MUCH ADO ABOUT THISTLES

Nodding Thistle flowering
head with pollinating skipper.

When you travel with a botanist, you never know where you will stop to examine wildflowers. Yesterday, as we returned from shopping in Winchester, Tennessee, my botanist friend, Vickie Sullivan, and I saw patches of beautiful purple thistles alongside the road, and noticing a hill on which the Musk Thistle (or Nodding Thistle) grew, we drove up the road leading to the patch to photograph the droopy purple flowers nodding on their long stems. As the Nodding Thistle loves limestone soils, Tennessee provides fertile ground for these musky smelling flowers. However, they’re regarded as spiky, nuisance flowers that crowd out other vegetation in pastures and also elicit a lot of roadside mowing.

I first encountered thistles during the 70’s while accompanying Vickie on a field trip near St. Martinville, Louisiana where I helped her shroud thistle flower heads with brown paper bags and tied tightly with a string to exclude pollinators so that she could determine if the thistles flowers could self-pollinate. I also returned with her to untie the bags and clip the heads after they had had time to set seeds. No seeds were produced so that she determined they were self-incompatible. But while we were in the field bagging the heads, she showed me that when she touched the top of a gigantic flower head, the spiky, purple flowers began to move. Of course, flowers aren’t supposed to move, and this was an eerie sight!

Vickie cut off several heads with her plant clippers and let them fall into a white plastic bag, then returned to her lab to determine if the whole flower or a part of the flower moved. When she examined a flower head under her dissecting microscope she saw that the slender, white filaments deep inside the flower shrank when she touched them with a dissecting needle. At the same time white pollen grains oozed out. At their bases, the filaments are attached inside the purple petal tube. As they contract the petal tube and, finally, the entire flower is pulled in the direction of the contracting filament. The flower moves in a circular path as filaments contract as they are touched by pollinators, and touch one another.

In her research, Vickie discovered that the famous botanist, James Small, had noticed this movement many times in other species of the sunflower family of plants while doing research as a graduate student in a Kew Gardens’ greenhouse in London, England during the early 1900’s. However, Small’s observations of this phenomenon weren’t re-examined or even mentioned again in the many hundreds of articles published on the sunflower family following his 1917 publication about the movement of the thistle.

Vickie and several colleagues at the University of Louisiana at Lafayette used chemical analyses, electron microscopy, and knowledge of the plants of the family to discover more about the movement in the complex flowers of the plant family to which thistle belongs. They discovered that thistle filaments require a ten-minute period of rest after contraction before they’ll again respond to touch. During that time, the filaments stretch out again to nearly their original length. When a pollinator comes along and touches the filaments while searching and probing for nectar inside the petal tube, the filaments contract, and the hairs of the style scrape the inside of the anther tube, dislodging pollen from the sacs that line the tube. The style emerges from the top end of the tube, its bristles covered with pollen ready to douse a pollinator.

This touch sensitive response is called thigmonasticity and operates like a nervous system in animals, with chemical messengers opening and closing channels through cell membranes for movement of water and chemical ions. Water is lost from cells of the filaments when they are touched and re-gained when they re-elongate, a process orchestrated by chemical communication. However, botanists still don’t know what the chemical messengers are.

After observing all of this, Vickie and two other colleagues wrote four scientific papers about this subject, published in the American Journal of Botany, Planta, and Protoplasma. The phenomenon of thigmonasticity is an intriguing subject, and it’s an amazing sight when a novice like me first sees thistle flowers responding to touch.

Pliny and other medieval figures thought that the milk of the thistle could restore hair to bald heads, so the plant was even used for medicinal purposes, but its greatest claim to fame involves the Scots who adopted the plant as their national flower. There’s an old legend touting that the reason the Scots love the wildflower so much is that in a battle with the Vikings the ancient Scots were saved because a field of spiky thistles drove the Vikings back, and the flower became an ongoing symbol of protection for the Scots. I think that Vickie should be admitted to the Ancient Order of Thistle in Scotland and England and acknowledged as one of the sixteen Knights and Ladies of that order!

Photograph by Victoria I. Sullivan