Nature Notes
by Bob Thomas

Thigmotropism is a concept in which living organisms move in a certain direction in response to touch (thigmo refers to touch; tropism to turn).

I was first exposed to thigmotrophy in a general botany class (thanks to the late and great Dr. Larry Erbe at the University of Louisiana at Lafayette, then the University of Southwestern Louisiana) when learning about how tendrils coil around a vertical support in order to climb. I observe this phenomenon in my backyard each summer as I watch passion flower (Passiflora incarnata) vines climbing about, and we will cover this in a future Nature Note.

For the moment, let us take a look at the most obvious thigmotropic response seen in New Orleans. Have you noticed it when driving down The Avenue?

Next time you are on St. Charles Avenue, notice how the bases of many southern live oak trees (Quercus virginiana) seem to cover curbing at the edge of the street. They appear to have melted and oozed along and over the upper surface of the curb.

This is a thigmotropic response to the plant tissue touching the impenetrable surface of the concrete.

The first time I saw this response of a tree to a hard surface was in Arthur R. Marshall Loxahatchee National Wildlife Refuge in Palm Beach County, Florida. Along the refuge’s fabulous swamp boardwalk, a number of bald cypress trees (Taxodium distichum) have similar associations in which they seem to flow over the boards, including hand rails. The refuge had a nice interpretive sign naming the process. I took a couple of photos and stored the concept in the deep recesses of my brain.

One day, as I sat at a traffic light on St. Charles, I was surprised to see that our live oaks often have the same response.

Note as you look that many of the tree bases seem to end abruptly at the edge of the curb. This may be due to several phenomena. One is that maintenance folks may have used a chain saw or stump grinder to keep the tree from invading the surface of the street. A second is that crews often cut the tree base and roots down about a foot below the street surface when building new or replacing old curbs. A third way is that cars often sideswipe the trees and scar the bases by chipping off chunks of bark.

By the way, if you want to observe thigmotrophy on The Avenue, it is best to park and take a walk. Otherwise, you are endangering yourself and others around you – no gawking and driving!
 

Thigmotropic response from the base of                                      A thigmotropic response of a southern live
a bald cypress tree (Taxodium distichum)                                    oak tree (Quercus virginiana) to a curb on
to an adjacent boardwalk at the Loxahatchee                               St. Charles Avenue in New Orleans.
National Wildlife Refuge in Palm Beach                                         Photo by Bob Thomas.
County, Florida. The tree appears to flow
over the boards.
Photo by Bob Thomas.

This example of thigmotropism has been                                    This oak tree is making its way over the 
clipped by a car or two.                                                             curb onto The Avenue.
Photo by Bob Thomas.                                                             Photo by Bob Thomas.

This is an example of the impact passing
cars have on the bases of oak trees that
spread over the concrete toward the roadway.
The cars hit the base of the tree and produce
such gnarled edges.
Photo by Bob Thomas.