By Jake Landers
Published August 2011
Sensitive Briar and Fern Acacia are cousins that have similar looking leaves that behave very differently. The leaves of Sensitive Briar fold up when touched while those of Fern Acacia stay put. Few plants have movements as quick as Sensitive briar, and it’s done without nerves as in animals. Botanists understand that these movements in plants come about by changes in water pressure, but they can’t explain the reason why. Why one and not the other! Perhaps the leaf movements might surprise a grazing animal to make it move to another plant for a bite. It’s a quirk in some plants that someday may entice a young scientist to find a good explanation.
Both plants are legumes and are first choice for grazing by goats, sheep, and deer because of their high nutrient content. For this reason both have become scarce even in pastures where moderate to light stocking has been practiced. They persist in much of central Texas in parks, roadsides, and rocky sites where plants are somewhat protected from heavy grazing.
The flower clusters are similar in construction, but quite different in size and color. Fern Acacia has marble-sized white puffballs too numerous to count on the multiple sprouts in a typical colony. Sensitive Briar has a few big pink ones at the ends of sprawling runners. Seed pods are quite distinct also, with those on Fern Acacia small and flat, and those of Sensitive Briar bean-like and covered with soft spines. I am having a hard time finding seeds to get either plant reestablished in my part of the ranch.
The scientific name of Fern Acacia is Acacia angustissima var. hirta. In plain language this translates to “Thorny plant with the narrowest leaves”. Most of its other cousins are thorny, including Sensitive Briar, but Fern Acacia it is soft to the touch. Sensitive Briar is named Mimosa roemeriana, although long ago I learned it as Schrankia roemeriana. At least the name-changing experts kept half of the original which is in honor of Ferdinand Roemer, the German geologist who checked out the ruins of the old Spanish Presidio near Menard 150 years ago, and wrote a book about his adventures in Texas.
Whatever the reason for the sensitive leaf movements in one and not the other, they are fascinating to watch. Several years ago one of my colleagues who also enjoyed teaching kids about native plants had a fun technique when he found a Sensitive Briar. He told the kids it was a “Scream” plant. “If you screamed at it, it would fold its leaves, and you had to get close to it because it was hard of hearing.” Soon all the kids would be on hands and knees screaming and watching the movement, not realizing at first it was their breath, not the sound that caused the movement. Kids and a few grown-ups like to be tricked, occasionally, so that they can pull the same trick on someone else. But you need to be smart enough to tell the difference between the two plant cousins.