Xylem in the centre of the root
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.