Tissue “Histology is the study of tissues!”

Syllabus l An understanding of the relationship between structure and function as exemplified by the following: l parenchyma, collenchyma, sclerenchyma, l xylem & phloem l Epithelium, connective tissues, muscular tissues & nervous tissues.

Suggested references l Advanced Biology principles & applications p. 218 – p. 223 l Toole, Glenn., Understanding Biology for Advanced Level (3 ed.). Stanley Thornes (Publishers) Ltd l Green, N. P. O., Biological Science. Vol. 1 (2 ed.). Cambridge University Press 1991

Definition: tissue l a TISSUE is a group of cells, usually of one kind, & their intercellular substance (e.g. middle lamella in plant & intercellular matrix in animal) which are linked together & perform a particular function

Plant tissues

Types of plant tissue (1): 1. Simple 2. Compund l simple plant tissue (tissue that consists of one type of cell only) l compound plant tissue (tissue that consists of more than one type of cells)

Types of plant Tissue (2) 1. Meristematic 2. Permanent l Meristematic tissue l isodiametric l without any intercellular spaces l thin cell wall l dense protoplasm l large nucleus l small or no vacuoles l occurrence : apical and lateral meristem

l Apical meristem l growing points of stem and root l give rise to primary permanent tissue (primary growth) l Lateral meristem l in cambium l producing secondary permanent tissues (secondary growth)

l Permanent tissue! l 1. simple 2. compound l types of simple permanent tissue l ~parenchyma l ~collenchyma l ~sclerenchyma

A. Parenchyma l Distribution l ~in cortex, pith & packing tissue of vascular tissues ( xylem and phloem) l Shape l ~usually spherical l ~unspecialised cells l ~thin cell wall

Parenchyma

A. Parenchma (continu.) lMlMetabolism l~l~metabolic active l~l~potentially meristematic lFlFunction l~l~store food ( starch ) l~l~provide support for herbaceous plants l~l~intercellular space around parenchyma cells allow exchange of gases

l Modified parenchyma n Green, N.P.O., Biological Science. Vol.1 (2 ed.). Cambridge University Press 1991: p.219 fig.8.3

Some parenchyma cells: l Epidermis l Structure: n One cell thick covers the whole of the primary plant body n Secrete a waxy substance to form a layer of cuticle l Function: n Reduce the water loss by evaporation l Guard cells l Structure: n exist in pairs, kidney shape, contain chloroplast l Function: n Control the size of the stoma

Epidermal Cells

Some parenchyma cells: l Mesophyll cells l Structure: –palisade mesophyll, column-shaped, contains many chloroplasts. l Function: –Carry out photosynthesis efficiently l Structure: –Spongy mesophyll, irregular in shape, contains fewer chloroplast l Function: –provide large space to allow efficient gaseous exchange

Some parenchyma cells: l Endodermis l Structure: –layer of cells surrounding the vascular tissue l Pericycle l Structure: –One to several cell thick, lied between the central cell and endodermis l Function: –produce lateral root

B. Collencyma l Green, N.P.O., Biological Science. Vol.1(2ed.). Cambridge University Press 1991: p.221 fig.8.a, b

Collenchyma

B. Collencyma l Distribution l outer region of cortex in young stems l Shape l shape similar to parenchyma but thickened cell wall at he corners of cell l elongated

B. Collencyma l Function l Give mechanical strength –especially in young plant & herbaceous plant –it is distributed towards the periphery of the region

C. Sclerenchyma (fibre) l Green, N.P.O., Biological Science. Vol. 1 (2ed.). Cambridge University Press 1991: p. 222fig.8.5a,b

Sclerenchyma

C. Sclerenchyma (fibre) l Distribution n found in pericycle of dicotyledonous stems & vascular bundle!

C. Sclerenchyma (fibre) l Structure l heavily thickened with deposits of lignin (a complex substances with great strength in the cell wall), even thickening of cell wall. l (stained red under microscope)

C. Sclerenchyma l Metabolism l ~mature sclerenchyma cells are dead l Function l ~provide main mechanical support

C. Sclerenchyma (fibre) l Shape l Fibres –Elongated sclerenchyma cells –Roughly spherical –Compound plant tissues

Compound plant tissue l Plant tissues consists of more than one types of cell l e.g. Xylem l consist of –parenchyma cells –fibres(elongated sclerenchyma cells) –vessels –tracheids

xylem: Tracheids l Structure l single cell l elongated & lignified l with tapering end l empty lumen with no cell content

Tracheids l Metabolism l ~they are dead with empty lumens when mature l Function l ~provide mechanical strength l ~give support to plant l ~conduct water and mineral salts

Xylem: Vessels lSlStructure ltltubular cells lplperforated at both ends lflformed by joining end to end lslshorter & wider than tracheids ltlthick lignified cell wall

Vessels l Metabolism l they are dead when mature l Function l Conduct water & minerals l because they are long & empty l xylem is distributed in centre of the root --> to withstand uprooting force l xylem is distributed in the peripheral region of dicotyledonous stem--> to withstand bending force l Support

Xylem: parenchyma l Structure l thin cellulose cell walls & living contents

Xylem: fibres l Structure l ~similar to sclerenchyma fibres

Compound plant tissue: phloem l Consist of: l ~parenchyma l ~fibres l ~sieve tube element l ~companion cells

Phloem l Function l ~translocation of food l ~no mechanical function

Phloem l a. Sieve tube cells l Structure l no nucleus(degenerated when mature) l cytoplasm are restricted to edges of the cell l long sieve tube are formed by fusion of sieve tube element & partial break down of the cross walls between them to form sieve plate

Phloem l b. Companion cells l Structure l closely associated with the sieve element l Functions n to give energy to sieve tube cells

THE END