Roots of Plants
When a seed germinates, its first root is called the radicle or embryonic root. This serves to anchor the plant to the soil and provide a supply of water and nutrients absorbed from the soil to the plant. As the seedling continues to grow, the radical develops into a primary root which in turn branches into secondary or lateral roots.
There are two main types of root systems in plants:
- Taproot systems: Those where the primary root grows downward into the soil branching into just a few secondary roots (e.g. carrots)
- Fibrous root systems: Those that branch early into many secondary roots which in turn repeatedly branch again (e.g. rye grass)
For trees with taproots, commercial nurseries will often cut their taproot at the apical root meristem early in a tree’s growth to stimulate branching. This promotes a more fibrous root system increasing the likelihood that transplanting them into a landscaped area or orchid will be successful.
Fibrous roots typically grow close to the surface of the soil. Grasses are a great example of plants with fibrous root systems. Because the roots are close to the surface of the soil, plants with fibrous roots are great for controlling erosion.
In addition to plants that have either taproot systems or fibrous root systems, there are other plants that have both or that, depending on the availability of water, may develop one or the other type of root system.
Roots in some plants can become specialized so that they store food for the plant to use. Carrots, turnips, and parsnips are great examples of plants with taproots which specialize in storing food. Other plants have secondary roots called tuberous roots that develop fleshy structures for food storage like sweet potatoes.
The two important external parts of roots are the root cap and root hairs. The root cap exists at the very tip of the root. It protects the apical root meristem that is right behind it. The root cap is believed to detect gravity so that it can tell the apical root meristem which direction it needs to elongate to grow down into the soil. Above the root cap, roots are covered by a layer of tissue called the epidermis. Root hairs extend outward for from epidermis cells increasing the root system’s surface area and its capacity to absorb of water and minerals from the surrounding soil.
There are three major regions inside of roots. The apical meristem just behind the root cap at the tip of the root is referred to as the region of cell division. Behind/above the meristem is the region of cell elongation, where cells elongate pushing the root through the soil. Behind/above the region of cell elongation is the region of cell maturation, where cells mature and fully differentiate into various tissues like epidermis, cortex, and vascular tissue.
The epidermis, as mentioned previously, is the outer layer of cells that make up the root. Because root hairs extend from the epidermis, the region of cell maturation is often referred to as the root hair zone. The layer of cells immediately inside the epidermis is called the cortex. This layer lets oxygen and water move between cells and are also responsible for storing food. Just inside the cortex is a single layer of cells called the endodermis, which regulates the amount of water and nutrients allowed to enter the root from the soil. Just inside the endodermis is another layer of cells called the pericycle, the meristematic tissue responsible for generating lateral roots in the region of cell maturation. The pericycle surrounds the stele or vascular column, a column of vascular tissue in the center of the root made of primary xylem and primary phloem.
In general, xylem is responsible for transporting water and dissolved minerals from the roots to the rest of the plant. Phloem is responsible for transporting food (sugars/carbohydrates) created through photosynthesis as well as amino acids from the leaves to all parts of the plant including the roots.