The macroevolutionary leap from water-dwelling to land dwelling plants was revolutionary, but it also required major adaptions for success. Water-dwelling plants had no need of a system for bringing in or transporting water. They did not need a supporting structure, since long columns of plants could be held up by water. Land-dwelling plants needed a means of collecting water and nutrients from soil, and the evolutionary change that made this possible was roots.
The roots of vascular plants, the plants that have tissues for transporting water or sap, serve several functions. The root is the organ of a vascular plant that provides water and mineral support and anchors the plant to the soil. It absorbs water and nutrients from the soil and may store quantities of carbohydrates. The root system is the complete mass of roots belonging to a plant, such as the one found on prairie grasses. It holds soil in place and reduces soil erosion. In general, the mass of a root system may be far greater than the shoot system, which includes the stem, branches, and leaves of a plant. The shoot system comprises the above-ground plant. In areas where water may be scarce, extensive root systems collect as much water as possible to keep plants healthy. Root hairs provide added surface area for water absorption and are responsible for absorbing most of the mineral solutes in soil. A solute is any a substance dissolved in water, and, for plants, it is the source of the nutrition plants need to thrive. Some typical solutes for plants are nitrogen, phosphorus, magnesium, potassium, and iron.
Root systems for plants vary. The roots of a fern may be thin and hair-like. Roots of trees, such as conifers, may be thick and woody. The roots of angiosperms vary depending of the type of angiosperm: monocot or eudicot.
Monocots are angiosperms that produce single embryonic seed leaves, such as grasses, grains, and corn. Plant roots for most monocots are fibrous root systems. They are thin, hairy, pale colored roots. Roots for prairie grasses, such as switchgrass or big bluestem, for example, may account for as much as 80 percent of the mass of a total perennial plant—a plant that lives for more than two years. Fibrous roots of these grasses develop a dense, mat-like structure and may reach two to five times deeper than the height of the above-ground plant.
Eudicots are angiosperms that produce two embryonic seed leaves, and include all plants that produce fruit and nuts, such as pear, plum, or avocado trees, as well as tomatoes, cucumbers, and green peppers. Roots for many eudicots are taproots, which are thicker, dominant, central roots that extend deep into the soil to obtain water. Eudicot taproots may be long, fat, round, or short. The taproot of a six-ton saguaro cactus may reach only five feet into the ground. Many taproots are edible, including carrots, parsnips, beets, turnips, horseradish, and radishes. Other plants with taproots include dandelions, parsley, and trees, such as elms or gums.