least specialized member of the system   compactly arranged   cells of various shapes   possess living protoplast   with cutin in cell walls as incrustation and

adcrustation

  Sometimes the entire epidermis consists of specialized cells   solid layer of sclereids  cells contain chloroplastids (Polypodiaceace)

  are apertures in the epidermis bounded by 2 guard cells

  may be functionally associated with subsidiary cells or accessory cells

 

  basipetal- monocots   mosaic- dicots   both types occur in

vascular cryptogams

Types according to function

  a. Glandular   stalk and head (produces secretions)

  b. non -glandular

Development

  outermost layer with cuticle   inner layers

  differentiated commonly as water-storage tissue lacking chlorophyll

  expand more than the outer (undergo more anticlinal divisions)

  Lithocysts keep pace with increasing depth of the epidermis by expansion and intrusion into the mesophyll

  cortex with chloroplastids   velamen is for absorption of water   composed of dead cells, the walls of which are

strengthened by band-like or reticulate thickening. When the air is dry, these cells are filled with air, but when the rain falls, they become filled with water. The exodermis situated at the inner edge of the velamen is interrupted with passage cells through which the absorbed water may be transferred.

  Special structures termed pneumatodes are present in the velamen. The function is to enable gas exchange during periods when the root is saturated with water. Some say that the function is not water absorption because exodermis and velamen are impermeable to water and certain solutes. The principal role is mechanical protection and prevention of excess loss of water from the cortex