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Preface
Contents
Introduction
1.
Pitcher
Plants
2.
Cobra
Plant
3.
Sundews
4.
Venus
Flytrap
5.
Butterworts
6.
Bladderworts
References
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Pitcher Plants
Sarracenia
PHOTOGRAPHY
General
There
are eight species of eastern North American pitcher plants all occurring in the Atlantic
coastal regions of North America. Of these, seven species are confined to
the southeastern part of the United States where they typically inhabit wet,
sandy areas in the pineland, sometimes localized and rather isolated, but often
with two or more species sharing the same habitat. This often results
in various hybridization. One species extends its distribution all the way
northward deep into a large part of eastern Canada. (List
of Species)
The genus name Sarracenia was
adopted in honor of Dr. M. S. Sarrazin of Quebec, an early discoverer. The
common name for the genus came from their hollow, tubular leaves which are
shaped and function like pitchers.
Pitcher plants are herbaceous (non-woody)
perennials consisting of a rhizome with thick fibrous roots. The hollow
trap leaves arise directly from the rhizome above the ground. The pitcher
leaves form a rosette and are erect or nearly so in most species but are
decumbent in some. The lid develops at the upper end of the pitcher. The lid is
typically reflected over the pitcher opening, but can develop to form a domed
hood in some species. The mature pitchers range in height from 10 to 100cm, or
even more at times, depending on the species and growing conditions.
A peculiar
structure and a bizarre appearance of the pitcher leaves are, for centuries, the first to have
attracted the attention of people. Linnaeus, who adopted the name Sarracenia,
is one of the 18th century botanists who believed -- erroneously,
as many others did at the time -- that the lid of the pitcher is capable of
movement in order to conserve the water within. Another botanist, Catesby,
thought that the hollow leaves were a refuge for insects fleeing from
animal predators. It was not until the beginning of the 19th century
that the more serious observations started to reveal the true carnivorous
nature of the plants.
Trap Structure and Attraction
A close look at the pitcher plant shows that the
hollow leaves are carefully constructed pitfalls designed to attract and capture
small animal prey with amazing efficacy. Sometimes the leaves are mistaken for
flowers by visiting insects and uninitiated humans
alike. In fact, the pitcher leaves have evolved
to exhibit all alluring elements of real flowers: Their visual lure of
striking colors and patterns, copious nectar secretions, and a convenient
landing site for flying insects. Some pitcher plants are known to
release sweet attracting odor in addition to the nectar secretions. The ultra-violet photography
of the pitcher also shows distinct UV
absorption patterns for insect guidance as are commonly found in many
insect-pollinating flowers.
Although the shape and the size of the pitchers are characteristic for each species, the basic structure
and the function
of the leaves are common to all species. The tubular pitcher
leaf has a lid at the top, which is immobile. The brilliant colors and the nectar secretions
along the lid margins and the lip of the
pitcher mouth attract various kinds of small animal prey including bees, flies,
moths, mosquitoes, butterflies, spiders and ants. In fact, nectar
is
scattered over much of the outer surface of the pitcher where they form nectar trail for the prey leading to the pitcher opening. The lid
-- which
often does not actually prevent rain in some species due to varying degree of
reflection over the pitcher opening -- may
provide a convenient ramp and feeding ground for the insect visitors. Trying to
lick nectar is a risky business, however, for venturing insects.
The inner surface of the pitcher is divided into several
zones by various authors. The lid portion is the first
zone
(1) characterized
by having many nectar glands. The inner surface of the lid is also covered by
stiff, short hairs all pointing toward the pitcher opening. This is also
where UV absorption pattern is most eminent. Just below near the pitcher
mouth is the short region
(2) where the hairs become shorter. This is where nectar secretionsis most abundant. Down below extends smooth, gland-free zone
(3), half
way into the pitcher. This is followed by the area
(4) covered with long, thin downward-pointing hairs intermeshing each other.
This is the zone some calls "eel-trap". Finally, at the bottom of the pitcher,
(5)
there is a short hair-free,
gland-free region.
Some species of pitcher plants -- the ones with
the hooded lid -- develop areoles void of pigmentation around the hood. The fenestrations, as
they are called, are white patches of small windows scattered toward the direction of the pitcher tube as seen from the pitcher rim.
Insects have a tendency to fly their way out of the closed spaces in the
direction of light. As the insect rests at the pitcher rim, these deceptive
windows shine brightly. The
flying insect bounces against the areoles as it attempts to fly through, gets
exhausted, and tumbles into the pitcher bottom.
Digestion and Absorption
Pitcher plants possess digestive glands over
the mid- to lower portions of the pitcher leaf interior (zones 1, 2 and 3).
During the development of a pitcher leaf, the fluid is secreted in the pitcher
while the pitcher is still closed. In this passive, pitfall type trap, the digestion of prey takes place in the solution retained at the pitcher
bottom. The prey basically jumps into the pool of a pre-formulated bath of digestive fluid.
Although, in many species, the rainwater dilutes the
pitcher liquid, the acidity is known to be retained, at least in a younger leaf.
Studies showed chemical stimulation by beef broth resulted
in a multiple increase of fluid secretions of an unopened pitcher. The surface tension of the pitcher fluid is measured to be
considerably lower than that of water. This promotes swift drowning of the insect prey by acting as
a wetting agent to otherwise water-resistant
surface of the insect body.
Researchers have been trying to determine the origin of
enzyme during the digestive process. In spite of the studies
confirming the protease secretions in the pitcher plants, at least in the
younger leaves, it is generally believed that the digestion is heavily aided by bacterial
actions externally introduced in the open pitcher
during much of the pitcher leaf life cycle. The digestion process reduces the protein in the insect
body into amino acid. The products of digestion are promptly absorbed by
digestive glands.
Benefit
Pitcher plants typically grow in bogs, swamps, and wet
sandy savannas where soils are often acid and deficient in major nutrients for
the green plants, notably nitrates and phosphates. The
question as to whether the carnivorous habit is essential to the survival of
pitcher plants is not easily assessed. Experience shows the plants do well
without trapping any animal prey in cultivation. Comparative experiments under a
controlled situation also indicated, however, that in a long run, a group of
plants well fed with animal prey grew more vigorously and produced more seeds.
It is generally accepted that the habit of
carnivory acquired by the pitcher plants does benefit the
plants in
alleviating the environmental stress by providing an extra edge over other plants with conventional means.
Inflorescence
The floral structure of pitcher plants is basically the
same for all species. In the early spring, a tall scape emerging from the
rosette center supports a solitary, nodding flower with
showy coloration and rather odd appearance. The unique flower morphology of pitcher
plants leads one to speculate an advanced floral adaptation, as in
many an orchid flower, in terms of pollinator interactions.
Nature often
provides various devices that prevent a plant from being fertilized by its own
pollen. An obvious structural separation of pollen-receiving stigma outside the
pollen chamber seems to encourage cross-pollination. When the pendulous
flower opens facing down at the tip of the tall scape, a modified style assumes
the shape of an inverted umbrella, with five points each having a tiny stigma
lobe
projecting inward. The five petals hang along the umbrella between two points to
form a corolla chamber, leaving the five stigma points outside. Numerous stamens
surrounding the round ovary are confined inside the corolla chamber. This
arrangement structurally separates pollen-producing anthers from stigmas
located outside the corolla chamber. When an insect pollinator lands on the
flower trying to find an entrance to the corolla chamber in search of nectar, a
stigma at one of the umbrella points -- located at the
parting of the petals -- is bound to be brushed and
the pollen from the previously visited flowers are deposited. Once inside the
corolla chamber, the insect seeks nectar at the base of the stamens. As it does
so, the insect accumulates ample amount of pollen which probably has been
accumulated on the umbrella floor inside the corolla. When the insect is ready
to leave the flower, it is likely to push one of the hanging petals from the low
point of the umbrella, rather than retrace the same petal parting. This way, the
pollinator, now with the flower's own pollen, does not
touch the stigma again, thus avoiding self-pollination.
Some field observation on one species showed,
however, that the bees -- believed to be a dominant
pollinator for pitcher plants among many insects visiting the flower -- often
exited the flower the same way it had entered, presumably brushing the stigma
again on exit, thereby increasing the danger of self-fertilization.
In cultivation, it is well known among pitcher plants growers that artificial
self-pollination almost invariably produces a good crop of viable seeds. It seems reasonable to conjecture that the floral
structure of pitcher plants does encourage -- if not
enforce -- cross-pollination, which is more
advantageous form of sexual reproduction in creating more genetic variations
within the species.
Seeds mature in July through September in the southeastern
U.S. habitats, depending on the species and localities. In the warmer region, if
the seeds are shed before the fall sets in, the germination takes place in a
month or so, and tiny seedlings will emerge in that year, although the
germination is often delayed until the following spring in many localities.
In cultivation, it is a known practice to shed the seeds just before the full
maturity and force the germination before or during the fall season. After twin
cotyledons, a seedling produces tiny juvenile leaves which are already hollow,
pitcher leaves. Shapes of the juvenile leaves are more or less the same for all
species and do not exhibit distinct leaf characteristics of each species for a
year or so. Pitcher plants usually mature from seedling to flowering age in 4 to
5 years. In nature as well as in cultivation, the bulb reproduction is
common. The plants are said to live for 20-30 years.
Pollinator/Prey Dilemma
Pitcher plants being insect-pollinated, they must rely on
the visiting insects to play the role of pollinating flowers on
the one hand for the successful continuation of the species, and yet at the
same time, must consume the insects as prey to supplement their nutritional
need.
How do the pitcher plants reconcile this apparent paradox?
There seem to be a few approaches taken by carnivorous plants in general. The
"temporal" solution
is one whereby flowers and traps are produced in different times of the season.
This is the case for many pitcher plant species. Generally the inflorescence of the
pitcher plants precedes the new pitcher leaf production by a month or so in the
early spring. In the southeastern United States where many pitcher plants can be
seen in savanna, mid-April through May is the height of flowering season for
many species. It is observed that there aren't many
new, active, pitchers produced at the time of flowering.
Down south in Mobile, Alabama, S. alata blooms profusely
in late April. There are no new leaves produced at that time. S.
flava, which has a wide distribution from the Florida panhandle all the way
to the north, produces golden yellow blossoms of large, dangling flowers in mass
in North Caroline habitats in early May. The leaves from the previous year
are all but completely blackened and decayed being pilled up at the base of the
plants. There are practically no new leaves sprouting at the time of flowering
for this species at this northern limit.
In addition to temporal separation, some carnivorous
plants deploy "spatial" separation
to resolve, at least in part, the dilemma of pollinator/prey differentiation. In S. psittacina and S. purpurea species, a tall scape
(relative to their leaf height) positions the flower well above the trapping
space occupied by the pitcher openings. Many erect species also produce their
flowers on the scape taller than their pitchers. < Soap
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PHOTOGRAPHY
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