After spending several hours stalking and photographing butterflies on Flagstaff Mountain, I returned home and spent about 10 minutes removing burs from my jeans and wool socks.
I was annoyed, for if I only knew what plant bore those burs, I could give them a wide berth. A year later, when processing my photo of a western tailed-blue (Cupido amyntula) on a cluster of flowers, I found the answer in the photograph. In the foreground were the flowers of houndstongue, Cynoglossum officinale. In the background were the maturing burs or seeds that are so annoying.
Houndstongue takes its common name from the similarity of its leaves to the tongue of a panting dog — the distal or outer part of each leaf hangs.
Houndstongue is native to the UK, northern Europe and northern Asia. It was thought to have been introduced to Canada in a contaminated shipment of seed before 1859. The contaminated shipment was never noticed, but specimens of houndstongue were collected in 1859 and deposited in an herbarium in Ontario. It was first reported in the U.S. in Oregon in 1893 and Montana in 1900. Today, they are in most of the 48 states and are listed as noxious weeds in seven western states, including Colorado.
A fertilized flower produces a fruit that is composed of four nutlets, thick-walled shells covered with barbed prickles and each containing one seed. (For simplicity, seeds, burs and nutlets will be called seeds). The seeds in the accompanying photo have a dense coat of straight green prickles, but as seeds dry, the prickles become barbed hooks, enhancing their ability to cling to animal hair.
The seeds, which hitchhike on sheep, cows, horses, dogs and hikers, are thoroughly annoying. Sheep wool can be so thoroughly packed with nutlets that the fleece loses value. In the United Kingdom, people who take their retrievers to walk or to retrieve ducks may spend an hour removing seeds from the dog’s pelt and from wool hunting jackets. In the western U.S., the seeds slow the marketing of cows, for they must be removed before they can be sold. The seeds are annoying, but its plant defenses cause even greater problems.
Like most species of plants, houndstongue has defenses that have evolved to discourage herbivores. Although it has no thorns, it has chemical defenses that are ingested by herbivores that eat its flowers, seeds, leaves and stems. It synthesizes four different pyrrolizidine alkaloids (PAs). The liver converts the PAs to pyrrolic metabolites that cause tumors, damage nuclear DNA and halt protein synthesis in liver cells.
So, PAs poison the liver and turn off the cell replication that may have replaced damaged liver cells. PAs have killed humans, cows, horses, farmed deer, pigs sheep, goats, poultry and quail. Horses are most susceptible, and sheep and goats are much less susceptible, with lethal doses 20 times higher than most animals.
Most species of wildlife are not killed, but they learn not to graze on houndstongue. Because wildlife learn to avoid houndstongue, it has a competitive edge over the healthy sources of food eaten by both wildlife and cattle. In places, this competitive edge allows houndstongue to become the dominant species, which greatly degrades the value of pastures for grazing.
PAs are synthesized by about 5% of all flowering plants and occur in 20 plant families. More than 600 forms of PAs have been described from the approximately 6,000 species that have been examined. None of these compounds are necessary for plant development, growth or reproduction; their functions are limited to plant defense against herbivores. The efficacy of these compounds can be seen in the speed of their invasion of Canada and the 48 contiguous states, despite efforts to eradicate or control them.
While clinging seeds and PAs give houndstongue a bad reputation, I must say that they have pretty flowers. Five petals fuse to form funnel shaped flowers that appear from leaf axils and the tips of branches. The flowers are not one hue, but each flower has a combination the hues of red, blue and purple, so the flowers on one plant exhibit a range of colors. I will continue to enjoy the flowers, but I will approach only tentatively, avoiding seeds, and I certainly will not try the leaves in a salad.
Jeff Mitton is an emeritus professor of the University of Colorado Boulder Department of Ecology and Evolutionary Biology.
