i have a very big doubt in botany.......please help me immediately?

[Mystery]

imagine a tracheid.one tracheid has many pits .so is one tracheid is made up of many cells or only one cell?not only tracheids also vessels.i are pits formed only in the secondary wall?
please clear my doubt

[The Watcher]

A tracheid is a single elongated cell with pointed ends and a secondary, cellulosic wall thickened with lignin inside the xylem of vascular plants and having it makes them different from non-vascular plants.

Tracheids are one of two types tracheary elements, vessel elements are the other.

In living cells, simple pits are holes in the secondary wall and it is to let tracheids "communicate" plant signals via plasmodesmata. Pits between tracheids have no plasmodesmata (the cell are dead and have no cytoplasm), but they do contain a pressure valve, a ball of lignified tissue (the torus). If the pressure in an element becomes too high, it will force the torus trapped between the two rings of thickened tissue into the hole, blocking it.
Pits between tracheids and parenchyma cells in the medullary rays are termed window pits, and allow water to enter the living storage parenchyma.

[maverick]

1) One tracheid is NOT made up of many cells . One tracheid is ONE cell only. IT is elongated with tapering and perforated ends .

Two adjacent tracheids are juxtaposed in such a way that they form a continuous system.

It is like a railway line !! A railway line may have indefinite length ; but a single rail segment has very very definite length.
These segments of iron are linked together to construct a railway!!

2) Vessels = They are also single cells with end walls dissolved to form a continuous ' water pipe' . Thus , Vessels are the ' Improved version ' of tracheids . They are also broader than the tracheids !

3 ) The pits , bordered pits and other 'designs' that one comes across on these xylem elements are depositions of lignin and are formed later on on the original cellulose walls to strengthen them . This is reinforcement to make them to withstand the force of water moving inside . But at the same time some areas are required to be unthickened to allow the movement of water from these elements and the adjacent tissue .
So they are formed on the secondary wall !

Click on the links below to learn more =

http://www.palaeos.com/Plants/Lists/Glossary/Images/Tracheid.gif
http://www.palaeos.com/Plants/Lists/Glossary/GlossaryT.html

http://www.ucmp.berkeley.edu/IB181/VPL/Ana/AnaD/AnaD3n.gif

http://waynesword.palomar.edu/images/trachei1.gif

http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/Xylem/Xylem-1.htm

From = A Botanist