The Acacia’s Army

10 Jul

Growing up in Texas, I was afraid of mesquite trees. The scraggly trees dotted the rolling plains. Their branches were twisting and delicate, tipped at the ends with fronds of elegant leaves. Their heartwood was rose-red and they smelled like barbeque. But I didn’t walk underneath them, no sir, because I was still small and I was afraid of the thorns. I had been told, as a little child, of a man who stepped on a fallen branch and run a three inch thorn through his foot.

Now, of course, I miss them. Washington has many fine trees, but no mesquite.

But I digress, because today isn’t about mesquite trees, but a very similar plant: the acacia. Visually, it’s quite a match. Both are thin, twisting trees mounted with thorns. But acacia thorns, at least do not frighten me. Rather, they fascinate me. Thorns, you see, are only the first part of the acacia’s defense.

Certain species of acacia trees have become symbiotically linked with their own private ant armies. The ants live inside large, hollowed out thorn-like stipules on the acacia trees, feeding of a rich sugary sap. At rest, one might mistake the ants for an odd kind of parasite. But when the tree is attacked by herbivores, be they insect or mammal, the ants rush out to sting and bite the attackers away. Some species of ants even go so far as to murder in the tree’s name. They’ll find nearby competing plants and systematically strip them of their leaves, killing them.

This induced, indirect defense may seem too brilliant for a nonsentient, immobile tree to evolve, but plants, in fact, boast some of the most unique and interesting defenses you can find in biology. The acacia’s insect mercenaries are just a single example.

Mesquite trees, luckily, do not harbor any such defenses. I never had to worry about being attacked by their insect armies. I mean, we did live in Texas after all, so fire ants provided enough trouble as it was. Still, those mesquite thorns were intimidating and I think it took until I was eight before I went anywhere near one without my boots.

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Posted by on July 10, 2012 in Natural History


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