BOTANY Higher Secondary Second Year - Textbooks Online

metaxylem. Xylem is made up of four kinds of cells - tracheids, vessels
or tracheae, xylem fibres and xylem parenchyma.
Tracheids
Tracheids are elongated with blunt ends. Its lumen is broader than
that of fibres. Their secondary wall is lignified. In cross section, the
tracheids appear polygonal and thick walled. The pits are simple or
bordered. There are different types of cell wall thickening due to deposition
of secondary wall substances. They are annular (ring like), spiral (spring
like), scalariform (ladder like), reticulate (net like) and pitted (uniformly
thick except at pits). Tracheids are imperforate cells with bordered pits on
their end walls. They are arranged one above the other. Tracheids are
chief water conducting elements in gymnosperms and pteridophytes.
Here, the conduction of water and mineral salts takes place through the
bordered pits. They also offer mechanical support to the plants.
Vessels or Tracheae
Vessels are perforated at the end walls. Its lumen is wider than that
of tracheids. The perforated plates at the end wall separate the vessels.
They occur parallel to the long axis of the plant body. Due to dissolution
of entire end wall, a single pore is formed at the perforation plate.
It is called simple perforation plate eg. Mangifera. If the perforation
plate has many pores, then it is called multiple perforation plate.
eg. Liriodendron.
The secondary wall thickenings of vessels are annular, spiral, scalariform,
reticulate, or pitted as in tracheids. Vessels are chief water
Annular Spiral Scalariform Reticulate Pitted
Fig. 2.5. Types of secondary wall thickenings in tracheids
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