I see in the paper that Winnie the Pooh is 75 years old this year.
I really hadn”t come in contact with Pooh until a few years ago
when our grandson was at the age to appreciate him. I must say I
probably enjoyed meeting the Pooh characters even more than he
did. Pooh”s enjoyment of honey from the bees” nest was always
amusing. I myself always enjoyed eating honey from the comb
more than just spooning it out of a jar. As I”m writing this, I”m
trying to figure out how this can possibly tie into a column
related to science. Ok, I think I”ve got it. How does that honey
get there in the first place? It”s an example of symbiosis which,
according to my dictionary, is “the intimate living together of
two kinds of organisms, especially where such association is of
mutual advantage”.
Of course, the symbiotic relationship between the honeybee and
the flowering plant is a classic example. The flower supplies the
nectar for the bee to make the honey. In return the bee pollinates
the flower, thus aiding in the growth of the fruit and the
subsequent seeds that ensure the reproduction and propagation of
the plant species. In recent years, there”s been considerable
concern over a bug or disease of some sort that has been killing
off substantial numbers of our honeybee population. This has
not made our fruit growers happy to say the least.
I was intrigued with another case of plant-insect symbiosis
described in an article by Eric Hansen in the October 2001 issue
of Discover magazine. The title of the article is “Plants Eat
Animals”. This hardly sounds like the arrangement is
particularly symbiotic for the “animals”! The article deals with
carnivorous plants. Probably the most familiar carnivorous plant
is Venus”s flytrap, Dionaea muscipula to you biologists out there.
This delightful plant has fanged “jaws” that close and squeeze
tight around any unsuspecting insect that wanders into its
embrace. The plant then digests the insect as a dietary
supplement.
Why did such carnivorous plants evolve in the first place? After
all, virtually all other types of plants on the earth”s surface seem
to get along fine with just water, carbon dioxide from the air,
light from the sun and nitrogen and other nutrients from the
ground. These are the elements of photosynthesis. It turns out
that carnivorous plants generally grow in rather hostile
environments with acidic soils – swamps, bogs or on sandy
shores. In such environments, the supplies of nitrogen and other
nutrients are either unavailable or pretty scarce. The carnivorous
plants do use photosynthesis but, in order to obtain the missing
nutrients, they”ve developed a way to supplement their diets with
insects.
There are apparently over 500 species of these carnivorous plants
and many of them have evolved independently. This surprised
me. I would have thought they were all related in some fashion
back to a common ancestor. Some of them can be quite large.
One of them, Nepenthes northiana, grows large enough to hold a
quart of liquid. In some plants, not only insects but also even
mice have been “eaten” by a larger plant. Some plants actually
suck their prey into their digestive chamber. Others just rely on
the insects falling into their pools of acidic juices that do in the
poor critters.
You”re possibly saying, “Hey, where”s the symbiosis?” I admit
that the critter that”s being digested doesn”t benefit at all from the
arrangement. But let”s follow the author, Eric Hansen, on a trip
to Borneo with Ch”ien Lee, a field botanist searching out various
threatened carnivorous plants in that far corner of the world.
Hansen describes his experience in a yucky peat-swamp forest
harboring the Nepenthes bicalcarata, also known as a fanged
pitcher plant. The name arises from two protrusions that look
like fangs that are attached to the leaf that hovers like a lid over
the top of the pitcher part of the plant.
While traipsing through this sticky, muddy swamp, our friend
Eric finds a cockroach that he decides to feed to a fanged pitcher
plant. After dropping the slightly injured (missing legs)
cockroach into the pitcher, Eric drops to his hands and knees in
the muck to observe the course of events through a magnifying
glass. The pitcher contains the acidic digestive fluid of the plant
and the enzymes in the fluid start to do their digestive work on
the struggling cockroach.
But wait. Help is on the way! A troop of Camponotus schmitzi
ants has appeared just inside the lip of the pitcher. These
schmitzi ants aren”t affected at all by the juices of the plant and
sure enough, one of the ants dives into the pool to rescue the
roach. Just one ant, mind you. The others wait at the shoreline
for the swimmer to haul the roach to within reach. This done, the
ants laboriously pull and drag the uncooperative roach up the 2-
inch high vertical wall. This is not an easy job and scaling those
two inches takes more than an hour! I don”t know about you, but
I”m not sure I could have gotten up if I were Eric hunkered down
for that long watching this drama unfold.
Finally, the schmitzis have completed the rescue and have the
roach safely on a ledge of the pitcher. But whoa! The carnage is
only beginning. Those ants didn”t rescue that roach to set it free.
They immediately begin tearing it into pieces and eating it! The
parts the schmitzis don”t like they “toss” (author”s word) back
into the pool.
Oh, you still don”t see the symbiosis? It seems as though all that
happened was that the ants robbed the fanged-pitcher plant of its
rightful meal, leaving just a few crumbs behind. Would you
believe that the symbiosis occurs because the schmitzi have
actually done the plant a favor by preventing it from overeating?
It seems that one Charles Clarke spent years in the rain forests of
Brunei and Sarawak and came to that startling conclusion.
Clarke found that if there are too many dead insects floating
around in the digestive fluid, the chemistry of the fluid is
changed. The mix of fluid and insects putrefies and the plant
dies.
So, the schmitzis serve as a sort of diet pill for the plant. In
return, the plant provides the schmitzis with both free food and
lodging. The lodging is in the form of a hollow leaf tendril
where the ants can nest. True symbiosis.
The article cites other examples of symbiosis involving
carnivorous plants. One example concerns birds. Some time
ago, I wrote a column on Fritz Haber and nitrogen fixation in
connection with fertilizers and explosives. In that column, I
mentioned that for many years a prime source of nitrogen came
in the form of guano, bird droppings from the Southern
Hemisphere. Remember, our carnivorous plants need nitrogen to
supplement their diet. One plant has developed wide leaves
shaped like a trumpet. The trumpet helps catch falling leaves to
be digested. But, to further supplement its nitrogen needs, the
plant secretes a sugary concoction in special glands. The sugary
stuff attracts birds but the glands are positioned such that the bird
has to sit on the edge of the trumpet with its rear facing the throat
of the pitcher. Voila! Guano, and more nitrogen, for the plant to
chew on.
Happy birthday, Pooh!
Allen F. Bortrum
