| 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  |