Roots are one of the major vascular systems of plants, along with shoots, stems and leaves. This system provides the rest of the plant with water and dissolved minerals essential for proper growth. By burrowing downward through soil, plant roots obtain additional water and nutrients. The process of photosynthesis that occurs in leaves supports growth and metabolism in plant root systems.
Root growth begins as soon as seedlings germinate. The first root that appears is called the primary root, with other roots emerging from this area called secondary roots. Fuzzy hairs on the plant roots contain growth-control mechanisms that allow the hairs to elongate and move through the soil. A protein called RHD2 at the tip of root hairs produces a self-reinforcing chemical cycle that stimulates the uptake of calcium, which in turn, produces more RHD2. Root hairs arise from epidermal tissues and contain a cell wall, are lined with cytoplasm and have a vacuole containing cell sap.
This area consists of the root cap, a thimble-shaped sheath that protects the root from damage as it pushes its way through the soil. Behind the root tip is the quiescent center, a region of inactive cells that organizes patterns of primary growth and replaces cells from the meristem directly behind it. The meristem is a region of rapid growth where cells constantly divide to form new root hairs and other specialized tissue, including cells for the next section of root, called the region of elongation. Water intake through the vacuoles provides the means for elongation.
The Mature Region
Primary root tissues emerge just behind the region of rapid growth. Roots become thicker here and give rise to not only secondary, but also tertiary roots. Anchoring the plant to the ground is one of the primary functions of this area. Specialized cells, including those that form root vascular systems, arise here. All secondary and tertiary roots have their own root hairs, root caps and growth structures. A protective cork layer covers roots in this area.
Under the epidermis are specialized tissues common to all types of roots. These include the cortex, which stores starch and permits the flow of water without entering other root cells. Vascular tissue, such as the xylem and phloem, transport water and nutrients to shoots, stems and leaves. The endodermis provides a barrier between the cortex and vascular tissue. A coating called the Casparian strip prevents water from seeping between cells, forcing it to enter the endodermal cells before passing through to vascular tissue.
Dicots, plants that have two seed leaves, have a long main root or taproot system from which smaller, lateral roots emerge. Taproots are important adaptations for finding water or for storage, such as in carrots or beets. Monocots, plants with one seed leaf, are characterized by a fibrous root system. Adventitious roots, which form on parts of the plant other than roots themselves, also characterize monocots.