intercellular spaces. Although epidermis is a continuous outer layer, it is interrupted by stomata in many plants. Leaf Epidermis The leaf is generally dorsiventral . It has upper and lower epidermis.
The epidermis is usually made up of a single layer of cells that are closely packed. Generally the cuticle on the upper epidermis is thicker than that of lower epidermis. The minute openings found on the epidermis are called stomata (singular: stoma) . A stoma is surrounded by a pair of specialised epidermal cells called guard cells.
In most dicots and monocots the guard cells are bean-shaped. While in grasses and sedges, the guard cells are dumbbell- shaped. The guard cells contain chloroplasts, whereas the other epidermal cells normally do not have them. Subsidiary cell Figure .
: (a) Stoma with bean-shaped guard cells. (b) Stoma with dumb-bell shaped guard cells Check Your Grasp! In which group of plants the guard cells are dumb-bell shaped? Grasses and sedges Subsidiary Cells Stomata are minute pores surrounded by two guard cells.
The stomata occur mainly in the epidermis of leaves. In some plants addition to guard cells, specialised epidermal cells are present which are distinct from other epidermal cells. They are called Subsidiary cells . Based on the number and arrangement of subsidiary cells around the guard cells, the various types of stomata are recognised.
The guard cells and subsidiary cells help in opening and closing of stomata during gaseous exchange and transpiration. Epidermal Outgrowths There are many types of epidermal outgrowths in stems. The unicellular or multicellular appendages that originate from the epidermal cells are called trichomes . Trichomes may be branched or unbranched and are one or more one celled thick.
They assume many shapes and sizes. They may also be glandular (Example: Rose, Ocimum ) or non-glandular. The trichomes on the leaves of insectivorous plants secrete mucopolysaccharides that trap insects. Piliferous layer of the root has two types of epidermal cells, long cells and short cells.
The short cells are called trichoblasts . Trichoblasts are elongate into root hairs. Epidermal hairs can also