29.3.3: The Evolution of Roots in Seedless Plants
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Learning Objectives
- Explain how roots provide support for plants
Roots: Support for the Plant
Roots are not well preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants. Roots provided seed plants with three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Importantly, roots are modified to absorb moisture and exchange gases. In addition, while most roots are underground, some plants have adventitious roots, which emerge above the ground from the shoot.
Types of Root Systems
There are mainly two types of root systems. Dicots (flowering plants with two embryonic seed leaves) have a tap root system while monocots (flowering plants with one embryonic seed leaf) have a fibrous root system. A tap root system has a main root that grows down vertically from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds; they can regrow another shoot from the remaining root.
A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, forming a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). In addition, some plants actually have a combination of tap root and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water tend to have shallower root systems.
Root Growth and Anatomy
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip; it is made up of the actively-dividing cells of the root meristem. The zone of elongation is where the newly-formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip.
The vascular tissue in the root is arranged in the inner portion of the root, which is called the vascular cylinder. A layer of cells, known as the endodermis, separates the vascular tissue from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots, serving as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.
Root Modifications
Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch. Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage.
Key Points
- There are two main types of root systems: tap root systems consist of one main root that grows down vertically with smaller lateral roots growing off of the main root, while fibrous root systems form a dense network of roots near the soil surface.
- Roots can be modified to store food or starches and to provide additional support for plants; many vegetables, such as carrots, are modified roots.
- A zone of cell division, a zone of elongation, and a zone of maturation and differentiation make up a root tip, where the root cells divide, grow, and differentiate into specialized cells.
- The vascular system of roots is surrounded by an epidermis, which regulates materials that enter the root’s vascular system.
Key Terms
- endodermis : in a plant stem or root, a cylinder of cells that separates the outer cortex from the central core and controls the flow of water and minerals within the plant
- suberin : a waxy material found in bark that can repel water
- pericycle : in a plant root, the cylinder of plant tissue between the endodermis and phloem