Plant Organs: Roots Chapter 6

KEY TERMS TAPROOT SYSTEM FIBROUS ROOT SYSTEM A root system consisting of one prominent main root with smaller lateral roots branching from it FIBROUS ROOT SYSTEM A root system consisting of several adventitious roots of approximately equal size that arise from the base of the stem

Root Systems

LEARNING OBJECTIVE 1 Describe the functions of roots Describe two features of roots that shoots lack

Functions of Roots Anchorage Absorption Conduction Storage

Storage Roots

KEY TERMS ROOT CAP ROOT HAIR A covering of cells over the root tip that protects delicate meristematic tissue directly behind it ROOT HAIR An extension of an epidermal cell of a root that increases absorptive capacity of the root

Root Cap Each root tip has a root cap A protective thimblelike layer Many cells thick Covers delicate root apical meristem May orient root so it grows downward

Root Hairs Short-lived, unicellular extensions of epidermal cells near the growing root tip Increase surface area of root in contact with moist soil, increasing root’s absorptive capacity

Structures Unique to Roots

(a) The root cap of an onion (Allium cepa) Root apical meristem (area of cell division) Root cap Figure 6.3: Structures unique to roots. (a) The root cap of an onion (Allium cepa) root. The root cap protects the root’s apical meristem. Fig. 6-3a, p. 114

in contact with the soil. Root hairs Soil air Soil water Figure 6.3: Structures unique to roots. Soil particles Epidermis (b) Root hairs on a radish (Raphanus sativus) seedling. Each delicate hair is a unicellular extension of the root epidermis. Root hairs increase the surface area in contact with the soil. Fig. 6-3b, p. 114

LEARNING OBJECTIVE 2 Contrast the structure of a primary eudicot root and a monocot root Describe the functions of each tissue

Primary Eudicot Roots 1 Outer protective covering Ground tissues Epidermis Ground tissues Cortex Pith (in certain roots) Vascular tissues Xylem Phloem

Primary Eudicot Roots 2 Epidermis Cortex Protects the root Root hairs help absorb water and dissolved minerals Cortex Consists of parenchyma cells Usually stores starch

KEY TERMS ENDODERMIS Innermost layer of the cortex of the root that prevents water and dissolved materials from entering the xylem by passing between cells

KEY TERMS CASPARIAN STRIP A band of waterproof material around the radial and transverse cells of the endodermis Ensures that water and minerals enter the xylem only by passing through the endodermal cells

Endodermis and Mineral Uptake

Endodermis Cortex Epidermis Casparian strip Endodermis Movement of water through the endodermis to the center of the root Figure 6.5: The endodermis and mineral uptake. Note the Casparian strip around the radial and transverse walls that prevents water and dissolved minerals from passing into the stele along endodermal cell walls. To reach the vascular tissues, water and dissolved minerals must pass through the plasma membranes of endodermal cells. Root cross section Casparian strip Fig. 6-5, p. 118

KEY TERMS PERICYCLE A layer of cells just inside the endodermis of the root Gives rise to lateral roots

Lateral Root

Fig. 6-7, p. 120 Figure 6.7: Lateral root. A multicellular lateral root emerges from a larger root. Lateral roots originate at the pericycle. Fig. 6-7, p. 120

Ruptured epidermis Lateral root Cortex Pericycle Willow Figure 6.7: Lateral root. A multicellular lateral root emerges from a larger root. Lateral roots originate at the pericycle. Willow Fig. 6-7, p. 120

Primary Eudicot Roots 3 Xylem Phloem conducts water and dissolved minerals Phloem conducts dissolved sugar

Herbaceous Eudicot Root

(a) Cross section of a buttercup (Ranunculus) root. Epidermis Cortex Figure 6.4: Structure of an herbaceous eudicot root. Stele (a) Cross section of a buttercup (Ranunculus) root. Note that the bulk of the root is the cortex. Fig. 6-4a, p. 116

(b) A close-up of the stele of the buttercup root. Cortex cells filled with amyloplasts Endodermis cell Pericycle cell Phloem cell Xylem vessel elements Figure 6.4: Structure of an herbaceous eudicot root. Intercellular space (b) A close-up of the stele of the buttercup root. Note the solid core of vascular tissues. Fig. 6-4b, p. 116

Comparing Monocot and Eudicot Monocot roots often have a pith in the center of the root In herbaceous eudicot roots, xylem and phloem form a solid mass in center of root Monocot roots lack a vascular cambium Do not have secondary growth

Monocot Root

Monocot Root

Greenbrier Epidermis Cortex Stele Fig. 6-8a, p. 121 Figure 6.8: Structure of a monocot root. (a) Cross section of a greenbrier (Smilax) root. Note the extensive cortex. (b) Close-up of a portion of the center of the root, showing the vascular tissues and pith. Stele Fig. 6-8a, p. 121

Endodermis Pericycle Phloem Vascular tissues Xylem Pith Figure 6.8: Structure of a monocot root. (a) Cross section of a greenbrier (Smilax) root. Note the extensive cortex. (b) Close-up of a portion of the center of the root, showing the vascular tissues and pith. Pith Fig. 6-8b, p. 121

LEARNING OBJECTIVE 3 Trace the pathway of water from the soil through the various root tissues

Water Movement 1 In a primary eudicot root, water moves from soil into center of root: Root hair → epidermis → cortex (symplast or apoplast pathway) → endodermis → pericycle → xylem of root Water is transported upward through root xylem into stem xylem and rest of plant

KEY TERMS SYMPLAST APOPLAST A continuum consisting of the cytoplasm of many plant cells, connected from one cell to the next by plasmodesmata APOPLAST A continuum consisting of the interconnected, porous plant cell walls, along which water moves freely

Symplast and Apoplast

Movement upward Endodermis Xylem vessels Casparian strip Epidermis Cortex Symplast: interconnected cytoplasm of living cells Phloem cells Pericycle Figure 6.6: The symplast and apoplast. Water and dissolved minerals that enter the root travel from one cell’s cytoplasm to another through cytoplasmic connections (the symplast) or from cell to cell along the interconnected porous cell walls (the apoplast). On reaching the endodermis, water and minerals can continue to move into the root’s center if they pass through a plasma membrane and enter an endodermal cell. The Casparian strip blocks the passage of water and minerals along the cell walls between adjoining endodermal cells. Plasma membrane Water and dissolved nutrient minerals Plasmodesma Cell wall Apoplast: interconnected cell-wall spaces Root hair Fig. 6-6, p. 119

LEARNING OBJECTIVE 4 Describe several roots that are modified to perform unusual functions

KEY TERMS PROP ROOT CONTRACTILE ROOT An adventitious root that arises from the stem and provides additional support for the plant CONTRACTILE ROOT A specialized root, often found on bulbs or corms, that contracts and pulls the plant to a desirable depth in the soil

Prop Roots

Contractile Roots

(a) Plants that produce corms or bulbs often have Contractile roots Figure 6.13: Contractile roots. (a) Plants that produce corms or bulbs often have contractile roots that lose much of their length as root cells shorten and broaden. Fig. 6-13a, p. 124

Contractile roots Figure 6.13: Contractile roots. Fig. 6-13b, p. 124

KEY TERMS PNEUMATOPHORE A specialized aerial root produced by certain trees living in swampy habitats May facilitate gas exchange between the atmosphere and submerged roots

Pneumatophores

Other Modified Roots 1 Buttress roots Swollen bases or braces that hold trees upright Aid in extensive distribution of shallow roots Found in some tropical rainforest trees

Buttress Roots

Other Modified Roots 2 Suckers Aboveground stems that develop from adventitious buds on the roots Asexual reproduction method of some roots Certain epiphytes have roots that are modified to photosynthesize

Parasitic Epiphytes

(b) Micrograph of a parasitized juniper (Juniperus) bark Mistletoe root Juniper wood Figure 6.12: Specialized roots of parasitic epiphytes. (b) Micrograph of a parasitized juniper (Juniperus) branch, showing a mistletoe root penetrating the wood (secondary xylem) of the juniper. Fig. 6-12b, p. 123

LEARNING OBJECTIVE 5 Discuss the significance of roots to humans

Important Foods Roots which store the products of photosynthesis are important sources of food for human consumption Some roots are used as flavorings Example: root beer flavoring (dried greenbrier roots)

Root Crops Predominantly taproots Some fibrous roots carrots, beets, sugar beets, parsnips, turnips, rutabagas, radishes Some fibrous roots sweet potatoes, cassava

KEY TERMS MYCORRHIZA NODULE A mutually beneficial association between a fungus and a root that helps the plant absorb essential minerals from the soil NODULE A small swelling on the root of a leguminous plant in which beneficial nitrogen-fixing bacteria (Rhizobium) live

Mycorrhizae

(a) Cross section of root showing ectomycorrhizae, Sheath of fungal hyphae encircles root Fungal hypha between plant cells Figure 6.14: Mycorrhizae. Mycorrhizae enhance plant growth by providing essential minerals to the roots. (a) Cross section of root showing ectomycorrhizae, fungal associations that form a sheath around the root. The fungal hyphae penetrate the root between cortical cells but do not enter the cells. Fig. 6-14a, p. 124

(b) Cells of a root cortex showing endomycorrhizae, Fungal hyphae within plant cortical cells Figure 6.14: Mycorrhizae. Mycorrhizae enhance plant growth by providing essential minerals to the roots. (b) Cells of a root cortex showing endomycorrhizae, fungal associations in which the fungal hyphae penetrate root cells of the cortex to aid in delivering and receiving nutrients. Endomycorrhizae colonize roots of most vascular plant species. Fig. 6-14b, p. 124

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