt found that most trees in high winds tilt less than .5o off its axis – link). Deeper, extensive root systems better anchor a tree in the soil, but these root plates can’t develop in all soil types. Roots need oxygen to fuel their growth, and in water logged soils trees keep their roots near the surface where oxygen is present. The result is a shallow root plate that only loosely anchors the tree. You can also get shallow root plates where a thin layer of soil mats bedrock.
Occasionally the roots are not anchored well enough (microbursts are particularly destructive: link) and a tree is knocked over. If wind knocks a living tree over, its roots pull up a giant chunk of earth with it. If the tree is already dead when the wind knocks it over, it’s likely that the roots are already in a stage of decomposition and will snap rather than pull up the soil. The resulting mound of roots, organic material, and soil becomes a fertile substrate for new vegetation to grow (paper and yellow birches are particularly well suited to this).
- Coming soon