Transcript 6- Epidermis

6- Epidermis
It occurs on the surface of plants
and it is a complex tissue
consisting of epidermal cells,
stomata and trichomes (hairs).
Function of epidermis:
1- Protect the plant from water loss
in transpiration.
2- It acts as boundary tissue
surrounding the plant.
3- Exchange of gases through the
stomata.
4- Storage of water and metabolic
products.
1- Epidermal cells
Shape: tabular in
side view,
isodiametric or
elongated in
surface view.
Cell wall:
straight, wavy
walls.
Cuticle: epidermal cells are covered by
an impermeable layer known as cuticle
which varies in thickness. The cuticle
may be smooth as in Stramonium or
striated as in Belladonna.
The cuticle is formed mainly of cutin
which is an aggregate of modified fatty
acids, partly combined with alcohols.
2- Stomata
These are openings in the epidermis of
stem and leaves. They occur in both
surfaces or only on lower surface. In
water plants they occur in upper
surface.
Types of stomata:
The types are indicated either with
regard to the characters of the guard
cells or with regard to the epidermal
cells surrounding the stomata.
Types according to the characters and
arrangement of surrounding cells (subsidiary
cells)
1- Ranunculaceous or
Anomocytic stomata
(irregular celled): The
surrounding cells
(subsidiary cells) are of
varying number have no
special arrangement
and generally do not
differ from other
epidermal cells e.g.
Digitalis.
2- Rubiaceous or
Paracytic (parallel
celled): the stoma is
surrounded by two
or more subsidiary
cells, two of which
have their long axes
parallel to the
osteole e.g. Senna.
3-Caryophyllaceous or
Diacytic (cross
celled): the stoma is
surrounded by
subsidiary cells
having their long
axes perpendicular to
the osteole e.g.
Mentha.
4- Cruciferous or
Anisocytic (unequal
celled): the stoma is
surrounded by
usually three or
more subsidiary
cells one of which is
distinctly smaller
than the others e.g.
Belladonna.
3- Trichomes (Hairs)
Epidermal cells are sometimes extended
outwards forming projections of variable
shape and size. If it is short and conical it is
called papillae and the epidermis is
described as papillosed e.g. Coca, but if the
projections are long and well protruding
they form trichomes or hairs.
The part of trichome embedded in
the epidermis is called foot while
the free part is the body. Hairs are
classified into glandular and nonglandular or covering hairs.
Non-glandular hairs
Multicellular
Unicellular
Unbranched
1- Cottony
2- Cystolith
3- e.g. Senna
Branched
Unbranched
Branched
11- Uniseriate
22- Biseriate
33- Pluriseriate
4-
Simple
Stellate
Peltate
Candelabra
A- Covering trichomes or nonglandular hairs
I- Unicellular:
formed of one
cell
a- Unbranched or
simple: e.g.
cottony hair,
cystolitic hair
b- Branched: e.g.
Lavender.
II- Multicellular: that
is formed of more
than one cell
a- Unbranched or
simple:
1- Uniseriate:
formed of one row
as in Belladonna,
stramonium .
2- Biseriate: formed
of two rows, each
row of one (twin
cell) as in Arnica
flower, or more
cells as in
Calendula.
3- Pluriseriate: formed
of many rows of cells
(Shaggy hair) as in
Cumin.
b- Branched:
1- Simple branched:
with uniseriate body
ending in two
branches e.g.
Tobacco.
2- Stellate:
radiating
unicellular
hairs e.g.
Karkadeh.
3- Peltate: with very
short axis
surmounted by a
plate-like
structure of
closely joined
cells e.g. Olea.
4- Candelabra:
having uniseriate
axis from which
arise numerous
unicellular
branches of hairs
e.g. Verbascum.
B- Glandular hairs
I- Unicelluar: not of common occurance,
present in Piper.
II- Multicellulr: the most common type. They
consist of several cells and are classified
into:
A- Unicellular stalk:
and;
*Unicellular head
e.g. Mentha.
*Bicellular head e.g.
Digitalis.
*Multicellular head
e.g. Labiaceous hair
in Mentha and
clavate hair in
Belladonna.
B- Multicellular
stalk:
a- Uniseriate stalk
e.g. Belladonna.
b- Biseriate stalk
and biseriate
head or
compositae
type e.g.
Chamomile.
c- Pluriseriate
stalk and a
head with
numerous
mostly eight
radiating cells
e.g. Cannabis.
d- Branched
stalk: the
branch ending
in head as in
Hyoscyamus
muticus.