1. The cell wall
Functions
• upport the plant against force of gravity
S
• rotection from desiccation
P
• aintains shape of the cell
M
• revents excessive uptake of water
P
• ommunication between cells
C

2. Classification of wall layers
 1. Primary wall
 2. Secondary wall

4. Middle lamella
 Composed of Ca pectate
 Commonly lignified in woody
tissues

5. Primary wall
 Contains cellulose, hemicelluloses and
some pectin, may become lignified
 Passes through a period of growth in
surface area and possibly increase in
thickness
 Associated with living protoplast

6. Secondary wall
 Consists mainly of cellulose and hemicelluloses,
and lignin
 Generally laid down after the 10 wall ceases to
increase in surface area
 Surface growth is not a characteristic of the 20
wall
 Supplementary wall whose principal function is
support
 Associated with dead cells or highly specialized
cells

7. Pits/primary pit fields
Primary pit fields
 Found in primary walls
 Primary wall is relatively thin but is
continuous across the pit field area
 Show concentrations of plasmodesmata

10. Pits/primary pit fields
Pits
 Found in cells with 20 walls
 20 wall layers are completely interrupted
at the pit
 2 types of pit
 a. Simple pit
 B. Bordered pit

12. simple pit
 consists of the pit
cavity and the pit
membrane
 may coalesce as
wall thickens
forming a
ramiform pit

14. bordered pit
 With pit chamber and pit aperture
 in gymnosperms, with torus and margo
 aspirated pit-pair – the displacement of pit
membranes

17. vestured pits
 found in some dicots
 outgrowths develop
on the secondary
walls of pits giving it
a sieve-like
appearance

18. Arrangement of bordered pits
 Scalariform pitting- pits elongated or
linear and form ladder-like series
 Opposite pitting- pits arranged in
horizontal rows; crowded pits appear
rectangular in face view
 Alternate pitting- pits in diagonal
rows; crowding gives the borders
hexagonal outlines in face view

20. Types of pit-pair
 simple pit-pair
 bordered pit-pair
 half-bordered pit-pair – a simple pit
complemented with a bordered pit
 blind pit- a pit without a complementary
structure /occurs opposite an intercellular
space
 unilaterally compound pitting- 2 or more pits
opposite one pit in the adjacent cell

22. Plasmodesmata
 cytoplasmic strands interconnecting the living protoplasts
of the plant body
 concerned with material transport and conduction of
stimuli
 arise during cell division because of the persistence of ER
tubules in the organizing cell plate; the desmotubule
appears solid through the plasmodesmata
 plasmodesmata multiply by splitting; during growth of
the wall in surface area the plasmodesmata are stretched
laterally and then split by interposition of wall substance

27. Chemical composition of
walls
 cellulose
 hemicelluloses
 pectin
 arbohydrate constituents
c
 mucilages
 Gums
 lignin (impregnation of the wall)

28. Chemical composition of
walls
 silica
mineral substances
 calcium carbonate
 tannins organic compounds
 resins

29. Chemical composition of
walls
 cutin
 suberin fatty compounds
 waxes
 water- found in microcapillaries or
associated with hydrophilic substances

30. Microscopic and submicroscopic
structure of the wall
 Structural elements
 Cellulose – amicroscopic component
 Elementary microfibril- contains 100
cellulose molecules in a transection;
 Micelle – crystalline aggregates of cellulose
separated longitudinally by amorphous
regions or regions of less perfect molecular
order

31. Microscopic and submicroscopic
structure of the wall
 Microfibril- contains 2000 cellulose
molecules in transection; the basic
structural unit of the cell wall
 Macrofibril – contains 500,000 cellulose
molecules in transection
 Secondary wall of a fiber- contains
2,000,000,000 cellulose molecules

36. Orientation of microfibrils
Primary wall- when first formed shows a
predominantly transverse orientation of
microfibrils but the orientation becomes more
disperse as the wall increases in surface area
during cell enlargement; primary wall shows
an increasing degree of parallelization of
microfibrils in the centripetal direction
 Secondary walls have parallel texture

38. PRIMARY WALL
• hin
T
• rientation of microfibrils is random
O

39. SECONDARY WALL
• arallel texture of microfibrils
P

40. Properties of the walls
 *Cellulose would have a major influence
upon the properties of the wall because of
their abundance
 1. Plasticity- property of becoming
permanently deformed when subjected to
changes in shape or size
 e.g. permanent extension in certain stages
of growth of cells in volume)

41. Properties of the walls
 2. Elasticity- property of recovery of the
original size and shape after deformation
 (illustrated by the reversible changes in
volume in response to changes in turgor
pressure)

42. Properties of the walls
 3. Tensile strength
 one of the remarkable features of the
cellulose
 lignin increases resistance of walls to
pressure and protects the cellulose
fibrils from becoming creased

43. Formation of walls
 Cell plate – is the first evident partition between
new protoplasts; it arises in the equatorial plane of
a fibrous spindle, the phragmoplast
 In highly vacuolated cell
 The nucleus comes to occupy the region
formerly occupied by the vacuole and is
surrounded by dense cytoplasm (cytoplasmic
plate- phragmosome)
 The phragmosome forms a living medium in
which the phragmoplast and cell plate develop

51. Growth of walls
 In thickness
 By apposition- successive deposition of wall
material, layer upon layer
 usually centripetal; centrifugal in pollen grains
 By intussusception- intercalation of new
particles among those existing in the wall

52. Structure of cell wall
• econdary wall
S
-synthesized after most cell growth has ceased
-microfibrils arranged in parallel within wall
-composed of:
 1- shallow helix of microfibrils
S
 2- thickest layer; steep helix of microfibrils
S
 3- shallow helix of microfibrils
S

54. Growth of walls
 In surface area
 By mosaic growth – synthesis of wall material
occurs in localized regions scattered over the wall,
in which the cytoplasm pushes apart the existing
microfibrils and weaves in new ones.
 By multinet growth- apposition of successive layers
of microfibrils, with the earlier becoming modified
in microfibril orientation by wall extension during
cell enlargement

57. Formation of intercellular spaces
 Schizogeny- intercellular spaces result from
separation of cell walls from each other
 intercellular substance partly dissolved but does
not disappear; lines the intercellular space
 In water plants the big aerenchyma develops
similarly but divides perpendicularly to the
circumference of the air space; resin ducts and
secretory ducts of Compositae

61. Formation of intercellular spaces
 Lysigeny –intercellular space arises by
dissolution of the cells themselves so
that the breakdown products are
released in the resulting cavity
 e.g. secretory cavities of Eucalyptus,
Citrus

63. Formation of intercellular spaces
 rhexigenous- result from tearing or
breaking of cells