Roots
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Transcript of Roots
3/21/2010
1
ROOTS
Functions• absorption of
water and minerals
• transport of water and minerals
h f h• anchorage of the plant to the soil
• holds soil particles in place
• storage of food
Types of root systems Origin of ROOTS
1. taproot/ primary root‐ from the radicle
• e.g. gymnosperms and dicots,
Origin of ROOTS2. lateral roots/branch roots –from the pericycle
• secondary root – from pericycle of
/ /main/tap/primary root
• tertiary root‐ from pericycleof secondary root
• quaternary root‐ from pericycle of tertiary root
• Ultimate lateral branches
10 state of growth
absorption of water
feeder roots‐‐remain short and fragile and short‐lived
3. adventitious roots‐ roots that arise from unusual places
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• Root cap
• Epidermis
• Exodermis
• Cortex
Primary State
• Endodermis
• Pericycle
• Vascular Cylinder
PRIMARY STATE1. ROOT CAP
• Protects the root meristem
• assists the root in the penetration of soil during its growth
• With mucilage
• Consists of living parenchyma cells containing starch and are believed to be involved in gravity perception
PRIMARY STATE
2. EPIDERMIS• closely packed elongated cells with thin walls, uncutinized
• If it persists, may become cutinized or suberized, or lignifiedor lignified
• typically uniseriate• with root hairs which are typically short‐lived
Velamen
• multiseriate (in Orchids);
• during dry weather, the cells are filled with air; during rain, they become filled with water;
f i h i l i d• functions: mechanical protection and prevention of excessive loss of water from the cortex
3. Cortex• Degree of differentiation is related to the longevity of the cortex
• With schizogenousintercellular spaces
a. Exodermis• Differentiated as a protective tissue
• May have casparian strip suberin lamella cellulose layers and be lignified
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b. Endodermis
• Universally present in roots
• With casparian strip
• Casparian strip is part of the primary wall; composed of lignin or suberin or both;
• In the 2nd stage
suberin lamella covers the entire wall on the inside of the cell
casparian strip is separated from the cytoplasm
• In the third stage,
a thick cellulose layer is deposited over the suberin lamella
• The thick wall+ original wall (with casparianstrip)
may become lignified
• The wall modificationsThe wall modifications
appear first on the face of the phloem strand and then spread toward the xylem
• Passage cells
opposite the xylem allow a limited transfer of material between the cortex and the vascular cylinder
4. Pericycle
• Consists of thin‐walled parenchyma
• Concerned with meristematic activities
• Origin of lateral roots and phellogen
• Origin of part of the vascular cambium
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• In monocots may undergo sclerification in older roots
• In gymnosperms typically multiseriate
5. Vascular cylinder—(+assoc. parenchyma)
• More clearly delimited from cortex than the• More clearly delimited from cortex than the shoot:a. Not interrupted by leaf gaps
b. This tissue is surrounded by the pericycle
c. Endodermis surrounds the pericycle
Vascular System
• Typically has an exarchxylem
• Typically differentiation of phloem is centripetal (protophloem pole near (p p pthe periphery of pericycle)
Pith consists of parenchyma, may become sclerified
Development Roots without 20 growthMONOCOTS– completion of 10 growth • Sclerification of parenchyma cells associated with the vascular elements
• Development of thick 20 walls in the d d iendodermis
• Differentiation of exodermis• Cortex is retained and no periderm develops• The protective tissues are the epidermis and the exodermis(replaces epidermis if former is destroyed)
Roots with 20 growth
• Vascular cambium develops partly between 10
xylem and 10 phloem
• Partly, vascular cambium develops from the y, ppericycle
• Pericycle divides periclinally and the outermost gives rise to the phellogen
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Protective tissue:
• Persisting thick‐walled epidermis (Ranunculus)
• Exodermis (Compositae)
• Dead and collapsed but persisting cortex (Linaria, Polygonum)
• Subdivided and suberized endodermis• Subdivided and suberized endodermis (Gentiana)
• Polyderm (Potentilla)
• Periderm of deep seated origin (Saxifragaceae)
Development of lateral roots• In gymnosperms and angiosperms, originate from the pericycle
• Endodermis may participate in the initial growth of the branch root
• The derivatives of the endodermis combined• The derivatives of the endodermis combined with those of the cortex may form a rootcap‐like structure called pocket
• In lower vascular plants
the branch roots originate from the endodermis
• If parent plant has more than 2 xylem poles, lateral roots emerge opposite the xylem or phloemp
• if diarch,
in between X and P
Development of Adventitious roots
• Most of them arise endogenously
• Plants with adventitious roots:
lower vascular – main root system
monocots
dicots –propagated by means of rhizomes or runners; water plants
• Most of them arise endogenously
• In young stems, adventitious roots derived from interfascicular parenchyma, phloem, in perivascular positionperivascular position
• In older stems from the vascular ray; sometimes from the cambial zone
• parenchyma vascular elements
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Little water enters the rootcap and the apical meristem
Maximum rates of absorption of water 10xylem is mature and endodermis has casparian
stripstrip
Maximum accumulation of salts close to the apical meristem
Root hairs
10 roots or roots with limited 20 growth cortex
with 20 growth parenchymatic and some sclerenchymatic cells of X
and P
The hypocotyls and base of the taproot is fleshy (Daucus, Beta)
Formation of many branches in the branched type of root system
Many adventitious roots in fibrous root system ( t t th il l d l b t bi d th il (penetrate the soil less deeply but binds the soil more tightly)
Root hairs play a part in binding the soil
Apical meristem of the root is subterminal because it is covered by the root cap
Apical meristem of the shoot is i lterminal
The epidermis of the root has more varied ontogenetic origin than that of the shoot (dermatogen; dermatocalyptrogen; protoderm)
The leaf primordia arise directly from the apical meristem of the shoot and the branches more or less directly; and both are exogenous
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The lateral roots arise independently of the apical meristem and are endogenous
Vascular system of the shoot differentiates largely or entirely in relation to the leavesThe vascular system of the root develops as an axial structure independent of the lateral organs
Leaf gaps and pith are characteristic of the vascular system of stems except in certain lower vascular plants
There are no leaf gaps in the root and frequently no pith
The relation between the regions of the primary body and the apical initials is often more precise in the root than in the shoot;
The 10 vascular tissues of the shoot more or less discrete bundles (units combining xylem and phloem) while in the Root the xylem alternates with the phloem
SHOOT 10 xylem differentiation is centrifugal in the shoot (endarch xylem) ROOT centripetal in the root (exarch xylem) lower vascular the 10 xylem is exarch in both root and stem, in ferns commonly mesarch in the stem
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The boundaries between the tissue systems are quite precise in the root.Endodermis and pericycle are universally present in roots
The root has shorter elongation region than the shoot and frequently develops no extensible types of
t l l t ( ith l d h li l 0protoxylem elements (with annular and helical 20walls)
Differences are quantitative rather than qualitativeThe 20 vascular tissues of the root higher proportion of living to non living cells Roots have higher bark to wood ratio
Roots have a lower percentage of the area of the bark occupied by fibersRoots have smaller number of fibers in the xylem
Larger vessels of more uniform size although sometimes fewer in numberA poor differentiation of growth increments
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First periderm arises in the pericycle while in the stem in the peripheral layers of the axis
A larger ratio of area of living cells to area of nonliving cells in both the phloem and the xylemMore starch and less tannic substances
region of the plant axis where some features are intermediate or transitional between those of the shoot and the root