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The Root: Structure & Developmentp

Ch 24

Root systems

• Two major functions:– Anchorage & absorption.

• Accessory functions:St & d ti– Storage & conduction

• Primary root– 1st root that originates in the embryo

• In seeds plants this is called taprootG di tl d d– Grows directly downward

– Gives rise to branch roots or lateral roots– Youngest roots are nearer the tip

• Commonly called taproot-system

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• Root systems can extend far into the soil– Prosopis juliflora – 53.3 m– Tamirix – 30 m

Maize 1 5m deep 1m wide– Maize – 1.5m deep – 1m wide

• Total area of 4 mo. old rye plant – 639 m2

– 130 times the surface area of the shoot

Root cap

• Rootcap– Thimble-like mass of living parenchyma cells

that protect the apical meristem– Aids the root in its penetration

• As root grows longer cells on periphery slough– Covered by slimy sheath, mucigel

• Lubricates root on its passage through soil

• Controls response of the root to gravity

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Apical organization

• “Closed-Type”– Rootcap, vascular cylinder, and cortex

• Interpreted as having its own initials

• “Open Type”• Open-Type– All regions (at least rootcap & cortex) arise

from one group of initials

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“Closed type”: three distinct layers of initials

“Open type”: three distinct layers of initials

• Region of initials in root apical meristems are mitotically active early in development– Later divisions become infrequent

Known as quiescent center– Known as quiescent center

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Growth in Length

• Region of cell-division– Actively dividing region

• Behind this region R i f l ti– Region of elongation

– Above this region does not increase in length• Beyond region of elongation

– Region of maturation or differentiation

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• Internal structure of root is relatively simple.– Three primary tissues

• Epidermis (dermal tissue layer)• Epidermis (dermal tissue layer)• Cortex (ground tissue system)• Vascular (vascular tissue system)

• Epidermis– Root hairs

• Aid in water and nutrient uptake• Short-lived; constantly being replacedShort lived; constantly being replaced

• Layer of soil bound to root by mucigel and root hairs contain microorganisms and sloughed rootcap cells– Rhizosphere

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• Cortex (ground tissue) in most roots

• Plastids of cortical cells store starchDevoid of chlorophyll– Devoid of chlorophyll

– Gymnosperms & Angiosperms• Shed cortex early• Cortex remains parenchymatous

– Monocots • Cortex retained for the life of the root

• Innermost layer of cortex is compactly arranged – endodermis

• Vascular cylinder – consists of primary vascular tissues and one or more layers of nonvascular cells

Pericycle– Pericycle• Completely surrounds the vascular tissue

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• Aerial roots and Air roots– Prop roots

• Contact with soil results in branching and function in absorption of water and minerals.p

• Air roots “pneumatophores”– Grown upward

providing adequateproviding adequate aeration

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• Adaptations for food storage:– Fleshy roots– Abundance of storage parenchyma

• Permeated by vascular tissue• Permeated by vascular tissue

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Summary• Roots are organs specialized for anchorage,

absorption, storage, and conduction• Root tip can be divided into regions of cell

division, elongation, and maturation• Root epidermis and cortex may be modified• Root epidermis and cortex may be modified

with age• The vascular cylinder consists of the primary

vascular tissues and pericycle• Root modifications include aerial roots, air

roots, and fleshy roots