Weird and wonderful members of the plant kingdom - plant hitchhiker

Probably everyone has at some point come home with cockleburrs in their socks or clothing. Cocklebur plants (Xanthium strumarium) produce hundreds of little football-shaped burrs, about one inch long and covered with stiff, hooked spines. Each cockleburr fruit contains two seeds that may remain viable for many years - up to fifty, in fact. The prickly burrs hook into your clothing and become tightly attached, like Velcro. Often the vicious burrs form tangled clots in the fur of animals, and must be cut away. There are many varieties of smaller burr, and a few larger ones - the Chestnut, for example, is technically a burr, but it is the Cockleburr that is the hardest to remove from your clothing, and this 'stickability' has turned out to be a huge evolutionary advantage.

There is some disagreement among botanists as to exactly how many varieties of common cockleburr exist, and precisely where their native, or original, habitat might have been, because its ingenious adaptation allows it to hitchhike on both people and animals and it has become almost universally widespread.

It is a classic example of a short-day plant (i.e. it only flowers when the nights are long). At least one leaf of a cockleburr plant needs fifteen hours of darkness to undergo various complex biochemical changes, this means they can bloom in the tropics where the days are short and the nights are long, thus greatly increasing their range and potential for seed production. In Europe cockleburrs tend to bloom during the autumn months when the days are shorter and the nights longer. They will not bloom during the long days of summer or near a street lamp.

Burr photograph by Dr Swan, used under a creative commons attribution licence