Patterns of Growth in Plant Roots
Investigating Primary and Secondary Root Growth
Growth in roots is apparently a continuous process that stops only under such adverse conditions as drought and low temperature. During their growth through the soil, roots follow the path of least resistance and frequently follow spaces left by earlier roots that have died and rotted away.Roots will not and cannot grow in dry soil. However, they will grow in any direction and to great lengths to reach favorable conditions of mineral nutrients and/or water availability.
The extent of a root system – the depth to which it penetrates the soil and the distance it spreads laterally – is dependent upon several factors, including the moisture, temperature, and composition of the soil. The bulk of most “feeder roots” (roots actively engaged in the uptake of water and minerals) occurs in the upper meter of soil, the part of the soil normally richest in organic matter.
The lateral spread of tree roots is usually greater than the spread of the crown of the tree. The roots of a 35-foot elm may have a radial spread of over 60 feet outward in all directions. The root system of corn plants often reach a depth of 1.5 m (5 ft), with a lateral spread of about a meter (3.3 ft) on all sides of the plant. The roots of alfalfa may extend to depths of up to 6 m (20 ft) or more making it an ideal crop in dry areas of the Central Plains states.
Primary Growth in Roots
Primary growth in roots occurs in an area of cells at the tip of the root known as the apical meristem. The apical meristem is an area of cells that form more meristem cells, root cap cells, and undifferentiated root cells. The undifferentiated cells, after first undergoing elongation (which is what pushes a root downward through the soil), gradually mature into the primary tissues of the mature root.
The tip of the root is covered by a root cap, a thimble-like mass of cells that protects the apical meristem (end growth point) behind it and aids the root in its penetration of the soil. As the root grows longer and the root cap is pushed forward, the cells on the periphery of the root cap are sloughed off.
As quickly as root cap cells are sloughed off, new ones are added by the apical meristem. The longevity of root cap cells ranges from four to nine days, depending on the length of the cap and the species. The growing root tip is covered by a slimy sheath, or mucigel, which lubricates the root during its passage through the soil.
Covered by the root cap, the apical meristem is the region of cell division. Here small thin-walled meristematic (growth) cells are undergoing rapid replication. Behind the region of cell division is the region of elongation. The elongation of cells in this zone, which is usually only a few millimeters in length, results in most of the increase in the length of a root.
The region of elongation is followed by the region of maturation. In this zone most of the cells of the primary tissues mature. Root hairs are also produced in this region, and sometimes this part of the root is called the root hair zone. As one travels upward from the root cap, the regions blend and transition one into the other with no points of sharp delineation separating them.
Secondary Growth in Roots
Commonly, the roots of monocots consist entirely of primary tissues. In addition, the roots of many herbaceous dicots undergo little or no secondary growth and remain largely primary in composition.
However, in the larger dicots, such as trees, secondary growth occurs. In secondary growth additional kinds of tissue arise from the primary tissue. The most notable are a vascular cambium and periderm.
Vascular cambium is a layer of meristematic (growth) cells that increase the diameter of the root as they replicate while the periderm is a thick, tough outer covering that replaces the primary epidermis. Because of these secondary structures, tree roots somewhat resemble tree branches in their internal structure and outer covering layer.
Roots will grow in any hospitable environment, often going to great lengths to find necessary minerals and/or water. As they grow, plant roots not increase in length but also in diameter.
Roots are Unseen and Their Extent is Unappreciated
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