1. Copyright Pearson Prentice Hall
23–2 Roots
Copyright Pearson Prentice Hall

2. Copyright Pearson Prentice Hall
Types of Roots
Types of Roots
Taproots: found mainly in dicots
Fibrous roots: found mainly in monocots
Copyright Pearson Prentice Hall

3. Copyright Pearson Prentice Hall
Types of Roots
In some plants, the primary root grows long and thick. This primary root is called a taproot.
Ex: oak and hickory trees, carrots, dandelions, beets, and radishes.
Plants have taproots, fibrous roots, or both. Taproots have a central primary root and generally grow deep into the soil. Fibrous roots are usually shallow and consist of many thin roots.
Taproot
Fibrous Roots
Copyright Pearson Prentice Hall

4. Copyright Pearson Prentice Hall
Types of Roots
Fibrous roots branch to such an extent that no single root grows larger than the rest.
Fibrous roots are found in grasses.
Helps prevent top soil from being washed away.
Plants have taproots, fibrous roots, or both. Fibrous roots are usually shallow and consist of many thin roots.
Fibrous Roots
Copyright Pearson Prentice Hall

5. Root Structure and Growth
Roots contain cells from dermal, vascular, and ground tissue.
A mature root has an outside layer, the epidermis, and a central cylinder of vascular tissue.
Between these two tissues lies a large area of ground tissue.
The root system plays a key role in water and mineral transport.
Copyright Pearson Prentice Hall

6. Root Structure and Growth
The epidermal subsystem provides protection and absorption.
The root’s surface is covered with cellular projections called root hairs.
Root hairs provide a large surface area through which water can enter the plant.
Root hairs
A root consists of a central vascular cylinder surrounded by ground tissue and the epidermis. Root hairs along the surface of the root aid in water absorption.
Copyright Pearson Prentice Hall

7. Root Structure and Growth
Inside the epidermis is a layer of ground tissue called the cortex.
Ground tissue (cortex)
A root consists of a central vascular cylinder surrounded by ground tissue and the epidermis.
Copyright Pearson Prentice Hall

8. Root Structure and Growth
The cortex extends to another layer of cells, the endodermis.
The endodermis completely encloses the vascular cylinder.
Endodermis
Copyright Pearson Prentice Hall

9. Root Structure and Growth
The vascular cylinder is the central region of a root that includes the xylem and phloem.
Phloem
Vascular cylinder
Xylem
Copyright Pearson Prentice Hall

10. Root Structure and Growth
Roots grow in length as their apical meristem produces new cells near the root tip.
These new cells are covered by the root cap that protects the root as it forces its way through the soil.
It secretes a slippery substance that helps to progress the root through the soil.
Only the cells in the root tip divide. In the area just behind the root tip, the newly divided cells increase in length, pushing the root tip farther into the soil. The root cap, located just ahead of the root tip, protects the dividing cells as they are pushed forward.
Apical meristem
Root cap
Copyright Pearson Prentice Hall

11. Copyright Pearson Prentice Hall
Root Functions
Root Functions
Roots anchor a plant in the ground and absorb water and dissolved nutrients from the soil.
It takes energy on the part of the plant to absorb water.
Copyright Pearson Prentice Hall

12. Uptake of Plant Nutrients
Root Functions
Uptake of Plant Nutrients
To grow, flower, and produce seeds, plants need a variety of inorganic nutrients in addition to carbon dioxide and water.
The most important nutrients plants need include nitrogen, phosphorus, potassium, magnesium, and calcium.
In addition to these essential nutrients, trace elements are required in small quantities to maintain proper plant growth including sulfur, iron, zinc, molybdenum, boron, copper, manganese, and chlorine.
Copyright Pearson Prentice Hall

13. Copyright Pearson Prentice Hall
Root Functions
Active Transport of Minerals
The cell membranes of root hairs and other cells in the root epidermis contain active transport proteins.
The root does not actually pump water, but by pumping dissolved minerals into its own cells, the end result is almost the same: water moves from the epidermis through the cortex into the vascular cylinder.
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Root Functions
Transport proteins use ATP to pump mineral ions from the soil into the plant.
Root hairs
Roots absorb water and dissolved nutrients from the soil. Most water and minerals enter a plant through the tiny root hairs.
Copyright Pearson Prentice Hall

15. Copyright Pearson Prentice Hall
Root Functions
The high concentration of mineral ions in the plant cells causes water molecules to move into the plant by osmosis.
Roots absorb water and dissolved nutrients from the soil. Most water and minerals enter a plant through the tiny root hairs. Water moves into the cortex, through the cells of the endodermis, and into the vascular cylinder. Finally, water reaches the xylem, where it is transported throughout the plant.
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Root Functions
Movement Into the Vascular Cylinder
Osmosis and active transport move water and minerals from the root epidermis into the cortex.
The water and dissolved minerals pass the inner boundary of the cortex and enter the endodermis.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Root Functions
The endodermis stretches up and down the entire length of root, like a cylinder, and is composed of many individual cells.
Endodermis
Cells in the endodermis are made waterproof by the Casparian strip. The Casparian strip prevents the backflow of water out of the vascular cylinder into the root cortex.
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Root Functions
Each cell is surrounded on four sides by a waterproof strip called a Casparian strip.
Casparian strip
Casparian strip
Copyright Pearson Prentice Hall

19. Copyright Pearson Prentice Hall
Root Functions
The Casparian strip prevents the backflow of water out of the vascular cylinder into the root cortex.
Water moves into the vascular cylinder by osmosis.
Because water and minerals cannot pass through the Casparian strip, once they pass through the endodermis, they are trapped in the vascular cylinder.
As a result, there is a one-way passage of materials into the vascular cylinder in plant roots.
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
Root Functions
Root Pressure
As minerals are pumped into the vascular cylinder, more and more water follows by osmosis, producing a strong pressure.
This root pressure forces water through the vascular cylinder and into the xylem.
As more water moves from the cortex into the vascular cylinder, more water in the xylem is forced upward through the root into the stem.
Root pressure is the starting point for movement of water through the vascular system of the entire plant.
Copyright Pearson Prentice Hall