|Themes > Science > Botanical Sciences > Plants and their Structure > Roots|
Roots are equally important in plant growth as leaves and stems because they provide the chlorenchyma cells of stems and leaves with a steady supply of water and dissolved minerals.
They have four primary functions:
The first root to emerge from a seed is the radicle or primary root. In most dicots, the radicle enlarges and forms a prominent tap root. Smaller branch (lateral) roots grow from the taproot. A taproot system is common in conifers and dicots. It functions as food reserves such as carbohydrate storage or for reaching water deep in the ground. Taproots usually control growth and development of branch roots. Taproots grow longer than branch roots.
#1Fibrous Root System:
Most monocots have a fibrous root system consisting of an extensive mass of similarly sized roots. In these plants, the radicle is short lived and is replaced by a mass of adventitious roots which are roots that form on organs other than roots; for example the stem. Because the adventitious roots of monocots are so extensive and cling to soil particles, such plants are excellent for preventing erosion.
The adventitious roots of most monocots begin to grow soon after the seed germinates. Each node in the embryo usually produces 2-6 seminal roots that grow from the seed. These roots are soon supplemented by a system of crown roots that grow from nodes of the shoot.
Recent studies of monocot systems tell us they are more complex than originally thought. Corn plants have two types of adventitious roots, one of which is usually unbranched and has actively growing tips. These feeder roots are associated with a 1mm-thick soil sheath permeated by root hairs and cemented together by secretions of the root and the soil's microbes. Other roots of the root system are long, branched, and lack an encasing soil sheath. These roots lack actively growing tips; they grow determinately. There are several types of adventitious roots besides those of monocots. These roots are common along rhizomes of ferns, clubmosses, and horsetails. In a lot of plants, adventitious roots are a primary means of vegetative reproduction; prairie grasses and forests of aspen are often a single clone spread by adventitious roots. The formation of adventitious roots is controlled by hormones such as auxin, which are teh active ingredients in the rooting compounds sold in stores. Primary roots and adventitious roots have similar function and structure.
Anatomy of a root:
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.
Important functions of mucigel:
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.
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.
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.
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.
Primary tissues differentiate in or distally to the zone of cellular maturation.
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.
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, storage parenchyma cells, and the endodermis.
Lateral transport of minerals and water in roots-
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.