The seeds are enclosed in one-seeded fruit, called an achene. Unlike many other members of the family Asteraceae, whose seeds are airborne with a plume of silky hairs, a papus, resembling miniature parachutes, the achene of the Xanthium plants are enclosed in a small, double-chambered, bur about 3/4 inch long that is covered with stiff, hooked spines, burs that have allowed this plant to be carried all over the world by unsuspecting travelers. These plants reproduce only by means of their seed.
Xanthium plants are short-day plants, meaning they only flower when the nights are long. 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 North America, Xanthium typically bloom during the fall months when the days are shorter and the nights longer. They will not bloom during the long days of summer or near a street light.
The Xanthium genus is distributed throughout much of Europe and parts of North America. Some authorities consider that the genus contains about 15 species, while others say from 2 to 4.
The gebus belongs to the Asteraceae family.
Most taxomomists seem to agree on the taxonomy of the genus for the plants of North America, there are two species, one of them with two varieties, namely :
There is also some disagreement among botanists as to exactly how many varieties of Xanthium strumarium exist, and precisely where is their native (indigenous) habitat. There are several named varieties listed in botanical literature. Some authorities believe Xanthium strumarium is one cosmopolitan species with many highly variable populations around the world. Since it can colonize new areas quite easily (particularly disturbed areas), it is a good example of the founder effect.
The founder effect is genetic drift that occurs when a small number of individuals, representing a fraction of the gene pool, establish (found) a new colony and only certain alleles (genes) of the original population are passed on to the next generation. The founding colony does not have the genetic variability of the main population, and the frequency of certain traits may increase greatly by genetic drift compared with the much larger ancestral population.