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Sundews
Drosera
PHOTOGRAPHY
General
Glistening in the sun like a cluster of diamonds,
with their dew-covered leaves emanating the full spectrum of the rainbow, the
name "sundew" aptly describes the beauty of these carnivorous plants
that use a "flypaper" or adhesive trap to catch small animal prey. The
genus name Drosera is derived from a Greek word droseros for
"dewy". Sundews typically grow on the moist surface of bogs and marshy
savannas, often creating a large patch of shining red carpet.
There are about 150 species of sundews worldwide over both
the Southern and Northern Hemispheres. By far, the heaviest concentration of the
species occurs in Australia -- particularly South Australia and the southern part
of Western Australia -- with nearly 50 endemic species of sundews in this
continent alone. The southern part of Africa is also known for a large number of
sundew species. Seven species grow in the continental United States. (List of U.S. Species)
The north temperate forms of sundews are generally
perennial plants, with several leaves forming a rosette. The plants have a short
stem with fibrous roots. The leaves of the sundew vary in shape and size in
different species, but the basic function and structural characteristics are the
same for all species. The leaves are of two parts: a narrow petiole (leaf stem)
at the leaf base, and a leaf blade modified into an adhesive trap.
Adhesive Trap
The upper surface of the leaf blade is covered with
numerous glands raised on a stalk. These hair-like stalked glands, often
referred to as "tentacles", hold a large
droplet of sticky mucilage at the tip. This is the basic trapping mechanism of
the adhesive trap.
The secretions of viscous substances is widespread in the
plant kingdom, and many insects are known to be attracted by glistening
droplets. Unlike the case of the pitcher plants where nectar
is offered as a reward, the adhesive trap provides no rewards.
When prey, usually a small insect, lands on a sundew leaf,
it becomes mired in the sticky secretions. As the prey struggles to free itself,
tentacles begin to bend over the prey. In many sundew species, the leaf itself
also slowly folds around the prey, if the prey is sufficiently large or strong
to justify further securing.
The mucilage appears to contain no lethal substances and
the trapped insects typically die of suffocation as mucilage covers their
trachea during the struggle to escape.
Structure of Tentacle
A closer look at the sundew tentacles reveals that, unlike
a usual plant hair which is of epidermal origin, the tentacle has a far more
complex structure exhibiting all the elements of the leaf itself. It is composed
of a tall, tapering stalk of multi-cellular structure, topped with two layers of
glandular cells. A slender vascular strand can be seen in the stalk center to
support gland-to-leaf linkage.
Mucilage is secreted from the glands through
their cuticular discontinuity. In addition to these stalked glands (tentacles), there are
numerous sessile (stalkless) glands. These are found on the leaf surface as well
as on the stalks of the stalked glands. Actual functionality of the tentacle, however, appears
far more complex and diverse than its apparent structural manifestation.
The movement of the tentacle plays a crucial role in the
actual entrapment of prey and the subsequent digestion-absorption sequence in
sundews.
The tentacles are sensitive to physical stimulation,
exhibiting the fast tentacle movement. Upon stimulation of tentacles, the action
potential has been measured by researchers. The outer, longer tentacles tend to
respond more readily. The inner, shorter tentacles require stronger stimuli.
This initial, fast movement is followed by a slower phase of movement, often as
a result of sustained physical simulation along with certain chemical
stimulation.
If the tip of the tentacle holding the gland is cut off,
the tentacle does not respond to the direct stimulation, though the remaining
stalk portion of the tentacle does bend in response to the indirect stimuli from
nearby tentacles. This led to the traditional hypothesis that the receptor of
the tentacle is located in the head. It is now noted that the cells structurally
homogeneous to the sensory cells of Dionaea's trigger
hairs are the epidermal cells of the upper part of the tentacle stalk where the
stress of the physical stimuli is most pronounced, replacing the previous
hypothesis. (Juniper et al, 1986). Illustration
Tentacle Movement
Detailed studies have been conducted on the behavior of
sundew tentacles by various authors, including Charles Darwin and F. E. Lloyd.
On the most widely known species of sundew, Drosera rotundifolia, the
tentacles can be divided into three groups based on their response
characteristics to stimulation. Those tentacles found on the periphery of the
leaf blade -- usually the longest tentacles of all -- are known as the
"marginal tentacles". Those near the leaf center are termed "central
tentacles" and are usually the shortest. Between the marginal and the
central tentacles grow tentacles of intermediate height called "outer
tentacles". By and large, a similar observation
holds for behavioral characteristics in most sundew species, although some
differences may be noted due to the particular posture of tentacles in different
species with varying leaf shapes. Illustration
The marginal tentacle, when stimulated directly, always
bends toward the leaf center. This type of bending motion toward a
pre-determined direction, regardless of the direction of outside stimuli, is
known as the "nastic" movement. The tentacle also responds to indirect
stimulation -- an impulse transmitted from outer or central tentacles. When a
direct stimulus is given to the marginal tentacles, however, only the stimulated
tentacle will bend, i.e., the marginal tentacle does not transmit stimuli to
other tentacles. This behavior immediately distinguishes the marginal tentacles
from the other two groups of tentacles.
The central tentacle, on the other hand, does not respond
to direct stimulation, like the other two groups of tentacles. When stimulated,
it does send out impulses to the neighboring tentacles in a matter of several
minutes to a few hours. When stimulated indirectly, the central tentacle bends
toward the point of the stimulus. This behavior of the tentacle -- bending
toward the direction which has a consistent correlation to the source of
stimulation -- is known as the "tropistic" movement.
The outer tentacle, located in the middle zone between the
marginal and the central tentacles, exhibits somewhat of a combined
characteristic of these neighboring tentacle groups. The outer tentacle responds
to the direct stimulation by exhibiting a rapid nastic motion, just like
marginal tentacles. It will transmit an impulse to nearby tentacles as well.
For indirect stimulation, the behavior of both the
marginal and the outer tentacles is more complex, typically showing an initial
nastic movement followed then by a tropistic reaction, as if to correct orbital
errors, as it were, to reach the prey. The outer tentacles are known to be more
responsive and agile to the indirect stimulus than the marginal tentacles. The
actual tentacle behavior may vary somewhat depending on where the stimulus has
originated, on different species, and on growing conditions. For instance, the
higher the temperature, the more likely the tropistic reaction is to dominate.
Tentacle Bending Mechanism
The mechanism of tentacle bending is not fully understood.
The bending is precipitated by a sudden drop of cellular pressure on one side of
the tentacle stalk. The resultant imbalance of pressure between the opposite
sides of the stalk causes the tentacle to bend. The pressure differential first
occurs near the tentacle base, and then gradually moves upward.
Sometimes a quite rapid tentacle movement can be observed
in some species -- bending more than 180 degrees in
less than 60 seconds -- with a proper stimulation
(mechanical and chemical) under an optimal condition.
Digestion and Absorption
Eventually the gland secretions are switched from sticky
substances to digestive juices. The prey, having forced down to the surface of
the leaf by flexing tentacles, is often submerged in the digestive fluids and is
suffocated. The bending motion of the tentacles allows the prey to be
transported to the pre-designated center portion of the leaf blade where the
digestion takes place. This brings about economies in quantities of digestive
enzymes. As the digestion progresses, products of digestion are promptly
absorbed into the leaf interior through the leaf surface and, in a matter of a
few hours, begin to be carried away to the other parts of the plant (Komiya et
al, 1978).
Numerous, tiny sessile glands found on the entire surface and on the tentacle
stalks also aid in the rapid absorption of the digested material.
Unbending of Tentacle
After the absorption is completed, the tentacle returns to
the original position. It is a reverse process of bending, again due to pressure
differentials. This unbending is a slow process usually taking a day or more. It
is believed the unbending is a growth phenomenon seen as the result of restoring
the cellular pressure. As such, the same tentacle is measured to be greater in
length by 10 percent or so after unbending. A given tentacle can repeat the
bending and unbending only a few times before reaching the growth maturity.
Inflorescence
For the most species in the temperate zone, a tall scape
appears from the rosette center in the spring, which bears numerous flowers.
Actinomorphic (radially symmetric) flowers have five petals and are typically
small and plain. Most of the sundew flowers are believed to be
insect-pollinated. Usually a flower opens for one day. If the pollination does
not take place, the anther and stigma are brought together as the flower closes
at the end of the day, thus effecting self-pollination. Many flower scapes are
produced and flowering continues through the summer season for most species.
PHOTOGRAPHY
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