5.3: Inside Roots

Learning objectives

By the end of this lesson you will be able to:

• Explain how roots elongate and increase in diameter via primary and secondary meristems.
• Identify the tissues in the root that originate from the root meristems and the cells into which they eventually mature.
• Distinguish between monocot and dicot roots.
• List the functions of the cells in plant roots.

Please read the OpenStax pages summarizing root biology. This resource reinforces several of the topics addressed below, so you may read it either before or after you complete this chapter. You don’t need to memorize the information there; use it to reinforce the content below.

Roots. Leonard J Matthews – CC BY-ND 2.0

Root review

The functions of roots include:

• Anchoring the plant in the soil (and stabilizing the soil).
• Supporting the upright growth of the plant.
• Providing a site for the symbiotic relationship of the plant with particular beneficial fungi and bacteria.
• Absorbing water and dissolved minerals from the soil.
• Storing nutrients like starch for subsequent use by the plant (tap roots and tuberous roots are examples of geophytes that store nutrients in roots).

Angiosperm (flowering) plants are often classified by whether they rely on a primary root system an adventitious root system. A plant doesn’t necessarily have only primary or just adventitious roots. One of these systems, however, will be dominant. Whether a root is considered primary or adventitious depends on whether the root traces back to the radicle (embryonic root) or arises from (normally underground) stem tissue.

Tap root (or primary root) system

Tap or primary roots arise as a continuation of the embryonic radicle tissue and persist into maturity. Secondary roots (also called lateral roots) arise from the primary root, and tertiary roots arise from the secondary. This primary –> secondary –> tertiary root formation is the usual rooting system for dicots. Next time you see a dandelion or other weed that doesn’t look like a grass, yank it up and look at the root system. The main root you see (assuming that it didn’t break off when you pulled it up) is probably a tap root. The photo on the right, above, demonstrates this point. It shows a young taproot system with many secondary roots branching off the primary root.

This Desmanthus spp. plant has a strong central taproot with many lateral or secondary roots. Btcpg – CC BY-SA 4.0

Adventitious or fibrous root system

In plants of this type, the primary root, which originates from the radicle, weakens prior to maturity and new, vigorous, adventitious roots arise from stem tissue. Adventitious roots may grow from nodes or might arise from the internodes. They originate from parenchyma cells in the cortex of the stem. Dig up a clump of turfgrass and look at the roots. Turfgrass has a fibrous root system, as does corn.

Adventitious corn roots. NY State IPM Program at Cornell University. CC BY 2.0.

As noted above, although adventitious roots originate from the stem they don’t have to emerge from a node. While it’s not apparent in the photo, the adventitious corn roots you see above do trace back to a node. In contrast, in the photo of a tomato stem, below, we see emergence of adventitious roots from both node and internodal regions of the stem.

Tomato stem with adventitious roots. Mark. CC BY 2.0

Internal root structure

Watch this video to take a closer look at root structure (6:21):

Root with areas labeled. CNX OpenStax. CC BY 4.0 International

Root cap

Shaped like a thimble, this structure covers the tip of the root and provides protection as the root grows into the soil. These parenchyma cells are produced by the root’s meristem which is just behind the root cap. The outer cells of the root cap are continuously worn away through contact with the soil, and new cells are added to the inner portion.

In addition to protecting the interior of the root, the cap secretes a mucilage which stabilizes the water content of the surrounding soil, ensuring longer-lasting nutrition to the root system and making for easier root probing. Finally, the root cap contains statocytes, specialized cells that help the plant sense gravity and grow accordingly. These cells are full of starchy organelles which settle at the lowest part of the cell and encourage growth in that direction. If the root cap, with these statocytes, is removed, a plant may grow in random directions because it has lost the ability for gravitropism (growth in response to gravity).

Root meristem

The cells here divide rapidly via mitosis to form new cells. New cells are laid down toward the root cap to replace those worn away during root growth, and also laid down away from the root cap. These new cells laid away from the root cap elongate and then mature into more specialized root tissues.

Region or zone of elongation

In this region, the cells produced by the root meristem undergo rapid elongation — they expand in length and volume. Root growth is the result of two processes: new cell production by the root meristem, and subsequent elongation of those new cells. This growth pushes the root further into the soil and also expands the diameter of the root. Within the region of elongation just behind the meristem you will find the following undifferentiated tissues:

• Protoderm — new cells laid down toward the exterior of the root which will mature to become the root dermal tissue (primarily epidermis cells).
• Procambium — new cells in the central part of the root which will mature to become the vascular tissue (xylem, phloem, and vascular cambium), labeled in the illustration above this section as the vascular cylinder.
• Ground meristem — the new cells lying between the protoderm and procambium that will mature to become the cortex tissue (primarily parenchyma cells).

Region of differentiation (also called the region of maturation)

Here the root becomes thicker, and secondary or lateral roots are initiated. In this region the protoderm, procambium, and ground meristem cells undergo differentiation into the specialized cells associated with the dermal, vascular, and cortex tissues, as noted above.

Root-hairs begin to form in the region of differentiation; these are fine outgrowths of epidermis cell walls and membranes, and increase the area of absorption of the root.

Here is a magnified image of a root, and a drawing to point out the various components. Jen Dixon – CC BY-NC-SA 2.0

Root tip. Image credit: Tom Michaels

Review questions

1. What two processes result in root growth?
2. Protoderm will differentiate or mature into what type of tissue? How about procambium? And ground meristem?
3. Do adventitious roots arise from stem tissue or from the primary root? Are they always found emerging from nodes or from internodes, or does it depend on the type of plant?