Xen Fungus

The Rock Tree is a kind of plant found on the border world, Xen.

Appearance
Rock Trees are massive plants similar to Earth's own trees in terms of basic design, but lacking leaves or branches. They can reach massive heights of over Five-Hundred feet, and seem to be coated in a grayish, rock-like pattern of scales.

Variations
Sometimes, Rock Trees appear in slightly different ways. Some are slightly shorter and stouter, whereas one noteworthy subspecies deviates from the standard appearance by possesing a somewhat whitish coloration, and a rib-like design running down the "trunk", rather than the usual plated scale design.

On noteable Rock Tree in the chapter Interloper, seems to have been modified by the Xenians for their own use (see below).

Use By the Xen Armies
One Rock Tree in Interloper, when accessed from underground, seems to be built as a sort of makeshift elevator for use by the Xenians. When Gordon Freeman reaches the underground base of this tree, he can use his crowbar(or any other weapon) to shatter the webbing holding it into place, causing it to sink deep into the ground. This in turn allows Freeman to get on the top and hitch a ride as it rises again moments later.

Other Rock Trees have been sighted inside the Xen base, clearly cut down for lumber.