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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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