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Roots
Roots are plant parts that can bear other roots and stems but never leaves. They have a characteristic structure, including a growing tip, or root apex, which does not produce leaves but is surrounded by a cap of cells, the root cap. They also have a characteristic internal structure. Roots have several functions. They anchor a plant to a site, absorb water and nutrients from the soil, store nutrients, and support bacteria or fungal hyphae, which work with the root to the benefit of both organisms.
Root Systems
Roots are formed in two ways during the development of the plant. Primary root systems are roots and their branches derived from the hypocotyl, the portion of the embryo below the seed leaves (cotyledons). They are the first root structures formed. In monocots, the primary root system does not develop well; instead, an adventitious root system forms, in which roots arise from stem or leaf tissue. At the bottom of a corn plant, for example, are a number of these adventitious roots growing out of, and acting as a prop for, the stem.
Root Types and Modifications [Illustration]
There are many different types of roots based partly on structure and partly on the major function of the root:
- Aerial roots: Roots formed and remaining above ground. They often have functions other than absorption. For instance, poison ivy (Toxicodendron radicans) has aerial roots that attach the vine to the trunk of a tree.
- Fibrous roots: Thread-like, numerous roots, such as those of many grasses. They are usually adventitious in origin.
- Haustoria: The areas of a parasitic plant that attach to its host, through which the parasite absorbs nutrients. These may be derived from roots or stems. Dodder (Cuscuta species), the yellow or orange parasitic plants that look like string, form haustoria on their stems that penetrate the host plant's stems.
- Nodules and mycorrhizae: Connections between a plant and either bacteria or fungi. Many members of the bean family form root nodules. These nodules contain a bacterium that converts atmospheric nitrogen into a form the plant can use. Alfalfa (Medicago sativa) is one so-called nitrogen-fixing plant. One reason farmers plant alfalfa as part of their crop rotation is to fertilize the soil with nitrogen. Mycorrhizae are fungi that are connected to a plant via haustoria. The fungi gain some nutrients from the parent plant, and they benefit the parent plant by absorbing nutrients more efficiently. Some plants, such as rhododendrons and native orchids, are dependent on the mycorrhizae and will not grow without them.
- Pneumatophores: Roots found on plants in swampy areas in which there is not enough oxygen in the water for the root to survive. These roots grow toward the water surface and then bend over, forming a knob above the water. Pneumatophores are almost hollow, allowing air to pass to the living tissue of the root. The knees of cypress (Taxodium distichum) are examples of pneumatophores.
- Prop root: A root that acts as support for the parent plant. Mangroves (Rhizophora species) have extensive prop roots that keep the plants above salt water.
- Root hairs: Small, hair-like projections from the epidermis or outermost layer of a root. Most absorption of water and nutrients occurs via root hairs.
- Tap root: Primary roots that have more or less enlarged and grown downward. The carrot is a typical tap-rooted plant.
- Tuberous root: A swollen root in which nutrients (usually starch) are stored for the parent plant. Dahlias and Dioscorea (yams) are examples of plants with tuberous roots.