Adventitious Root, Aerial Root, Apical Meristem, Fibrous Root, Lateral Root, Root Cap, Root Hair, Taproot

adventitious [ ad-vuhn-TISH–uhs ] root noun: a root that arises from any point other than the radicle or the root axis, such as stems or leaves of the plant

aerial [ AIR-ee-uhl ] root noun: an adventitious root exposed to the air, especially one found in epiphytes and climbing plants

apical meristem [ EY-pi-kuhl, AP-i- ]  [ MER–uh-stem ] noun: the area of actively dividing cells at the tip (apex) of a shoot or root that produces new tissue, mainly to increase length

fibrous [ FAHY-bruhs ] root noun: a root system consisting of fine branches of similar size, especially one found in grasses

lateral [ LAT-er-uhl ] root noun: the smaller often horizontal side roots branching off from the main root

root cap  [ root kap ] noun: a thick, protective mass of cells that is the terminal one centimeter covering the end of most root tips 

root hair [ root hair ] noun: the fine hairlike structures that grow from the elongated epidermal cells of young roots and rootlets, through which water and nutrients are absorbed 

taproot [ TAP-root, -root ] noun: the main root axis, often bulky, that extends downward from the radicle and which can serve as a food storage area in some plants 

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There are two major types of root systems. Gymnosperms and dicots usually have roots based on taproot predominance, while monocots (grasses) tend to have fibrous or adventitious root structure. 

A taproot is the primary root, the central axis of the root system, which usually burrows straight down into the soil to anchor the plant. Lateral or secondary roots grow horizontally off of the taproot in search of good soil to feed the plant. They can spread laterally up to five times the width of the crown of a woody plant. 

A fibrous root system also develops from a primary root, but as its growth decelerates or ceases altogether, adventitious roots emerge from the stem tissue. Nodes on stolons and on underground stems such as rhizomes and tubers are the starting points from which new plants produce adventitious roots. Aerial roots are modified adventitious roots that grow out from the above ground stem but do not have contact with the soil. Instead, they help a plant absorb moisture from the air, anchor the plant to a support, and sometimes function in photosynthesis. They are found in orchids and vines like English ivy. Those aerial roots that do eventually penetrate the soil to provide stabilizing support to a plant are often referred to as brace, prop, stilt, or buttress roots.

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In both system types, the root hairs play the critical role of absorbing moisture and nutrients from the soil. In growing through the soil, if it is excessively hard or dry or contains obstacles, root hairs are destroyed and have to be replaced. They grow from the elongated cells of the roots’ epidermis, an ongoing, active process. The root cap is relatively sturdy, in effect armoring the root tip (apical meristem) to advance through the soil, to better absorb moisture and nutrients. These roots are lubricated and protected by a secretion called mucigel, a hydrated polysaccharide that helps prevent roots from drying out, an especially important function during droughts. In some cases, mucigel contains inhibitors that prevent the growth of roots from competing plants. Mucigel also contains nutrients that can aid in establishing mycorrhizae. 

Growth also depends on the roots’ ability to absorb oxygen from the soil. Whatever a plant’s inherent ability to absorb oxygen and nutrients and transfer them to shoot growth, if the soil is extremely compacted and lacking in oxygen and/or moisture, growth will be affected. When possible, root systems will spread farther from the plant’s base in an attempt to find soil that will meet the plant’s need for food and water. If it cannot find enough, the plant may be stunted in growth or even die. Gardeners should be careful in transplanting to preserve delicate roots and their root hairs as much as possible in order for the plant to establish in its new location and continue to grow optimally.

As plants mature, roots branch and expand to form root systems. Hence gardeners will find their perennials, whether herbaceous or woody, benefit from attention to the places where they are planted. Good soil, space for expanding root systems, and healthy growing conditions will pay dividends in stronger and healthier plants. There is a reason gardeners are so often advised to test their soil, to consider its chemical components, its texture, and its access to air, light, and water.

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References

Bender M. March 19, 2024. These Researchers Are Digging Into the Understudied Science of Roots; After centuries of neglect, botanists are using new techniques to understand roots. Smithsonian Magazine, Science.

Michaels T, Clark M, Hoover E, Irish L, Smith A, Tepe E. 2022. Chapter 3.3 Roots. The Science of Plants. University of Minnesota Department of Horticultural Science. 

Ryan P, Delhaize E, Watt M, Richardson AE. August 2016. Plant roots: understanding structure and function in an ocean of complexity. Annals of Botany. 118(4): 555–559. doi:10.1093/aob/mcw192.

Schalau, J. February 2020. Backyard Gardener: Understanding Plant Roots. University of Arizona Cooperative Extension, Yavapai County.