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Secondary walls in tracheary elements


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Secondary walls in tracheary elements

Postby botanygirl » Mon Nov 10, 2008 7:50 pm

I hope somebody can help me out here.

I understand that secondary walls are laid in xylem tissue in one of the following ways: annular thickenings, helical thickenings, scariform thickenings, reticulate thickenings.
My questions are as follows:
1. Are these only present on primary xylem and there is a different kind of deposition pattern in secondary xylem? Or are they also found in secondary xylem? Do tracheids or vessel elements favour one of these types of thickenings?
2. When the secondary wall is laid down in areas other than where plasmodesmata are does it leave all of the plasmodesmata areas free? Or does the secondary wall sometimes favour only pits at ends of cells (I'm thinking tracheids especially here) and cover up the other plasmodesmata areas?

Sorry if it sounds a bit long-winded!
Many thanks
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