After looking at Tennis's picture of catch roots, I agree that the roots would be more of a deterrent then a landing place for small insect eating birds. They look more like the spikes used to fortify positions in war before barbwire and razor wire. Having thought about catch roots more, I don't think they have anything really to do with ants, and at least not protecting them. Most insect eating birds that look for insects on trunks and branches have no problem clinging sideways or upside down in search for food.
The search for moisture doesn't seem to be a good explanation for the straight upward growth as the moisture content a few inches up from the base of the orchid would not be that dramatically different then a few inches down. Besides the roots could just grow down or all around and collect just as much moisture from the air. In foggy environments the roots would become wet just about anywhere they were located. While I wouldn't discount it, moisture catching does not seem to explain the upward root growth.
When Nicholas wrote (in response to Paul): "O.K., can you expand on your hypothesis that upward growing roots in a plant that grow on ant nests are actually seeking a nutrient source external to the ant nest? What data do you base that idea on, and what nutrients might be involved?" It sparked an idea.
Maybe this upward root growth is simply phototropism. Grammatophyllums (and I imagine other species that develop catch roots) can stand full sun. As orchid roots have chlorophyll and can photosynthesis, maybe, during root growth if the light is of enough intensity it causes the auxin hormone in the root to pull toward the light and away from gravity.
This brings up a question which I may have already answered above (light), but I don't know for sure. What is the trigger that causes auxin to know if it is in a root to grow downward with the pull of gravity? Is is lack of light or the presence of some root cells? Light causes new growth to grow upward, but does less light with gravitational pull cause roots to grow downward? With epiphytes root growth maybe dictated by contact with a surface or otherwise light?
Mark Sullivan
Mark Sullivan
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