PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

 

COCKLEBUR

 

Common name:        Cocklebur

 

Scientific name:       Xanthium pensylvanicum

 

Explanation of scientific name:       

Xanthium            - from the Greek for yellow, in reference to the Cocklebur’s yellow-green leaves and yellow male flowers.

pensylvanicum  - of Pennsylvania.  In colonial America, it was common to name a plant after the colony or state in which it was first identified.  

The Cocklebur has a well-deserved reputation as a noxious weed.  Farmers and gardeners alike have been battling the aggressive plant since colonial times.  Besides its ability to out compete most garden and field crops, Cockleburs are quite toxic as young seedlings and have been known to poison livestock.

While most weeds are studied solely with the idea of learning how to kill them, the Cocklebur is an exception.  It is undoubtedly one of the most intensely studied weeds and has been instrumental in helping plant scientists understand diverse physiological topics such as flowering, seed germination, circadian rhythms, and plant hormones.  In addition, it kindled an idea for an invention that touches all of our lives on a daily basis.

The Cocklebur itself is a large coarse-appearing annual plant reaching 3 feet in height.  Its flowers are relatively inconspicuous, with the sexes separated in distinct inflorescences.  The male flowers are small yellow clusters that drop from the plant after shedding their pollen.  The female flowers are small, green, prickled capsules that eventually develop into inch-long brown fruits called “burs” by the end of the growing season. The burs are efficient vehicles for the dispersal of the plant’s seeds.  The recurved hooks on their surface cling to fur, feathers, and clothing.  It is a convenient way to get animals to do all the work of getting the offspring distributed to sites far from the parent plant.  An autumn walk through a field, meadow, or vacant lot will often result in one’s clothing or pet picking up at least a few burs.  Most people, upon discovering them, pull them off and discard them.  That is exactly what the plant wants to happen.

In 1941 a Swiss engineer, Georges de Mestral, noticed that his wool socks and jacket and his dog’s fur were covered with cockleburs after a walk in the woods.  Observing the burs under a microscope, he noted the hundreds of hooks and how easily they attached to fibers, especially if the fibers were looped.  By 1948 de Mestral had duplicated the hook and loop configuration in nylon and named it Velcro.  Since then it has been used for making hundreds of items from clothing to heart valves.  The basic patent for Velcro expired in 1978 and manufacturers throughout the world now make it.

The longevity of the Cocklebur’s seeds no doubt contributes to its status as a difficult weed to control.  In documented experiments, seeds buried 8 inches deep in soil for 16 years still showed a 15% germination rate after being recovered and sown on the surface.  This in part explains why cocklebur plants reappear in areas where they have been absent for decades.  Just how the seeds last so long has spurred investigations that have led to surprising conclusions.  Each bur contains two seeds that never leave the fruit until germination.  The lower seed germinates under normal conditions of moisture and temperature, but the upper seed will not.  Its seed coat prevents the uptake of oxygen, and oxygen is needed to break down a germination inhibitor that is found in the embryo.  Germination of the upper seed can only occur if the seed coat is punctured, removed, or allowed to decay over time.  If gardeners bury the bur during cultivation, the low oxygen concentration deep in the soil can hold the upper seed dormant for some time, only to have it germinate when cultivation brings it up to the surface again.

Probably no aspect of the Cocklebur is as fascinating as its response to day length.  While it has been known since the 1920’s that many plants bloom in response to the day length they experience, the Cocklebur holds the record for rapid response.  A single day of less that 15 ½ hours of light initiates floral development in this plant.  It is called a short-day (SD) plant because it is stimulated to bloom when the day length is less than its “critical light period” (for cockleburs:  15 ½ hours).  There are long day (LD) plant species as well as day-neutral plants (blooming under any photoperiod), but most LD and SD plants need many repeated cycles of specific photoperiods to initiate flowers.  Cockleburs, like many other photoperiod sensitive plants, can measure time quite precisely.  A Cocklebur grown under a 15 hour, 45 minute day length would not bloom and would remain vegetative indefinitely.

Because of its rapid response time, the Cocklebur has been used extensively to study photoperiodism in plants.  Its use as an experimental plant has led to the discovery that leaves are the receptors for measuring day and night, and an as-of-yet unidentified flowering stimulator (hypothetically referred to as the hormone “Florigen”) is made in leaves in response to specific photoperiods.  If just one leaf of a Cocklebur is exposed to a SD environment, the entire plant will bloom.  If a SD exposed Cocklebur is grafted to a LD exposed Cocklebur, both will bloom.  All this suggests that “Florigen” can be translocated throughout a plant.

There is still a great deal to be learned about how plants tell time and why plants bloom when they do.  The Cocklebur is an integral part of most studies in this area.  It is interesting that this bane of farmers and gardeners is quite useful to science, and promises to yield insights as to how plants work.