17.4–17.7

Most plants have common structural features.

Take a deep breath: a stoma—where carbon dioxide is taken in and water is lost—from the underside of an American beech tree leaf.

Figure 17-9Mostly unseen, but essential. Roots serve functions that are necessary for growth and survival.

Figure 17-10Taproots and fibrous roots.

Figure 17-11Organization of tissues within the root. In eudicots, the vascular tissue of the stem is arranged in a ring. In monocots, vascular tissue is dispersed throughout the ground tissue in the stem.

Roots don’t garner the attention enjoyed by flowers, fruits, or the trunks and branches of tall trees. But they are essential to plant growth and survival, and they perform three primary functions (FIGURE 17-9).

• 1. Absorption. Roots take up water and dissolved minerals from the soil. In deserts, absorbing enough water can be difficult: mesquite plants send roots straight down as deep as 175 feet to reach water, while some cacti send roots outward 50 feet in all directions to collect moisture. Roots also obtain oxygen from the soil.
• 2. Anchorage. Roots secure the plant in place. Also, without this anchorage, plants would not be able to stand upright.
• 3. Storage. Sugar produced in the leaves can be sent to fleshy roots, where it is converted to starch and packed away for later use, much like humans store energy as lipids in fat cells around their waists. Sweet potatoes, carrots, radishes, and turnips are examples of starch-filled roots. Water, too, is sometimes stored in roots.

There are two types of root systems: taproots and fibrous roots (FIGURE 17-10). Taproots often grow deep down into the soil. A taproot is a thick primary root with many smaller roots branching out from it. Found primarily in eudicots, taproot systems can be thought of as more expensive to produce (energetically) but providing greater storage and depth of penetration. Taproots commonly clog sewer pipes or burrow into and through house foundations, causing damage. Fibrous roots are more commonly associated with monocots (but also found in some eudicots). They consist of numerous, similarly sized roots all arising from the stem and branching out. They tend to occupy the upper, shallow parts of the soil and spread outward. Fibrous root systems are energetically less expensive to produce, characteristic of situations in which plants have greater spread and faster growth.

The organization of all root tissue is simple (FIGURE 17-11). A cross section of a root reveals the usual three tissue types. In most eudicots, the vascular tissue (xylem and phloem) is a central column, surrounded by ground tissue, and at the outer edge is dermal tissue. The xylem—a tube-like system in which water moves up into the plant—is in the center of the root, with several evenly spaced bulges swelling outward. Nestled between the bulges are phloem strands, which conduct high-energy sugars to the roots from the leaves. Most monocot roots have a different organization, with a central core of soft, spongy parenchyma tissue, called pith, surrounded by a ring of vascular tissue.

When the ground tissue in a root is used to store energy in the form of starch, the root becomes enlarged and fleshy. Plants store excess energy for the times when rapid growth or flower production demands a lot of energy. Having so much food all in one place, though, can be risky. Animals are often tempted to find such storehouses and eat them. Many humans, for instance, like to dig up and eat large roots such as beets, turnips, carrots, and sweet potatoes.

Figure 17-12“Snorkel roots.” Mangroves lift some of their roots out of the water for better “breathing.”

The outer surface of the root is epidermis. Perhaps the most important components of a root’s epidermis are the epidermal cells that elongate and become tiny hairs. These hairs greatly increase the surface area of the root that is in contact with the soil. Root hairs do virtually all of the absorption that occurs in the roots. The epidermal cells are extremely permeable to water and absorb it easily. The water then moves through and between the cells of the root’s ground tissue until it reaches the xylem, from where it is distributed throughout the plant.

The many spaces between root cells allow oxygen to diffuse in so that the root’s cells can respire (make ATP and exchange gases with the environment). Mangroves grow in water-logged soil, hence very little air can get into the submerged roots directly from outside. As a consequence, mangroves produce odd-looking long roots—with numerous little pores in them—that protrude above the water (FIGURE 17-12). The pores open into loosely packed cells, creating a column of air that extends down to the underwater roots.