Quiscent center

 QUIESCENT CENTER
 STRUCTURE OF ROOT
 ROOT CAP
 Function
 EPIDERMIS
 CORTEX
 EXODERMIS
 ENDODERMIS

1. 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

2. 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.

3. QUIESCENT CENTER SPECIES Q.C cells Zea mays 174 500-600 Vicia faba 292 1100 Sinapis alba 520 600

4. 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

5. 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

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

7. 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.

8. 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

9. 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

10. 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.

11. 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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