Presents:
Carnivorous Plants
Utricularia – Bladderwort.
Utricularia (bladderwort, Figure 2), a plant named for
its tiny bladders, or utricles. Unlike the other
carnivorous plants discussed here, Utricularia often
lives in open water, but again where the nutrient
concentration is relatively low. One common habitat
is in the nutrient-poor bog lakes. In the open water, it
supplements its nutrients by trapping insects in a
bladder that is like a suction bulb (Figure 1, Figure 4,
and Figure 5). Tiny hairlike projections at the
opening of the bladder are sensitive to the motion of
passing organisms like Daphnia (water fleas). When
they are stimulated, these hairs cause the flattened
bladder to suddenly inflate, sucking in water and the
passing animal and closing a trap door after it (Error!
Reference source not found.). If you ever remove a
bladderwort plant from the water, you may be
surprised to find audible crackling noises as the
bladders get triggered shut by the motion and removal
from water. Experiments indicate that growth will decrease by as much as one half if the
bladders are removed. Lollar and
coworkers (1971) fed labeled
Crustacea
(ostracods)
to
Utricularia and showed that
radiophosphorus
uptake
was
greater in plants with bladders than
in plants without bladders,
indicating that prey phosphorus is
absorbed by the plant.
Those hair-like structures at the opening provide a
nice habitat for a group of tiny rotifers, or wheel
animals (Figure 3). These miniature organisms have
two circles of cilia at their upper end, and they rotate
these cilia in such a way that it looks kike
two wheels spinning toward each other.
The result is that the rotifer is able to grab
small particles and organisms nearby and
direct them toward its mouth. By sitting
near the mouth of the bladder, they can
take advantage of the current when the
bladder closes and grab food particles
from the inflow water.
You may have noticed that many aquatic
plants have leaves that are finely divided.
Utricularia is no exception. These finely
divided leaves greatly increase the surface
area of the leaves. And if you are a plant and live in the water, this is a big help. You are
in need of CO2 for photosynthesis and of nutrients
for growth. But both are scarce in water. In the
atmosphere, CO2 is only ~0.03% of the air we
(and plants) breathe. It is hard enough for land
plants to get enough CO2 for photosynthesis. But
in the water, this gas is even harder to get. Little
of it dissolves in the water, and in the summer,
when aquatic plants are actively growing, the
water is warm and the gas easily escapes from the
water to the atmosphere. Having lots of surface
area relative to the leaf internal tissue permits the
plant to take maximum advantage of the very small
amount of CO2 that is in the water. And, that added
surface area helps the plant obtain nutrients also.
Unlike its terrestrial relatives that obtain their
nutrients with their roots, these plants typically float
and have no rooted connections to the soil. They
must get their nutrients through their leaf surfaces
like their cohabitants, the algae. Thus, these finely
divided leaves create more surface area through
which to obtain these nutrients.
Now, where does an unrooted aquatic plant go in
winter? Many, like this bladderwort, form special
bud-like structures called turions. These turions
are really just dense masses of plant with nearly
formed leaves, but in which the stem has not
elongated. They differ from typical buds like the
ones on trees because their leaves are much further
developed. Furthermore, they have no bud scales or
other protective outer tissues. They are, however,
very densely formed into a sphere and packed with starch in their tissues. This makes
them heavy and they sink to the bottom of the pond or lake. There they remain dormant
during the winter. But take one of these and put it in a warm aquarium with some mud to
provide nutrients. To your surprise, you are likely to find plants covering the entire
aquarium in just 24 hours! All it takes is elongation of the stem, and this is relatively
simple as it requires mostly water to elongate the cells.
So does this mean it doesn't need flowers and seeds? Well, producing only turions keeps
making more plants like the parents, and it means being stuck in the same lake or pond
nearly forever. Those bulky turions don't disperse well. So, yes, they do produce
flowers, and these flowers are specially designed to be sure they get pollen from a
different flower, thus permitting genes to mix and get lots of new combinations. They do
this by the special structure of the flowers. Note in Figure 7 that the flowers have a large
nectar spur. This spur, as its name implies, contains a nectar reward for its pollinators,
the bees. The face of the flower (Figure 8) attracts the bee with its bright yellow color.
Once the bee lands, its weight pulls the lower petal down somewhat, opening the flower
like a mouth. Just inside this mouth is the 2-parted stigma awaiting the delivery of
pollen. If the bee has already visited other flowers, it most likely has pollen on its back
and some of this will adhere to the open stigma as the bee enters the flower seeking the
nectar in the long spur at the back. Then the flower does a very "smart" thing. That 2parted stigma slowly moves its two parts together
and closes! No, it doesn't trap the bee, but it does
trap the pollen that the bee has left behind. This
accomplishes two things. First, it makes sure that
when the bee leaves the flower it cannot deposit the
flower's own pollen on the stigma, and the bee is
sure to brush the anthers further inside the flower as
it rummages down the throat to get the nectar. But
the second advantage is that now the pollen has a
moist, protected chamber in which to germinate and
grow, making its pollen tube. But what if the bee
had no pollen, or delivered pollen from some
unrelated plant? The stigma then reopens after
several hours and is once more ready to receive the
real thing. You can tell if a flower has been
pollinated in the past few days by examining the
stigma to see if it is closed.
Close ups of the images in this document are available at: http://images.botany.org