You're walking through a Borneo rainforest, keeping your eyes focused on shrubs and tree trunks that might be good opportunities for spider hunting, and suddenly your forward progress is halted. Three possible explanations: (1) You're in one of those nightmares where you keep trying to get somewhere but mysteriously can't move. (2) You've become entangled in vines across your path. (3) A rattan palm has you in its spiny clutches. No nightmares yet for me on this expedition, but the other two occur several times each day.
Vines (or "lianas") crisscross a tropical forest like a tangled spider web, at all heights from the ground to the forest canopy. Indeed, there are so many lianas among the trees, some as thick as a small tree's trunk, that when one tree falls from a storm or decay, it may pull down surrounding trees by their connecting lianas. For me, raised in the temperate zone, the idea of so many vines is unexpected. I expect plants to grow vertically, not diagonally or horizontally. I don't pay enough attention for lianas as I walk, and often I am suddenly held as in this photo of my entangled leg. I rarely trip, but I am sometimes befuddled as to how to untie the knot I am in.
Vines are stubborn foes, but not nearly so cruel as the terror of the Bornean jungle, rattan palms. The central rib of each multi-part leaf extends beyond the leaf as a long filament reaching into space. These filaments can cross a meter or more, linking to another plant like a liana's tendrils. It would be just another vine entangling me -- except that along the filament's length are many small backward-pointing spines. It's as if the forest were criss-crossed by meters-long hacksaw blades. When one catches my clothes, I feel a sharp tug and I'm stuck.
Sometimes, I can twirl around and I'll get unstuck, but other times I have to try to grab the spiny tendril (gingerly) and pull it away. Of course, sometimes it's not my clothes but my skin that the spines dig into as I charge through the forest. Luckily, most of the blood lost has been from my hands, not my neck. I'd mentioned in a previous post that Edy voted the leeches as her biggest annoyance; for me, it's the rattan palms.
Why do rattan palms have these tendrils that inflict suffering, apparently gratuitously, meters from the plant's body? They don't seem to make sense as protective devices against animals. Instead, the spines serve to keep hold on other plants that the palm uses for support. But, why might the palm want to borrow support from other plants?
Look at a forest, and a question might occur to you: why do the trees invest so much into those tall trunks that don't photosynthesize and don't pull in water and minerals? If only the trees could all agree to be short, they could divert the resources that make the trunk instead into flowers and fruit. But, of course, they don't cooperate, but rather compete, and their competition for precious light compels them to build massive trunks to lift against gravity and outreach their neighbors.
Lianas that rise up through the forest along the trunks of trees are, in one sense, parasites. They avoid investing in a trunk, but can still reach high into the light by using the infrastructure built by the trees. I like to think of them as "gravity parasites". (Actually, this name isn't quite right, as they are parasites of the effort against gravity, but "gravity parasites" is more poetic.) Rattan palms play the same game, maintaining with their spines their hold on upward-growing plants. Plants play cruel games with each other and with animals. And you thought plants were so innocent.
Previously in this series:
Text and images © W. Maddison, under a Creative Commons Attribution 3.0 license (CC-BY)