Dianella,
I'm kind of at a loss how to help you with your choices, Sprossachsen oder stachelige Stämme. Perhaps if I knew what Australian plant was being described I might be able to help.

In botanical English, a true thorn is a modified stem, often in the axil of a leaf with the leaf subtending the thorn. A spine is a modified leaf that is never subtended by another leaf. Both thorns and spines are defenses against browsing animals.

Your source says thorns "are replaced by pungent leaves or stem tips." Pungency indicates to me some sort of chemical defense against browsers, something strong-smelling that either repels the animals by its scent or by its bitter taste. I'm inclined to suspect that the source means "pungent leaves or [pungent] stem tips."

beißend? Now here's where my grammar goes to pot.
beißende Blätter oder beißende Stammspitze (Stielspitze?)

Hello Agalinis,
it is not the describing of a special plant, but of a region in Western Australia called Kwongan.

http://en.wikipedia.org/wiki/Kwongan

Dianella

Wow! What a great sounding place.