Plugging In Wall 1  - There at least seven ways that the vertical transport system can be plugged after injury and infection:  1) gums;  2) tyloses; 3) pit closure; 4) air pockets - embolism; 5) microorganisms and their products; 6) granular - like materials, especially at end walls, or where there are old end walls; 7) varnish - like membranes materials, especially where there were old end walls.  There are variations in these plugs.  Some tyloses have a definite boundary while others do not.  Many types of granular or coarse materials can be found in vessels.  Vessels do end and then connect with others.  The place where 2 vessels meet in the vertical direction is a prime position for plugs.  Some trees, such as birch, have scalariform or ladder - like end walls at the ends of vessels.  Materials collect at these places and plug transport of liquids.  A tree must maintain a vertical transport system.  Yet after wounding, the transport system near the wound must be plugged in some way, or pathogens would have easy access deep into the trunk.  How fast the tree can plug the transport cells is very important.  Living cells that surround the dead transport cells play an important role in plugging.  The cells that have pit connections with the transport cells and with the radial parenchyma are called contact parenchyma.  Contact parenchyma are usually axial parenchyma.

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For an explanation of the 4 walls please see "CODIT."

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