Complex Tissues 1. Xylem For translocation of water and minerals nutrients Consists mainly of vessels, tracheids, fibres, parenchyma cells, and xylem ray cells Occurs in association with, and usually internal to, the phloem

A vascular bundle

sclerenchyma cells (fibers) pit end walls of vessel elements pit vessel elements complete perforations in end walls of vessel elements tracheids

Vessels A continuous longitudinal tube composed of vessel elements. Vessel elements are joined end to end forming the long tube The vessel elements are perforated at both ends ( with either single/simple perforation or several perforations) Vessels elements are dead cells at maturity, with lignified secondary cell wall Usually larger in diameter but shorter than tracheids Salix vessel

Vessel elements scalariform perforation plate simple perforation plate Unsur-unsur vesel Vessel elements scalariform perforation plate simple perforation plate

Tracheids Similar to vessels, tracheids are dead cells at maturity, with thick lignified secondary cell wall Each tracheary element tapers at both ends and arranged end to end with other tracheids Without any perforations at the ends, instead there are pit pairs whenever two tracheids touch each other Smaller in diameter but greater in length than vessels The secondary cell walls show various types of thickening pit spiral pitted

Pit : the region in the cell wall without any secondary thickening, through which water moves from cell to cell.

2. Floem For translocation of food materials Composed of sieve tubes, companion cells, fibres, parenchyma cells and phloem ray cells

Phloem

Sieve tube Like vessels, they are composed of sieve tube elements that are arranged end to end Without any large perforation at the cell ends. Instead, the end walls are perforated by pores forming the sieve plate. The pores allow cytoplasmic strands movement between cells. Sieve tube elements are living cells, but do not possess a nucleus at maturity. sieve plate sieve tube element

Companion cell A specialised parenchyma cells, characterized by a dense cytoplasm and a conspicuous nucleus, and associated with a sieve tube element. The protoplast of the companion cell is connected to that of the sieve tube element by means of plasmodesmata. Companion cell helps regulate the flow of food materials in the sieve tube.

3. Epidermis The outermost cells found on all young organs of plants, the part that is exposed to the outside environment. The cells differentiate from the protoderm. Functions to protect the underlying tissues from excessive water loss, and from physical injury and pathogen attack.

Normally consisting of a single layer but sometimes several layers thick, e.g. epidermis of aerial roots of orchids and leaf epidermis of certain species to prevent from excessive water loss. Mostly secrete a fatty complex called cutin on the inner and outer wall, forming the protective layer called cuticle Wax can also be produced on the surface of the cuticle

In the epidermis of leaves are pores/openings called stomata In the epidermis of leaves are pores/openings called stomata. Each stomatal pore is surrounded by a pair of guard cells. Glands may be found in the epidermis for secretion of protective substances or other compounds. Some epidermal layer may produce hairs, needle-like structures or scale-like structures to help control water loss or to protect from attack by insects and other animals. guard cell

4. Periderm In woody plants, the periderm replaces the epidermis. Cork cells (phellem) are produced in stems and roots through the activity of the cork cambium (phellogen). The cork cells are rectangular in shape and are dead at maturity. When the cork cells are still alive, the cytoplasm secretes a fatty substance called suberin making the cork cells become water proof and help to protect tissues below the bark.

Periderm phellem phellogen phelloderm (cork cells) (cork cambium) (parenchyma)

Lenticel Small elliptical pores, consisting of loosely packed cells and without suberin, called lenticels, protrude from the periderm surface. Lenticels are analogous to stomata and use for gaseous exchange.

5. Secretory cells and tissues Cells that secrete either waste products or useful products such as hormones, nectar and perfumery substances. Formed from parenchyma cells and are found in specific parts of the plant body. Examples : secretory cell that produce nectar in flowers, oil in Citrus, latex in Euphorbiaceae and resin in Mangifera

Resin ducts in a stem tissue