Root cap: Tips of roots are covered by a thimble-shaped root cap, that has its own meristem that pushes cells forward into the cap. As they move through the cap, these cells differentiate into columella cells.
Columella cells each contain 15-30 amyloplasts that sediment in response to gravity to the lower side of the cell. Besides protecting the growing root tip and its meristem, the root cap senses light and pressure exerted by soil particles.
Within a few days, columella cells differentiate into peripheral cells. The peripheral cells of the root cap and the epidermal cells of the root produce and secrete large amounts of mucigel, a slimy substance made by dictyosomes.
Mucigel is a hydrated polysaccharide containing sugars, organic acids, vitamins, enzymes, and amino acids. Ê
Important functions of mucigel: Ê
- Protection-It protects roots from desiccation and contains compounds that diffuse into the soil and inhibit growth in other roots. Lubrication-It lubricates roots as they force their way between soil particles.
- Water absorption-Soil particles cling to mucigel, and increase the root's contact with the soil.
- These properties of mucigel help maintain the continuity between roots and soil water. Nutrient absorption-Carboxyl groups in mucigel influence ion uptake, and organic acids in mucigel make certain ions are more available to plants.
- Also, fatty acids, lectins, and sterols in mucigel may help establish beneficial symbioses with soil microbes.
Quiescent center:
- This structure is located just behind the root cap and consists of 500-1,000 seemingly inactive cells. These cells are usually in the G1 phase of the cell cycle and divide only about once every 15-20 days.
- Quiescent and meristematic cells are different in sensitivity to environmental problems such as radiation. For example, meristematic cells stop dividing when exposed to x rays while quiescent cells are unaffected by radiation and soon begin dividing to reform the meristem.
- Cells in the quiescent center function as a reservoir to replace damaged cells of the meristem. Its important because it organizes the patterns of primary growth in roots.
Subapical Region- This region of roots has traditionally been divided into three regions;
the zones of cellular division, cellular elongation, and cellular maturation. These divisions are useful for teaching but are not sharply defined. They do not always accurately define what is happening in a particular region of the root.
- Zone of cellular division- Surrounding the
quiescent center is a dome shaped apical meristem located 0.5-1.5 mm
behind the root tip. This meristematic region is the zone of cellular
division and its made of small densely cytoplasmic cells. Meristematic
cells in roots divide every 12-36 hrs, in some plants, the meristem
produces almost 20,000 new cells each day.
Ê - Zone of cellular elongation- This area occurs
4-15 mm behind the root tip. Cells in this zone elongate by as much
as 150-fold by filling their vacuoles with water. This zone is easily
distinguished from the root cap and zone of cellular division by its
long, vacuolate cells. Cellular elongation in the elongating zone shoves
the root cap and apical meristem through the soil at rates as high as
4 cm per day. Cells behind the elongating zone do not elongate. Ê
- Zone of cellular maturation- Differentiation is completed in this zone, which occurs 1-5 cm behind the root tip. This zone is easily distinguishable by the presence of several root hairs. Root hairs increase the absorptive surface area of the root several thousandfold and are usually less than a millimeter long. In most plants they form from asymmetric divisions of the protoderm and usually live only a few days, with old hairs farthest from the tip constantly being replaced by new ones closer to the tip. Root hairs only form in the maturing, non elongating region of the root. Because root hairs are fragile extensions of epidermal cells, they usually break off when plants are transplanted.
In the root proper.......Mature Region- Primary tissues differentiate in or distally to the zone of cellular maturation. Ê
Epidermis: This surrounds the root and is usually one cell thick. Epidermal cells differentiate from the protoderm and usually either lack a cuticle or have a thin cuticle that does not significantly affect water absorption. The epidermis covers all of the root except the root cap and usually has no stomata. Ê
Cortex: Just interior to the epidermis is the cortex, which is formed by the ground meristem and usually occupies the large cross-sectional area of a root and consists of three layers;
- the hypodermis (hypo = below) ,
- storage parenchyma cells, and
- the endodermis.
Lateral transport of minerals and water in roots- Ê There are two pathways in a root which can be taken in the plant for uptake of water and dissolved nutrients. Each way has its advantages and disadvantages.
