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Plant Biology - Roots

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    Study Guide Plant Biology – Roots 1. Primary Root Tissues and Structure The primary root has a simpler tissue arrangement than the primary stem . This is because roots do not produce leaves , so they do not need side connections to the vascular system. As a result, their internal structure is more straightforward. The primary plant body of the root is formed by three primary meristems and has three main regions: 1. Epidermis – the outer protective layer 2. Cortex – a broad middle region mainly for storage 3. Stele (vascular cylinder) – the central conducting region 1.1 Epidermis The epidermis is the outermost layer of the root. In young roots, many epidermal cells form root hairs . Why root hairs are important: • They greatly increase surface area • This allows more water and mineral absorption • Each root hair is an extension of the protoplast (living cell content) Root hairs: • Have very thin walls • Are extremely delicate and easily broken • Live only for a short time In some plants, the root epidermis is covered by a thin waxy cuticle . This layer is not thick enough to block the movement of water and dissolved substances into the root.

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    Study Guide 1.2 Cortex The cortex lies between the epidermis and the stele and makes up the largest portion of the primary root. It is composed mainly of parenchyma cells . In many dicots (eudicots) and gymnosperms that later develop extensive secondary growth, the cortex is eventually crushed , and its storage role is taken over by other tissues. The cortex has three distinct layers : 1. Hypodermis (exodermis) 2. Storage parenchyma 3. Endodermis 1.3 Hypodermis (Exodermis) The hypodermis lies just beneath the epidermis and is usually one or two cell layers thick . Key features: • Cell walls are heavily suberized • Walls are impermeable to water • Cells are tightly packed with no air spaces Functions: • Prevents loss of water and nutrients from the cortex • Protects against soil microorganisms • Especially well developed in arid - region plants and plants with shallow roots 1.4 Endodermis The endodermis is the innermost layer of the cortex and is easy to recognize because of special structures called Casparian strips . Casparian strips: • Are bands of suberin • Occur on the radial and transverse walls • Block movement through cell walls

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    Study Guide Why this matters: • Water and dissolved minerals must pass through living cell membranes • This allows selective absorption • Harmful substances in the soil are prevented from entering the plant The endodermis is: • Almost always present in roots • Rarely found in stems 1.5 Storage Parenchyma The storage parenchyma makes up most of the cortex. Characteristics: • Thin - walled, living cells • Loosely packed with many air spaces • Store starch and other substances Functions: • Food storage • Provides aeration , allowing roots to get oxygen • Cells expand and shrink as materials move in and out 1.6 Stele (Vascular Cylinder) The stele includes all tissues inside the cortex , namely: • Pericycle • Xylem • Phloem • Pith (in some plants) In most dicot roots , the center is filled with solid xylem . In most monocot roots , the center contains a parenchymatous pith .

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    Study Guide 1.7 Pericycle The pericycle is a thin layer of parenchyma cells located just inside the endodermis . Important roles: • Cells retain the ability to divide • Gives rise to lateral (branch) roots • During secondary growth, it forms: o Vascular cambium o Often the first cork cambium The pericycle can also produce more pericycle cells through division. 1.8 Vascular Tissues The arrangement of xylem and phloem differs between plant groups: • Dicot (eudicot) roots o Central, lobed xylem core o Phloem located between xylem arms o This arrangement is called a protostele • Monocot roots o Xylem forms a ring o Surrounds a central pith o This arrangement is called a siphonostele These vascular patterns are important because they help scientists trace evolutionary relationships among plants.

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    Study Guide Summary • Roots have a simpler structure than stems • The cortex is mainly for storage and regulation • The endodermis ensures selective absorption • The pericycle forms lateral roots • Xylem and phloem arrangement differs in dicots and monocots This organized structure allows roots to absorb, store, regulate, and transport materials efficiently while anchoring the plant firmly in the soil 2. Secondary Growth of Roots 2.1 What Is Secondary Growth in Roots? Secondary growth is the process that increases the thickness (girth) of roots. It is especially important in woody perennial plants , where secondary tissues make up most of the root mass . • Primary tissues continue to form only in young feeder roots. • The main supporting root system is made of secondary tissues . • These tissues are produced by lateral meristems : o Vascular cambium o One or more cork cambia Although roots grow underground and are not easily seen, they are as massive as the aerial parts of trees . They balance the weight of the plant above the ground and keep the tree upright and stable .

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