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From what I behold, spiral and annular thickening of xylem and trachied cell walls leaves a lot of not thickened regions of cell wall. Lignin is the material which prevents water from escaping these water conduits. If lignin is absent from certain regions, does not that mean that water flows out of the xylem vessels and trachieds through their cell walls? If so, is not this disadvantageous to a plant? Why can't a plant simply thicken the cell walls with uniform lignin deposition, only sparing pits to allow lateral flow into the required tissue? I would appreciate your kind response.

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  • $\begingroup$ Lignin adds mechanical strength. Leakage from one xylem lumen to another can be seen to be advantageous as it lets water tension to equalize radially - else a plant would tend toward one-leaf-to-one-root (I exaggerate since there are also pits to distribute water). Trachieds are dead cells. Additions to their walls come from nearby parenchyma. Wouldn't it instead be curiously remarkable if the lignin depostion was uniform?. $\endgroup$ – Jim Young Feb 23 '17 at 19:28
  • $\begingroup$ Exactly, if pits have the function of lateral flow then why is not lignin deposition uniform? That is what I am curious about. $\endgroup$ – Taimur Feb 25 '17 at 10:43
  • $\begingroup$ I think pits tend to be located at the ends of the trachaeids. So lateral flow is minor compared to axially. Water can flow laterally thru the unlignified cellulose walls by osmosis. Anyway, you pose an interesting question I cannot answer, though is does boarder on asking why there is air, IMHO. :) $\endgroup$ – Jim Young Feb 25 '17 at 17:04
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Non uniform lignification of xylem vessels is beneficial to a plant in that it enables growth. It is at the unlignified parts of plants that growth occurs since fully lignified tissues are dead and so can not grow. Spiral and annular thickening allows for vessels to elongate and thus allows tissues to expand without breaking the vessel walls.

Water cannot just go on flowing from plant cells because they are not lignified, plant cells have cellulose cell walls and cell membranes which prevent water loss. In addition, water in the xylem is not under pressure so I do not think water would be able to flow out of the xylem vessels except in cases where water is forced to move to adjacent phloem tissues by osmosis during phloem translocation.

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    $\begingroup$ Welcome to Biology.SE! Your answer sounds reasonable, however, it is likely to receive a more favorable response if you include supporting references (primary literature is best, but a textbook available online would seem sufficient for this question). You also should be able to easily remove the pictures by editing your post ...——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit your answer accordingly. Thanks! 😊 $\endgroup$ – tyersome Nov 2 '19 at 4:00

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