The bull, yellow, or purple thistle (Cirsium horridulum Michaux) is a member of the Asteraceae, the composite or daisy family. These large thistles are native from Maine down the eastern seaboard to Florida and across to Texas and Oklahoma. The thistle has two color forms, hence the common designation of yellow or purple. (Gray's Manual of Botany, M.L. Fernald, 8th Edition, Dioscorides Press, 1989)
The majority of plants growing near coastal areas tend to have yellow flowerheads, while more inland plants tend to have purple flowerheads. The pasture where the photographs were taken had over a hundred thistles, only two were the pale yellow forms.
The species epithet, horridulum, means "somewhat bristly". Obviously, André Michaux had a great flair for understatement. The botanical name was published in his book, Flora Boreali-Americana, in 1803 several months after his death. The spines on the leaves are rigid and sharp; it is not a plant that one bungles into a second time. Cows recognize this attribute and plants in the vicinity of a thistle go ungrazed.
The inflorescence or flowerhead is composed of hundreds of individual tubular flowers. It is a banquet for bees, beetles, and butterflies that come for the nectar. The tiny anthers do something surprising. They are capable of thigmonastic motion--when touched they respond by curving toward the disturbance. Any insect crawling between the flowers is contacted by dozens of anthers and dusted with pollen. ("Thistle", Plant Physiology Lab, University of Louisiana-Lafayette)
After tapping a dozen or so flowerheads, I witnessed the motion only in flowers that were shedding pollen and in sunlight. Shaded flowers in the early morning showed little motion; pollenless flowers had none. My 'experiment' with the flowerheads attracted the attention of a palomino mare. She followed me across the pasture and baring her teeth, gingerly nipped out the flowers and ate them!
