The discovery by Clowes (1954,1956) of a quiescent center in the root apex brought

about a fundamental change of view about the behavior of root meristems.

QUIESCENT CENTER

QUIESCENT CENTER: Clowes(1958)studied the root tips of Zea

mays and observed an inactive center in between root cap and active meristematic region.

fewer mitochondria ,ER , very small nuclei , low rate of DNA

synthesis and protein synthesis.

QUIESCENT CENTER

SPECIES Q.C cells

Zea mays 174 500-600

Vicia faba 292 1100

Sinapis alba 520 600

A quiescent center arises twice in primary roots:

1.Embryogeny 2.stages of seed germination The quiescent center is variable in volume: Thin roots- smaller or entirely absent long roots- distinctive centers short roots- no Q.c seedless vascular plants- it lacks qc

The relatively inactive state of the quiescent center cells does not mean that they have become permanently non functional.

Quiescent center cells do divide occasionally and serve to renew the more actively dividing regions around them, the cells of which are unstable and displaced from time to time.

In roots injured experimentally by radiation or surgical treatments the quiescent center is able to repopulate the meristem

STRUCTURE OF ROOT Arrangement of primary tissue in root:•Root cap•Epidermis •Root cortex•Vascular or central cell

ROOT CAP:•Site of geoperception.•Situated at the tip.•Made up of living parenchymatous cell.•Cells contain starch grains.•May be arranged in radiating rows or no

special arrangement.•Statoliths: starch containing plastids.

Function:• Protection• Penetration• Geotropism, (it controls the geotropic growth of root).• Root cap cells secrete a polysaccharide slime.(the process of secretion is accompanied by:Hypertrophy of dictyosome cisternae form large vesicles vesicle content released. )

,this secretion moves outwards through the wall.

•Root develops continuously.•Root caps are absent in parasites and mycorrhizal roots.•Root caps develop in true water plants but they degenerate early.eg. Eichornia

EPIDERMIS:

CORTEX:

Outermost cell layerRoot epidermis – epiblemma

or rhizodermisMade up of parenchymatous

cellsMay be cutinizedUniciliate or multiciliateChar. Feature: production of

root hair.Absorb water and minerals

It is found below the epidermisIn dicots,gymnosperm- paren.

cellsIn monocots- paren. + scleren.

cellsMajor role – storageInnermost layer – endodermisOutermost layer – exodermisChloroplast absent

EXODERMIS-Walls of the cells of outer subepidermal layer of cortex

become suberized ,this layer known as exodermis.• Protective tissue• Lignin present • Casparian strips may also present• Cells of the exodermis contain viable protoplasts .• In pteridophytes, no exodermis• Variation in thickness of the exodermis:• In Phoenix , exodermis is fibrous.

ENDODERMIS• Inner boundary of the root cortex.• Consists of a uniseriate cylinder of cells.• Casparian strips appear in the radial and crosswalls of the endodermal cells.• Band plasmolysis is found in endodermis.• Plasmodesmata is found.• Help in primary growth of roots.• In the formation of lateral roots.

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