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Carnivorous Plants Gilbert A. leisman THE KANSAS SCHOOL NATfJRAl/$T VoL 30 4
University
Kansas
Apr. 1984 2
The Kansas C 01 Naturalist Published by
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3
Carnivorous Plants Gilbert A.
"People were
and a little disgusted, to learn that the
was carnivorous, and that the flies and other in the cups were
by the sticky substance there.
We in temperate zones were not ig­
norant of insectivorous plants, but we
were unaccustomed to find them out­
side
hothouses, and apt to con­
sider them as in some way slightly inde­
cent, or at least improper. But actually
that the
was the
a triffid's stem could lash
out a slender
weapon ten feet
of discharging
long,
poison to kill a man if it struck squarely
on his unprotected skin ...
But there were a number of not unob­
vious characteristics which
comment for some little time. It was, for
instance, quite a while before anyone
drew attention to the uncanny accuracy
with which they aimed their
and
that they almost invariably struck for
the head. Nor did anyone at first take
notice of their habit of lurking near
their fallen victims. The reason for that
became clear when it was shown that
they fed upon flesh as well as upon in­
tendril did not have
sects. The
the muscular power to tear firm
but it had strength enough to pull
shreds from a decomposing body and
lift them to the cup on its stem."
The Day of the Triffids
John Beynon Harris
Man-eating plants? Rubbishl Science
fiction? Obviouslyl Pure poppycock?
There are, indeed, plants
Not
which are
capable of digesting
trapped insects, as indicated in the first
sentence of the above quotation, and
which can even digest other very small
animals as well. In view of their
somewhat varied
it is
more
to refer to them as car­
nivorous rather than insectivorous.
Most flowering plants, of course,
carryon photosynthesis and this is
usually the
process for producing
energy-rich compounds, like sugar. In
such plants the various nutrient
elements needed for metabolism are
typically absorbed from the soil
with water. Carnivorous plants carryon
and most of them (Blad­
derwort is an exception) are anchored to
the soil and absorb nutrients and water
from the soil just as ordinary plants.
Why then do carnivorous
use
animal
as a part of their diet?
Are animal proteins essential to such
plants? Do these plants grow in some
special type of environment that re­
quires a protein supplement? Questions
such as these have
scientists
for years; even Charles
in 1875,
published a book entitled Insectzvorous
Plants.
Most of the answers to the above
are probably contained in the
characteristics of the habitats in which
The cover illustration was prepared by Dr. R.P. Keeling, Biology, Emporia State University,
and is based on an inspirational (?) sketch by the author of this article. The sketch, in turn, was in­
spired by a well-known TV rom chip commercial.
4. these plants grow. Typical habitats in­
clude acid, mineral-poor
and
swamps, and freshwater marshes and
savannahs. The waters of such habitats
are usually dark brown in color and are
facilitates
very acid. This acidity
the leaching out from the soil of many
irreplacable minerals.
may also play a role in nutrient
availability. Warm temperatures en­
courage the growth of bacteria and
fungi and hasten the breakdown of
organic matter to available
the
most of which are utilized
bacteria and fungi
with the
remainder subject to
In
cool temperatures, decomposition slows
down perceptably and the nutrients re­
main bound and unavailable. The
factor in all of this is
poor
nutrition.
The next question is which nutrients
are in short supply. Since
critical component of
element most
in short supply
in soils, a great deal of attention has
been focused on it. And, indeed,
have confirmed
adds con­
siderably to the
of the plants.
However, current research seems to in­
dicate that other minerals are perhaps
Both have been shown to
an impor­
tant role in the rate of absorbtion of
from animal protein ..
Research has also shown that car­
can be grown in the
nivorous
absence of animal protein. It has also
been shown that suitable fertilizers can
be supplied to the roots, leaves, and
even to the traps
and be ab­
sorbed bv the plants. However, in both
cases (complete absence or artificial fer­
is slow,
tilization) growth of the
flowering and
and budding
is lessened.
there is
something in the insect or other animal
that we have not identified that is essen­
The search for
tial for optimum
that elusive something is the stimulus
for continuing research on these
unusual plants.
There are about 250,000
flowering plants on the earth. Of
about 400 are known to be carnivorous.
They belong to six families and 13
genera. In all cases it is the leaf that is
modified to form the
. With 400
different plants
would seem
variation in the leaf trap.
there is. However. most of them share
two things in common: there is some
mechanism or device to secure the insect
in or on the trap and there is the secre­
tion of
enzymes to break down
the animal
Basically, carnivorous plants can be
classified as active or passive trappers
(Figure 1).
5
Fig. 1. Active (A, B) and passive (C, D) trappers among carnivorous plants.
A is bladderwort which has bulbous suction traps. B is Venus' fly
trap in which the halves of the leaf dose together. The pitcher plant
(C) has a pool of
at the bottom of the funnel-like leaf.
Sundew (D) has sticky glandular hairs on its leaf surface. (From Car­
nivorous Plants, Y.
copyright 1978 by Scientific
American, Inc. All
9
7
ACTIVE TRAPPERS
These are plants which employ a
rapid plant movement to entrap and
secure the insect.
L Closing traps We have only one
in the western
that
qualifies under this
flytrap (Dz'onaea) (Figure
very restricted range. being found only
in the savannahs of southeastern North
Carolina and northeastern South
2). Each !eaf consists
like a
clamshell. Around the margins are
numerous rigid and pointed hairs or
teeth. The upper surface of the leaf
blade is covered with glands which often
serve two purposes. They frequently
become bright red in
thus attrac­
insects. and
also secrete
digestive enzymes. Also present on the
upper surface are trigger hairs, usually
three per half. When two of these hairs
are touched in succession (usually by an
the two halves of the leaf quickly teeth intermesh, close, the
3) and the inset is trapped inside
the glandular
to be slowly
enzymes. Once the insect is fully
digested, the leaf opens up and the un­
digestible skeleton is blown away.
2.
The only represen­
tative that we have of this category is the
bladderwort (Utricularia). However, it
IS a
genus, containing some 250
and most, if not all, are car­
nivorous.
are worldwide in
distribution and, while they do grow in
a variety of habitats, most are aquatic.
3.
Fig. 2. Distribution map of Venus' fly trap.
A. General view of Venus'
plant. B. Close up of
open leaf on left and closed
leaf with insect trapped in­
side on
8
The modified leaves or traps are
bulbous, very small (up to 3.0
and
have a trapdoor at one end. The open­
ing and closing of this
is quite
and several of the details
will be omitted here. The door is sur­
rounded by a ring of tactile hairs which
are very sensitive in their response. Dur­
ing the
stage, the door is closed,
and much of the water inside the blad­
der is absorbed so that a negative
pressure or suction results. If a water
for
were to swim along
and touch the
of
the door
would spring open inwardly, water and
the flea would be sucked into the blad­
and the door would slam shut
(Figure lA).
all of this takes
1/ 460 of a second I Once
in
inside the bladder, the flea is slowly
~.~;~'''.'-~ by glandular enzymes secreted
by the inner
This fantastic speed of the bladder­
wort trapdoor and the rapid closure of
the Venus' fly trap have been the subject
of much speculation. No normal
metabolic process would seem to ac­
count for such rapid response. Current
ideas center around an electrical action
potential which can trigger an instan­
taneous change in turgor pressure of the
cells involved in the response.
leaves are modified to form tubes or
closed funnels with a mixture of water
and (usually)
enzymes forming
a pool at the bottom. The prey is lured
one method or another to the mouth
of the pitcher, enters or falls in, and is
in the pool at the bottom.
The
Sarracenia is by far the
more common of the above two genera,
being found in bogs and marshes in
much of northern and eastern North
America
There are
unfor­
generally
tunately, they have a tendency to
which makes iden­
tification a bit sticky especially in areas
of overlapping
4. PASSIVE TRAPPERS
These are plants that, for the most part,
do not require any motion as part of
their trapping response. Some, like the
sundew, do respond, but only very slow­
ly, and the response is really not all that
essential to the trapping process.
These are the so-called
1. Pitfalls
pitcher plants, including such genera as
Sarracenia and Darlingtonia. Here the
Distribution map of most
widely distributed
of
pitcher plant (S. purpurea).
Darlingtonia is a western genus found
in the coastal bogs and mountain slopes
of Oregon and northern California
(Figure 5); there is only one
9
which prevents any kind of a foothold_
As a
the only possible conse­
quence is an unwelcome
in the en­
pool at the bottom (Figure Ie).
Special glands
the pool area
secrete proteinaceous enzymes, and the
amount of enzymes appears to be direct­
ly proportional to the number of prey.
To make matters worse for the prey
another
of downward-pointing
hairs is present near the bottom, ensur­
ing permanent entrapment.
5. Distribution map of Darl­
ingtonia.
---_ _---­
.•
The
structure of Sarracenia
pitcher plants is
similar from
to species. The tube or funnel­
like leaves are topped by a lid or hood.
in one
this hood is
reflexed over the pitcher opening and
may prevent the entrance of excessive
rainwater. Running down the length of
the outer surface of the leaf and facing
the center of the cluster of leaves is a
fairly
wing or ala whose ex­
act function is not known.
There are a number of
adaptations to both lure and entrap
prey. The pitchers are often brightly
colored green, various shades of red,
yellow, and white
6). The
of the hood has numerous nee·
taries which yield a sweet-smelling HOnce inside the opening the prey
finds it virtually impossible to get out.
The inside of the hood is covered with
stiff downward-pointing hairs which
prevent egress. Inside the pitcher itself
the inner lining of the upper third or
half consists of a smooth waxy cuticle
7.
Darlingtonia,
stalk at left.
with
flower
10
6.
Dark veins on leaves are bright red in nature,
Pitcher
to attract insects.
Darlingtonia
are similar to
those of Sarracenia except for a
of features. At the top of the pitcher is a
prominent
hood which is reflex­
ed so that the opening to the pitcher
faces downward. From the outside edge
of the
a two-lobed or forked ap­
pendage extends downward
Viewed from the side, the whole
and its
looks somewhat like
a coora, hence the common name cobra
plant. Finally, the pitchers of Dari­
zngton£a twist 180 a in either direction so
that they wind up
away from the
center of the cluster.
Internally. the major difference is the
glands any place in
absence of
the pitcher. Presumably the prey is
by
and the
There is at least one other pitcher
plant
of mention since one fre­
quently sees it
as a
This is the tropH:a!
plant Nepenthes. It is a vine-like
growing to 50 or more feet in
The vine produces long flat
leaves which
very
drUs from their tips. In turn, the
the tendrils frequently, but not
produce pitchers (Figure 8). Each pit­
cher has a hood or lid which is usually
brightly colored and covered with nec­
taries, undoubtedly to attract insects
and other prey. The mouth of the
11
---~"'-~~.--------------------------
Pitchers of Nepenthes borne at end of tendrils.
chef has a
ribbed
at
the base of each rib is a downward­
projecting tooth. Below the teeth is a
smooth waxy area, and below that a
with water and
1. "Flypaper" or adhesive traps
Sundew (Drosera) is the best known of
this group of carnivorous plants (Figure
Since many sundews are small in
size
are frequently less than 5
em
they are frequently
overlooked in their
habitats. The
upper surfaces of the leaves are covered
with numerous sticky glandular hairs
which immobilize any small prey that
comes in contact with them. Following
12
leaves are covered with glands, both
stalked and sessile. The stalked gland is
responsible for entrapment
and the sessile gland for digestion
link
dlg~stt ...c
lone,
51-lOwing gLmJs
Fig. 9.
section
of
entrapment, many of the surroundmg
glandular hairs bend toward the prey
and the leaf itself may
fold so as
to bring as many
as possible in
contact with the prey. This is a slow pro­
cess, however, and is part of digestion,
not entrapment. The
of course,
secrete enzymes, with those in the center
of the leaf being most
There
are about 100
and
in
plant is butterwort
are 48 known
America our species are
ed to Canada (Figure
southeastern coastal area of the United
States (Figure 13). A typical butterwort
a small rosette of flattened or
slightly fleshy leaves (Figure
Like
the sundew, the upper surface of the
Fig. 10.
Distribution map of one of
the northern
of
sundew, the roundleaf
sundew (D. rotundifolia).
Fig. II. Distribution map of the
southeastern
of
sundew, the shortleaf
sundew (D.
13
Fig . 12.
Distribution map of nor­
thern species of butterwon
(P. vulgaris).
Fig . 13.
Distribution map of south­
eastern coastal species of
butterwort (P. pumila).
Fig. 14.
Rosette of leaves of butter­
won (Pinguicula).
14
Fig. 15. Scanning electron micrograph (SEM) of surface of butterwort leaf show­
ing large, stalked, entrapment glands (black arrow) and small, sessile,
digestive glands (white arrow).
CULTIVATION OF
CARNIVOROUS PLANTS
Contrary to popular belief, a majority
of these plants lend themselves readily
to cultivation with a minimum of time
and effort. Space does not permit going
into detail, but an excellent source book
is Carnivorous Plants, Adrian Slack,
1979, MIT Press. Also contrary to
belief, you don't have to "feed" these
plants hamburger or filet mignon. Even
the most fastidious of households has
enough insect life to satisfy most of these
plants. As a matter of fact, why pay an
exterminator to come in each spring
and fall to get rid of the "bugs"? Put a
few carnivorous plants around the
house, they'll do the job efficiently, and
they won't charge even a farthingl
One final word of caution. Do not
collect carnivorous plants from the wild.
This is important because these plants
15
play a
role in a
type of
environment. The
are not com­
mon, in many places they are rare, and
in many states they are protected by
law. Man seems to have a
talent
for disrupting and even destroying
natural environments. And a car­
nivorous
bog is an
one. Please. let's leave this one
alone.
There are several nurseries that offer
carnivorous plants for sale. Some are
listed in the book by Slack
above; others are listed in Schnell, an·
notated in the reference list below.
These nurseries, of course, had to start
with wild plants, but all
pro­
pagation of them has been within the
A particularly
propagation technique that is
us­
ed today is tissue culture. This is a clon­
ing
in which small bits of
tissue from a parent plant are induced
to grow into completely new mature
plants, all just like the parent. Venus'
flytra p is
to
this technique, which accounts for its
frequent appearance in stores as a
novelty
OTHER SELECTED REFERENCES
Yolande. 1978. Carnivorous Plants. Scientific American 238:
104-115.
Lloyd, F.E. Carnivorous Plants. Dover Books.
Mellichamp, T. Lawrence. 1983. Cobras of the Pacific Northwest. Natural
April issue: 46-51.
Schnell, Donald E. 1976. Carnivorous Plants of the United States and Canada.
Blair, Publisher.
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