The Root System

The Functions of Roots - Absorption of water and nutrients (dissolved salts & minerals). - Conduction of absorbed materials into the plant body. - Anchorage of plant in soil. - Food storage (Carrots and Beets). - Many roots have relationships with bacteria and fungi in the soil in the rhizosphere (contact zone between soil and root surface, often a few millimeters thick). In the plant embryo, the radicle is the first root that is initiated.

The Root System and Shoot System support each other.

Types of Root Systems Fibrous Root System Tap Root System - Several main roots branch to form a dense mass of roots, which holds the plant firmly in place. As a result, grasses make fine ground cover because they minimize soil erosion. Basically, no common single primary root. Examples: Many monocots, Corn, Rye, Many Grasses, Small Garden Plants. Tap Root System - Posses one main root or primary root (Tap Root) from which lateral roots branch. Some taproots tap water deep in the soil. Tap Root Systems often have a large storage root (carrot, turnips and beets). Other Examples: Some desert plants, Hickory, Milkweed, Many Dicots, Dandelion. In your yard, how can dandelions and grass not compete for water?

Plants Have Different Types of Roots When seeds germinate, the embryonic root (Radicle) begins. In a fibrous root system, a single radicle forms but in addition several other embryonic roots (seminal roots) form just above the radicle. All roots then branch and from a fibrous root system. Example: Wheat. In a tap root system, the radicle forms the primary root then adds lateral roots (Branch Roots). Example: Pea.

Types of Adventitious Roots Adventitious roots originate on leaves and stems. These are often roots that arise above ground. Seminal Roots - Adventitious roots forming at the base of the seedling stem. Seminal roots are adventitious because they don’t emerge from another root. Prop Roots – In a young corn plant, soon after germination prop roots develop on the stem just above the soil. Prop roots support the plant and absorb water and minerals. Aerial Roots – These roots are exposed to air, they extend down from branches into the soil. They hold up branches of a tree and absorb water and nutrients. In English Ivy, aerial roots enable the ivy to cling to the sides of buildings. CORN

Types of Adventitious Roots Root Propagation can often occur with cut stems of some plants. Roots will grow off the stem. These roots are adventitious. Pneumatophores - Aerial roots that stick up out of the water and serve to aerate the root cells. These are common in trees that grow in swamps or salt marches like mangroves.

The Development of Roots Root Cap covers the end of the root tip for protection. It is the site of gravity perception & it controls the direction of root growth. Root tip is organized into 3 regions: 1) Root Apical Meristem (RAM) - Derives the cells and tissues of the root. 2) Region of Elongation - Cells are elongating in the roots. 3) Region of Maturation - Basal (Further Up) to the region of elongation where root hairs form and cells reach maturity.

The Development of Roots Primary Tissues Below Primary Meristems give rise to the above primary tissues. Primary Meristems RAM gives rise to above primary meristems.

Buttercup Root (Cross Section) The Development of Roots Primary Meristems Primary Tissues Some epidermal cells have root hairs. Parenchyma, Endodermis and Casparian strip. These tissues are within the vascular cylinder. Buttercup Root (Cross Section) Root Diagram (Cross Section)

Dicot Root Cross Section The Structure of Roots The Epidermis covers the entire root surface and is modified for absorption (Root hairs on epidermal cells help this). The Cortex cells (parenchyma) have many plastids for the storage of starch. The Vascular Cylinder (stele) consists of vascular tissues (xylem and phloem) surrounded by one or more layers of tissue called the pericycle. The Pericycle gives rise to lateral roots and contributes to the formation of the secondary meristems. The Endodermis is tightly packed layer of cells surrounding the vascular cylinder. Dicot Root Cross Section

Endodermis Each endodermis cell is wrapped with Casparian Strip, a continuous band of suberin, a waxy material that is impervious to water and dissolved minerals The function of the endodermis is to select what minerals enter the vascular cylinder and the body of the plant. Control of water and nutrient movement into the xylem is a function of the endodermis.

Some Roots Have Special Functions Haustorial Root – Modified root of certain parasitic plants, such as dodder, penetrates into the host plant. Root Nodule is a knot (swelling) on a root, caused by the infection of Nitrogen-fixing bacteria (N2 gas into NH4+) named Rhizobium sp. Plants benefit by receiving nitrogen in the correct form and bacteria receive housing and nutrients from the plant. Bacteria enter the plant by passing through a tiny infection thread that penetrates the root hairs. Once inside the host, the bacteria penetrate the cortex of the root forming a swollen mass (nodule) of cells filled with bacteria.

Some Roots Have Special Functions Mycorrhiza (plural, mycorrhizae) – a symbiotic association between a fungus and a root of a plant. These are short, forked root structures common to as many as 90% of seed plants. This relationship makes roots more efficient in mineral absorption. Fungi receive housing and carbohydrates from the plant. Mycorrhizae are not always essential but the fungus can help the plant fight disease. 2 Main Types of Mycorrhizae Ectotrophic – fungal cells do NOT penetrate plant root cells. However, it does form a sheath or mantle over the entire root. Endotrophic – fungal cells actually enter the cortex cells of the plant.

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