Introduction to the science of botany

ROOTSPLANT ORGANS

OBJECTIVES

Describe the functions of roots and its two unique features

Contrast the structure of a primary eudicot and monocot root

Describe the functions of the tissues of eudicot and monocot root

Describe several roots that are modified to perform unusual functions

ROOTS

The extent of a plant’s root depth and spread varies considerably among species and even among different individuals in the same species.

Soil conditions greatly affect the extent of root growth.

Anchors a plant securely in the soil.

The root systems are adapted to obtain water in a variety of ways.

Lack nodes and internodes and do not usually produce leaves or buds.

ROOT SYSTEM

Taproot System• Consists of one main root formed from the

enlarging radicle with many smaller lateral branch roots.

• Lateral roots often occur initially in regular rows along the length of the main root.

• Characteristic of many eudicots and gymnosperms.

• Most trees, have taproots when young and later develop large, shallow lateral roots from which other root branch off and grow downward.

Gymnosperms – seed-bearing plants that do not produce

flowersEx: Cone-bearing trees such as

pines, hemlock and firs

Eudicots – angiosperms that are flowering plants

Ex: Oak, Cherry, Bean and Daisy

ROOT SYSTEM

Fibrous System• Several to many roots of the same size that

develop from the end of the stem, which smaller lateral roots branching off these roots.

• Form in plants in which the embryonic root is short -lived

• Do not arise from preexisting roots but from the stem, they are called adventitious.

Adventitious – 0rgans occur in unusual locations, such as roots that develop on a stem or buds

that develop on roots

Monocots – grass or grass-like flowering plants

Ex: Onion, Crabgrass

FUNCTION

Anchorage• Roots anchor a plant

securely in the soil.• Firm anchorage is

essential to a plant’s survival so that the stem remains upright, enabling leaves to absorb sunlight efficiently.

Absorption• Roots absorb water and

dissolved minerals (inorganic nutrients), such as nitrates, phosphates and sulfates, from the soil.

• These materials are the transported throughout the plant in the xylem.

Conduction• The water and minerals

absorbed are conducted by the various parts of the root and shoot system

FUNCTION

Storage• Surplus carbohydrates produced in

the leaves by photosynthesis are transported in the phloem, as sugar, to the roots for storage, usually as sugar or starch, until needed.

• Although roots use some photosynthetic products for their own respiratory needs, most are stored and later transported out of the roots when the plant needs it.

• Taproot and fibrous roots may be modified for storing food.

Carrot roots – have an extensive phloem for this purpose

Taproots – Beets, Carrots, Radishes and TurnipsFibrous – Sweet Potatoes

Usually biennials, as part of the strategy to survive winter, store their food reserves in the root during the 1st

year’s growth and use these reserves to reproduce during the 2nd year’s growth

*Other plants living in dry regions, possess storage roots adapted to store water

STRUCTURE

Root cap• Covers the root tip• A protective thimble like that layers many

cells thick, covers the delicate root apical meristem

• As the root grows, pushing its way through the soil, parenchyma cells of the root cap slough off by the frictional resistance of the soil particles and replaced by new cells formed by the root apical meristem toward its outer side.

• Involved in orienting the root so that it grows downward

STRUCTURE

Root hairs• Short-lived, unicellular extensions of

epidermal cells near the growing root tip. Short, typically less than 1cm or 0.4in in length, but are quite numerous.

• Form continually in the area of cell maturation closest to the root tip to replace those that are dying off at the more mature end Of the root hair zone.

• They greatly raise the absorptive capacity of the root by increasing the surface area in contact with the moist soil.

STRUCTURE

Root hairs• Soil particles are coated with a

microscopically thin layer of water in which inorganic nutrient minerals are dissolved.

• The root hairs establish intimate contact with soil particles, which allows absorption of much of the water and dissolved materials.

• Modification of the root epidermis that enables it to absorb more water from the soil.

ROOT OF HERBACEOUS EUDICOTS

The root epidermis does not secrete a thick, waxy cuticle.

The lack of a cuticle and the presence of root hairs increase absorption.

Most of the water that enters the root moves along the path of least resistance – along the cell walls rather than entering the cells.

One of the major components of cell walls is cellulose, which absorbs water as sponge does.

Lacks a pith, a ground tissue in the centers of many stems and roots


Cortex• Ground tissue which is composed primarily of loosely

arranged parenchyma cells with large intercellular spaces, makes up the bulk of the root.

• Lacks supporting collenchyma cells, although it may develop some supporting sclerenchyma cells as it ages.

• Primary function of the root cortex is storage.• The large intercellular air spaces, provide a pathway for

water uptake and allow for aeration of the root.• The oxygen that root cells need for aerobic respiration

diffuses from air spaces in the soil to the intercellular spaces of the cortex, and from there to the cells of the root.


Endodermis• Inner layer of cortex which controls the

amounts and kinds of water and dissolved materials that enter the xylem in the root’s center.

• Endodermal cells fit snugly against one another and each cell has a special bandlike region, called a Casparianstrip, on its radial (side) and transverse (upper and lower) walls.

Casparian Strip – a band of waterproof material that ensure water and minerals enter

the xylem only by passing through the endodermal cells. It contains suberin


Endodermis• Water enters endodermal cells

by osmosis, whereas inorganic minerals enter by passing through carrier proteins in their plasma membranes.

• Thus, endodermis controls what kinds of dissolved minerals and how much of each kind move from the soil into the vascular tissue of the root and from there to the rest of the plant body.


Continuum consisting of the cytoplasm of many plant cells which is connected from one cell to the next by plasmodesmata.

Some dissolved mineral ions move from the epidermis through the cortex

Continuum consists of the interconnected porous cell walls of a plant, along which water and inorganic mineral ions move freely.

The water and mineral ions diffuse across without ever entering a living cell.

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Water and dissolved minerals that enter the root travel from one cell’s cytoplasm to another through

cytoplasmic connections or from cell to cell along the interconnected porous cell walls. 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 Casparianstrip blocks the passage of water and minerals along the

cell walls between adjoining endodermal cells.


Pericycle• Outermost layer of the stele which is

just inside the endodermis.• Composed of parenchyma cells that

remain meristematic, gives rise to lateral roots.

• Involved in forming the lateral meristems that produce secondary growth in wood roots.

Lateral roots – originate when a portion of the pericycle starts dividing.

- pushes through several layers of root tissue before entering the soil

- has all the structures and features of the larger root from which it emerges: root hairs, root cap, epidermis, cortex, endodermis pericycle, xylem and phloem


Xylem• Centermost tissue of the stele, often

has two, three, four or more extensions, or “xylem arms”

• Tracheids and Vessel Elements conduct water and dissolved minerals

Phloem• Located in patches between the

xylem arms.• Sieve-tube Elements conducts

carbohydrates (sucrose)

Root Hair Epidermis Cortex Endodermis

Root XylemStem Xylem

Sugar is made in photosynthesis

Sugar is used for growth and maintenance of root tissues or stored, usually as starch

Sugar is stored as starch

Sugar is used for growth and maintenance of

tissues

Vascular Cambium gives rise to secondary tissue in woody plants, is

sandwiched between xylem and phloem

ROOT OF HERBACEOUS MONOCOTS

More varied in internal structure than eudicot roots

Its xylem does not form a solid cylinder of vascular tissues in the

center

Phloem and xylem are in separate alternating strands that in cross section are arranged in a

circle around the centrally located pith, which consists

parenchyma cells

Do not have true secondary growth, therefore, no vascular

cambium exists

Have thickened roots produced by a modified form of primary growth in which parenchyma cells in the cortex divide and

enlarge

Cortex expands in such plants

ROOT OF HERBACEOUS MONOCOTS

ROOT OF WOODY PLANTS

Plants that produce stems with secondary growth (wood and bark) also produce roots with secondary growth

The production of secondary tissues in the roots of woody plants occurs some distance back from the root tips is the result of the activity of two lateral meristems, the vascular and cork cambium.

Major roots of trees are often massive and have both wood and bark.

In temperate climates, the wood of both roots and stems exhibits annual rings in cross section.

ROOT WITH UNUSUAL FUNCTIONS

• Adventitious roots often arise from stem nodes (regions of the stem where leaves are attached).

• Many aerial adventitious roots are adapted for functions other than anchorage, absorption, conduction or storage.

• Prop roots are adventitious roots that develop from branches or from a vertical stem and grow downward into the soil to help support the plant in an upright position.

• Prop roots are more common in monocots.

Monocots and herbaceous plants that produce prop roots

Examples: Corn and sorghum

Tropical and subtropical eudicot trees also produce prop rootsExamples: Red mangrove and

banyan


• The roots of many tropical rainforest tress are shallow and concentrated near the surface in a mat.

• Swollen bases and braces called buttress roots hold the trees upright and aid in the extensive distribution of the shallow roots.

Examples: Moreton Bay Fig Tree and Terminalia arjuna


• In swampy or tidal environments where the soil is flooded or waterlogged, some roots grow upward until they are above the high tide level.

• Even though roots live in the soil, they still require oxygen for aerobic respiration.

• A flooded soil is depleted of oxygen, so these aerial “breathing” roots, known as pneumatophores, may assist in getting oxygen to the submerged roots.

Pneumatophores anchors the plant, have a well-developed system of

internal air spaces that is continuous with the submerged parts of the root,

presumably allowing gas exchange

Examples: Back mangrove and white mangrove


• Climbing plants and epiphytes, which are plants that grow attached to other plants, have aerial roots that anchor the plant to the bark, branch and other surface on which it grows.

• Some epiphytes have aerial roots specialized for functions other than anchorage. Epiphytic roots may absorb moisture as well.

• Other starts its life as an epiphyte that produces long, hanging aerial roots that eventually reach the ground and anchor the plant in the soil.

Examples: Epiphytic Orchids –Photosynthetic roots

Examples: Strangler fig - the tree where it originally grew is often killed as the fig grows around it, competing

with it for light and other resources and crushing its second phloem

Becomes a self-supporting

tree


• Some parasites have roots that penetrate the host-plant tissues and absorb water and dissolved minerals from the host’s xylem.

• Other parasitic plants, take both water and sugars from their hosts

Examples: Mistletoe – does not take sugars from its host plant


• Contractile roots grow into the soil and then contract (the cortical cells shorten or totally collapse on themselves), thus pulling the plant deeper into the soil.

• The contraction is irreversible.• Contractile root are necessary for

some plants because each succeeding year’s growth is on top of the preceding year’s growth.

• Are more common in monocots, but eudicots and ferns also possess them.

Examples: Corm and bulb-forming plants such as Lily and Gladiolus

Corm and bulb tend to move upward in the soil over time.

Without contractile roots, they would eventually be exposed at

the soil’s surface


• Some roots reproduce asexually by producing suckers, which are aboveground stems that develop from adventitious buds on the roots.

• Each sucker grows additional roots and becomes an independent plant when the parent plant dies.

• These plants are difficult to control, because pulling the plant out of the soil seldom removes all the roots.

Examples: Black locust, pear, apple, cherry, red raspberry and

blackberry

Examples: Some weeds – field bindweed

The roots grow as deep as 3 meters in the soil. In response to a wound

the roots produce additional suckers

ROOT ASSOCIATION WITH OTHER SPECIES

• The two types of roots may grow together by secondary growth to form a natural graft.

• The root of most plant series form a mutually beneficial relationship with certain soil fungi.

• These subterranean associations known as mycorrhizae, permit the transfer of materials (such as sugars) from the roots of the fungus.

• At the same time, essential minerals (phosphorus) move from the fungus to the roots of the host plant.

• The threadlike body of the fungal partner extends into the soil, extracting minerals beyond the reach of the plant’s roots.

Examples: Between birch and maple

Because their vascular tissues are c0nnected in the graft, dissolved sugars and other materials such as hormones

pass between the two trees. Root grafts have been observed in more than 160

tree species

Ectomycorrhizae –fungal mycelium encircles the root

like sheath

Endomycorrhizae –fungus that

penetrates root cells

The relationship is mutually beneficial because when mycorrhizae are not

present, neither the fungus nor the plant grows as well




• Certain nitrogen-fixing bacteria, collectively called rhizobia form associations with the roots of leguminous plants.

• Swellings called nodules develop on the roots and house millions of the rhizobia.

• Like mycorrhizae, the association between nitrogen-fixing bacteria and the roots of plants is mutually beneficial.

Examples: Clover, peas and soybean

The bacteria receive the products of photosynthesis from the plants while

helping the plant meet its nitrogen requirements by producing NH3 from

atmospheric nitrogen.

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Introduction to the science of botany