Are Genlisea traps active? A Crude Calculation
(Carnivorous Plant Newsletter, 1994, 23:2, 40.)
I find the structure of the Genlisea trap very interesting,
although they are not well understood in the botanical literature.
Many questions remain unanswered. Are prey attracted or do they just
wander into the traps?
How is the prey retained in the utricle
(the trap's digestive chamber) and how is it digested?
How are the digested nutrients retained and then assimilated? With more
time and research, these questions will be answered.
The form of Genlisea traps is well known, and I described some
of its features in the previous article. In this article I
concentrate on a single aspect of the Genlisea trap, an aspect
that would seem to be a flaw in the trap's construction. An observation
has been made by Juniper, Robins, and Joel in the
book, The Carnivorous Plants (hereafter JRJ), which may
point to the plant's solution to this flaw--namely that
the trap is actually active. To complete my conjectures, I present an
approximate calculation exploring whether this is plausible.
It seems CP are fairly efficient digestion mechanisms. Dionaea
traps allow prey to escape if the prey are too small to be worth digesting.
JRJ report that Drosera erythrorhiza absorb a full 76% of the available nitrogen in insect
prey. Yet consider the fate of a rotifer (to choose a likely nutrient source)
swimming along the interior of a Genlisea trap. Because of strategically
located trap hairs it can only swim towards the utricle where it
dies and is broken down for absorption.
But what of the chemicals released by the
dissolving rotifer, before they are absorbed by the plant?
There are no one-way
valves at the entrance of the utricle (as there are in Utricularia
bladders), and inward pointing hairs have
no effect on individual molecules. So
what prevents a significant portion of the valuable nutrients
from diffusing through the utricle entrance, out of the trap, and away from
the plant?
How does Genlisea prevent a wasteful loss of nutrients
from the trap? Or does it simply operate inefficiently? JRJ make an
observation which may be important (pg 126). They note that utricles
contain not only the digested carcasses of prey, but also particles
of dirt. The traps of Genlisea hang downward, so it is difficult to
explain how sinking or drifting dirt particles could find their ways into
the utricle. After settling into the spiral trap entrance, the particles
would need to inexplicably rise into the trap mouth, through the trap
tube, and into the utricle. Instead of that unlikely scenario,
is it possible these bits of detritus have been sucked into
the trap by the plant's effort? Perhaps the plant is expelling water from
the trap through the
utricle walls. New water from outside the trap would flow up the trap tube
to replace the water removed from the utricle.
The expulsion would be comparable to the phase in which
water is removed from the interior of a sprung Utricularia bladder
and is excreted into its surroundings. This is not too implausible
since the two genera are
closely related and the traps of both genera contain similar internal
and external glands. The purpose of this expulsion might be to
suck valuable nutrients into the cell walls, and thus
prevent their escape from the trap.
Genlisea traps may be active and not passive.
I decided to make a few simple calculations to see if it is even
wildly possible that a Genlisea trap could function as a pump. Could
it remove water from its utricle at
a rate sufficient to overcome the molecular speed of nutrients diffusing
down the trap tube to the trap bifurcation, and
then into open water? Being a scientist, I know that approximate
calculations provide
insight to basic phenomena. You can get a rough idea of
what is going on, or if a mechanism is possible--then
let the next group of researchers worry about
the details! To treat this problem
I needed to calculate two velocities. First, what is the velocity of
liquid being sucked through the trap tube to the utricle? Second, what is a
typical velocity at which nutrient molecules diffuse out of
the trap? If the velocity of fluid up the tube (Vf) is greater than a
molecule's diffusion speed (Vd) then the plant could overcome diffusion
and thus maximize its efficiency. If you find math uninteresting or paralyzing,
skip the next three paragraphs and read the one starting with
"I don't expect you...." for the results.
First I estimated the flow velocity through the tube. JRJ note
work by various researchers who
measured that Utricularia bladders
expel about 40% of their fluid
volume in approximately 20 minutes. Assuming a
spherical bladder 1 mm in diameter, this corresponds to
1.74×10-7 cm3 sec-1 of water pumped
through its surface area. Some research suggests the glands scattered over
the entire exterior surface of the bladders are responsible for removing
the internal bladder fluid. Since similar glands are found on the exterior
of the Genlisea utricle, it is plausible they remove water from the
trap in the same way. Modelling a
typical large African Genlisea utricle as a sphere 4 mm in diameter,
it would have sixteen times the surface area of the Utricularia bladder
and could pump water sixteen times faster. As
this water is sucked through the narrow trap tube, which has an
inner diameter of about 0.05 cm, it
would produce a flow velocity of
Vf= 0.0014 cm sec-1.
And what is the diffusion speed of nutrient molecules through water? This
is a little more complicated. A molecule of mass m and at temperature
T (in Kelvins) will have a molecular velocity Vm approximately
given
by m(Vm)2 = 2kT,
where k is Boltzmann's constant. For a
typical nutrient like the phosphate ion (PO4)-3 at T=25°C,
Vm=2.3×104 cm sec-1.
As this ion races among the water molecules, it will travel only a short
distance L before colliding with one. This distance
is called the mean free path. (The mean free path can be estimated using
L3=m/p, where m and
p are the molecular mass and density
of H2O.) The time for a particle to traverse a mean free path is
given by t=L/Vm.
Because of all these molecular collisions, the ion
will not travel in a straight line. Instead it will randomly wander around.
It can be shown that after n molecular collisions, the ion will have
wandered a distance X from its starting point, where
X2=nL2. For
it to wander about 1.5 cm (the length of the trap tube for a large
Genlisea) the ion will suffer 2.3×1015 collisions!
To wander this distance will take the phosphate ion a total amount of
time equal to nt, so I can write the effective diffusion velocity as:
Vd =
n1/2L/(nt) =
L/(n1/2t) =
Vm/n1/2.
For our nutrient ion,
this gives a diffusion velocity of Vd=0.00048 cm/s.
My velocity calculations were admittedly crude and did not
consider a wealth of interesting details. But unless I made a fatal blunder
and neglected an important effect, the details that would
make these
calculations many times more difficult are unlikely to change the results too
much. I note for example that I did not treat the effects of
intermolecular forces at all. But these forces would only
conspire to decrease diffusion velocities, and therefore make the trap
even more effective. I think the strongest criticism against my argument
is that the methods of water excretion in both Utricularia and
Genlisea traps are not understood. In spite of its
greater size a Genlisea trap might pump fluid only at the same
rate as a Utricularia trap. But still the flow and diffusion
velocities would be roughly comparable and the pumping mechanism would
be useful for the plant. After all, diffusion is a random process
and the diffusion velocity I calculated is only a typical value for a
molecule--there will always be faster and slower particles. So the
precise value of Vd is not important. For this reason, I am not too
concerned with my choice of a phosphate ion as the
test particle--Vd is modified only
by the square root of the nutrient's molecular mass. I would be very surprised
if all my approximations would combine to change the ratio of velocities I
calculated by as much as 100.
I don't expect you necessarily followed that calculation. But the
point is the following: simple estimates show that a
Genlisea trap may be fully capable of generating a current into
its stomach with
a speed three times faster than
the speed at which nutrient molecules could escape. This tactic would allow
Genlisea to extract a greater percent of nutrients from its prey.
Perhaps the water-sucking
phase of a Genlisea trap only occurs when the trap is signalled by some
mechanical or chemical means, analogous to the 20 minutes of water expulsion
Utricularia bladders experience after they have been sprung. In fact,
a Genlisea would have to draw fluid through its utricle for 18 minutes
to completely change the fluid in its tube. It is striking that this is about
the same time period as for a Utricularia bladder's water suction
phase. Maybe Genlisea swallows!
Finally, while these calculations are interesting and even
evocative, they do not prove anything. It might just be that
despite any calculations Genlisea is a passive carnivore.
Proof must
await the laboratory and not the calculator. But an experimental
investigation to prove or disprove the hypothesis that Genlisea is
active would be relatively easy to perform. Place a
chemically killed but structurally intact Genlisea trap next to a
live and functioning one. Observations of how quickly dyes migrate through
the tubes of each trap should reveal if the live trap is drawing dye into
its utricle faster than the dead trap. Unfortunately I have neither the
facilities nor the familiarity with biological lab methods to do this
experiment to my own satisfaction, so I will leave that job to someone else.
Clearly, this is a field of study that is in need of
solid experiments for information and insights into the mechanism of this
fascinating plant.