1. Xylem

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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

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A vascular bundle
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sclerenchyma cells (fibers)
pit
end walls of vessel elements
pit
vessel elements
complete perforations in end walls of vessel
elements
tracheids
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• 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
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• 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
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• Pit
• the region in the cell wall without any
  secondary thickening, through which water moves
  from cell to cell.

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

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Phloem
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• 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
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• 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.

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• 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.

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• 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

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• 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.

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(cork cells)
(cork cambium)
(parenchyma)
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• 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.

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• 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

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Resin ducts in a stem tissue