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I want to ask a question about xylem in the centre of the root.

I am reading a book about transport in plants, and it reads this regarding the root structure:

Roots are subjected to vertical stresses - they have to be able to resist being pulled out of the soil.
A central core of strong xylem tissue gives ideal resistance to being uprooted and at the same time gives economy of space.

I'm confused about this passage. How does the centre of the root being xylem tissue exactly prevent the resistance to pulling forces upwards out of the soil?

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  • $\begingroup$ (+1) because this doesn't make any sense to me - I can't see why having xylem further to the centre of the root would withstand vertical stretching forces $\endgroup$ – Jam Apr 8 '18 at 19:06
  • $\begingroup$ Roots also must resist compression, as root tips push out against soil particles. $\endgroup$ – Jim Young Apr 8 '18 at 20:42
  • $\begingroup$ Further, I think your diagram of root structure only applies to the young growth near the root tip, just as near the apical meristems above ground are xylem-phloem bundles separate by cambium. Between, the core of xylem (wood) covered by cambium, covered by phloem, covered by various epidermal tissues prevails in woody plants $\endgroup$ – Jim Young Apr 8 '18 at 20:45
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The is something called the Pressure Stiffening Effect (turn on a coiled garden hose and watch it try and uncoil for instance), pressure in the fluid filled xylem stiffens the root, making it harder to pull out, Roots are rarely completely vertical or straight to pulling them from the soil generally involves them flexing, which the stiffening pressure resists. its the same reason an Inverted Y of steel can anchor in soil but an inverted Y of rope will not, the steel resists bending thus you have to displace all the soil above it to remove it.

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