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Quiscent center

The document discusses the discovery of the quiescent center in root meristems by Clowes, highlighting its characteristics such as fewer organelles and variable volume depending on root size. It details the structure and function of different root tissues, including the root cap, epidermis, cortex, and endodermis, as well as the roles of each in growth and protection. The quiescent center, despite its inactivity, plays a vital role in renewing the surrounding actively dividing cells and maintaining root structure.

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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: 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 layer  Root epidermis – epiblemma or rhizodermis  Made up of parenchymatous cells  May be cutinized  Uniciliate or multiciliate  Char. Feature: production of root hair.  Absorb water and minerals  It is found below the epidermis  In dicots,gymnosperm- paren. cells  In monocots- paren. + scleren. cells  Major role – storage  Innermost layer – endodermis  Outermost layer – exodermis  Chloroplast 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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