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question about position of proto and meta xylem in plant

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question about position of proto and meta xylem in plant

Postby karanmaster0007 » Thu Oct 22, 2009 1:32 pm

why protoxylem occur outside in roots and inside in stems and vice versa with metaxylem
.


is there any relation of them with the functions ,
then pls tell there separate functions
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Postby SauronTheConqueror » Thu Jan 21, 2010 3:33 pm

My guess is that as the name suggests, protoxylem is formed first and hence has more lignin deposition over time than the Metaxylem that forms later during a plant's growth. It is due to this that protoxylem has a narrower lumen due to excess lignification.
As for their relative positions, it depends on the pattern of growth- whether it is centrifugal( as in stems where the growth starts from the centre and spreads outwards resulting in the older protoxylem towards the centre and the newly formed metaxylem towards the periphery- Endarch Condition) or centripetal( as in roots, where the development of xylem starts from the periphery of the Stele with the formation of the protoxylem and gradually proceeds towards the pith or medulla with formation of larger, newer metaxylem- Exarch condition).

Regarding their functions. I think they are basically the same. As the protoxylem gets older, its lumen gets narrower until a stage where it becomes non-functional. That is why metaxylem is formed- to replace the ageing protoxylem. Hope my answer is not too lengthy or confusing but helpful. Cheers!
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