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 non-glandular or covering hairs.

Non-glandular hairs Multicellular Unicellular Unbranched Branched 1- Uniseriate 2- Biseriate 3- Pluriseriate 1- Simple 2- Stellate 3- Peltate 4- Candelabra 1- Cottony 2- Cystolith 3- e.g. Senna

A- Covering trichomes or non-glandular 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 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.