Roots

In most dicots, the primary root emerges from the seedling, increases in size, and grows downward. Lateral roots emerge sideways. The primary root plus lateral roots form the taproot system. A carrot would be an example.

In monocots, the taproot is replaced by adventitious roots that arise from the stem. These roots and their branchings form a fibrous root system. Grasses would be an example. Adventitious roots and their branchings are similar in diameter and length.

Cells in the apical meristem divide and then differentiate into epidermis, ground tissue, and vascular tissues. The dome-shaped cell mass at the root tip is the root cap. The apical meristem produces the cap. Cell divisions at the apical meristem cause the root to lengthen. Farther up the root, cells differentiate and mature. The root cap protects the apical meristem and pushes through the soil. Cells are torn loose as the root grows.

The root epidermis is the outermost layer and is an absorptive interface with the soil. Root hairs are slender extensions of specialized epidermal cells that greatly increase the surface area available for absorption. Certain symbiotic organisms work with plants to increase uptake (mutualism). Gaseous nitrogen is converted to forms useful to plants by bacteria, often residing in root nodules as symbionts. Mycorrhizae (fungi growing around plant roots) aid in absorbing minerals that are supplied to the plant in exchange for sugars (symbiosis).

This animation (Audio - Important) describes root structure.

This video from the Indiana University Plants in Motion web site illustrates root growth. Can you see the development of root hairs?

The root cortex is beneath the epidermis. The endodermis which is the innermost layer of the cortex and is a ring of cells surrounding the vascular cylinder, helps control water movement. Its cell walls are waterproof. Water can only enter the vascular cylinder by moving through endodermal cells. This allows the plant to control inward flow of water.

Roots of most flowering plants have an endodermis that surrounds the vascular cylinder and an exodermis just below the epidermis. Cells of both layers contain a Casparian strip that controls the flow of water and nutrients. The water-repellent Casparian strip forces water to move through the cytoplasm of the endodermal cells. Transport proteins and other membrane components control absorbed nutrient distribution throughout the plant. Within the endodermis is the pericycle, which is meristematic and can give rise to lateral roots that push through the cortex and epidermis.

This animation (Audio - Important) describes the function of the Casparian strip.

Monocot roots have their vascular bundles arranged in a ring. Dicot roots have their xylem in the center of the root and phloem outside the xylem.

Roots are stimulated to grow outward to greater concentrations of water and nutrients. Most root systems of flowering plants mine the soil to a depth of 2 to 5 meters. But in unusual climates, such as deserts, roots can go as deep as necessary to find water.

REVIEW: The Casparian strip is associated with the
Answer endodermis.