Dr. T. Ombrello - UCC Biology Department
Nature
makes "mistakes" all the time, producing animals and plants that are
different from typical members of their species. Many of these
"mistakes" are called mutations, and have a genetic basis that can be
passed on to future generations. Mutations that provide a favorable adaptation
can become incorporated over time into most if not all of the members of a
species. Unfavorable or non-adaptive mutations are often eliminated from a
population. This natural selection process by nature is the driving force behind
evolution.
One
interesting type of mistake that is occasionally found in plants is known as a
fasciated or crested growth form. It is usually the result of a growing point
changing from a round dome of cells into a crescent shape. Subsequent growth
produces a flat stem. In some cases fasciation is the result of several
embryonic growing points fusing together, with the same flat-stem appearance.
Fasciated
stems look strange. Leaves, flowers, and fruits often develop unusual shapes and
appear at odd angles to the stem. Some, looking like hand-held fans, have led to
the descriptive term "crested". Nature usually eliminates fasciated
tissues. Branches or even whole plants with this condition are overshadowed by
normal branches or plants of the species, and do not last long.
Humans
are fascinated by fasciated plants. Their unusual shapes make them prized by
many in horticulture. Most can be perpetuated by vegetative propagation, and are
designated as cultivars of the species. Some noteworthy examples are: crested
saguaro cactus, fasciated Japanese cedar, and fantail willow. Less frequently,
the fasciation is carried from generation to generation by seed, with the
fasciation only manifesting itself in the flowers and subsequent fruit. Some
examples would include crested cockscomb celosias and beefsteak tomatoes. (If
you have ever wondered why beefsteak tomatoes have such unusual shapes, look at
their flowers and you will readily see why).
What
causes plants to produce fasciated stems? For the most part, we just don't know.
Fasciation has been induced experimentally by applications of plant hormones,
severe pruning, wounding, and atypical day lengths. Most, however, appear by
chance with no obvious cause.
Rarely
does a year go by without my discovering fasciated stems on wild and
domesticated plants. Recent finds have included dandelion, asparagus, raspberry,
black-eyed susan, and the tree of heaven (Ailanthus). If you look
carefully you will see them too. Finding these fascinating fasciations offers
small but intriguing rewards for observing nature.
A "normal" stem (left) and a "fasciated" stem (right),
cut from a cactus plant growing in one of Union County College's greenhouses.