Focusing on U. prehensilis--an Inquisitive Plant
(Carnivorous Plant Newsletter, 1994, 23:2, 34.)
This is the second in a sequence of articles
showcasing Utricularia species. In this
series I integrate the information in Peter Taylor's
taxonomic monograph with my own carnivorous plant horticulture experience.
Since Utricularia in carnivorous plant collections are so often
misidentified, I will carefully describe the species being highlighted
so you can determine if
you are growing it yourself. I warn you against trusting the veracity of your
plants' name tags, even if they came from seemingly
reliable commercial sources (who may be more confused about Utricularia
than you!). Unfortunately this is probably
also the situation with Drosera, especially rosetted species.
The subject of this article is
U. prehensilis--a popular plant that is easy to grow
and fairly free flowering, although not as floriferous as
U. sandersonii or U. dichotoma. Its species name
comes from an interesting feature you usually do not associate with
carnivorous plants--but more about that later.
Once I showed a pot of a non-flowering
Utricularia to my family. When I did, they looked at
it and asked questions like "When will it germinate?" and "Died
on you, did it?" When I explained to them that the tiny leaves were
what I was growing ("No, not next to the 'tiny moss,' it is
the 'tiny moss'"), they shook their heads and walked away muttering. I knew then I was out of the will.
With U. prehensilis this would not have happened--even out of flower
it is unmistakable something is growing in the pot. The leaves are lime green and moderately
large--in cultivation they are commonly up to 2.5 cm long and Taylor
reports plants with 10 cm leaves. They are only about 3 mm wide, strap shaped,
and terminate with a blunt rounded tip. The
leaves are multiveined with a central nerve running along the middle
of the leaf and many smaller secondary nerves that branch to the
leaf margins. If grown in strong light the nerves and other parts of the leaves
are often reddish. They usually lie flat on the soil surface (but can be
semi-erect), and as in many small Utricularia do not form
a rosette--instead they emerge above ground in unpredictable spots.
The underground bladders are up to 1.5 mm across, and smaller bladders are
often sporadically formed on the undersides of the leaves, especially
if the plant is kept very moist.
Cared for properly, a small clump of U. prehensilis
will quickly colonize its entire growing area. When the leaves are so
densely packed they lie on each other in tangled confusion,
the plant may produce a green or golden brown scape about 1 mm thick
and round in cross-section. It grows vertically
until it is 15 cm or more tall and then it starts to twine. As it grows,
the upper several cm waves and wanders through the air in search of something
to spiral around. The motion is slow--it takes a few hours for it to
move appreciably--but when it finds something it winds tightly
about it. Strangely, sometimes the next day you may discover that the
plant has unwound itself and wound onto something else.
This prehensile nature is the origin of the
specific epithet prehensilis.
Only the most recently developed 5 cm or more of the scape is mobile, so
as the tip continues to grow, the length of non-twining scape increases.
The scape always twines to the right when viewed
from the side. In other words, a scape winding its way around
a stick as it climbed it would wrap itself
around the stick counter-clockwise as viewed from above.
About twenty Utricularia species may twine, and
all grow in this direction except the African plant U. appendiculata
which grows in the other direction.
Each U. prehensilis inflorescence produces one to several odorless
flowers. They are spaced by up to several cm, and mature slowly.
When each flower bud is ready to open, the portion of the peduncle
it is attached to is no longer twining and has stabilized. No matter what
orientation the peduncle might have gotten itself into, even
straight down, the pedicel twists around so the flower is borne level.
The corolla (see Figure to right) is pure yellow and is
remarkably similar to yellow-flowered species of
Linaria, commonly known as toad-flax or Butter-and-eggs. In my
previous article (Focusing on U. calycifida)
I defined some botanical terms useful in describing Utricularia
flowers and you may find it helpful to refer to my drawing in that issue.
The lower lip is about 1 cm long and 1 cm wide. It is approximately
obovate--sometimes with a suggestion of having two or four lobes--and
its edges often slightly curl towards the ground.
A striking feature of the lower lip is its large inflated
palate bulge located near where the lower lip emerges from between the calyx
lobes. Three grooved indentations parallel to the long axis of the flower
divide the bulge into four swollen lobes.
The upper lip is a few mm wide and up to several mm long.
Its shape varies, and can be strap-like or spathulate with an end that is
rounded or, as often the case
with my plants, forked to a depth of a few mm. It usually curves over the
inflated palate bulge.
The spur is conical, sharply pointed, and is as long as or longer than the
lower lip. The spur points away from the lower lip by nearly 180 degrees.
My flowers average about 1.5 cm long, and this agrees with Taylor's
stated range of 0.8-2.0 cm.
The two calyx lobes are both ovate or elliptical, and the upper calyx lobe
is always at least a little larger than the lower lobe. When the flower is
open the lobes are about 5 mm long, but when a fruit is developing they can
enlarge to twice this size. Each flower is attached to the peduncle
by a pedicel 1 cm long (Taylor reports a range
of 0.3-1.8 cm) that is distinctly flattened in cross-section.
Where it attaches to the peduncle is a small (1.5-2.0 mm) tapered bract
and two shorter and much narrower bracteoles.
Utricularia can be variable in many ways, especially flower
color. But as a rule
yellow flowered species like U. prehensilis
are almost always consistently yellow flowered.
In contrast, species that have flowers colored with whites, pinks,
lilacs, and purples are prone to great variation in flower coloration
(i.e. see the discussion of U.calycifida in the
last installment of this series). Of course there are some exceptions to this,
for example those yellow flowered plants which can produce small white
or reddish cleistogamous flowers (e.g. U. subulata), or two
species that are normally purple but with occasional yellow forms
(i.e. U. spiralis and U. tortilis). Still, it is a
useful rule to remember.
Cultivation of this species is easy. It will grow in pure milled
Sphagnum or a 2:1 peat-sand mix. I grow
mine in moss and my plants flower well. I do not advise live
Sphagnum, especially species with coarse and
large tufts, because they will overwhelm the Utricularia.
As with all my carnivorous plants, I use water purified by distillation or
reverse osmosis. I keep the water level 2-5 cm beneath the soil surface
but some growers raise the water level to the soil surface or even
submerge the plants when they become established. I have found this to
be a successful method but if you grow your pot of U. prehensilis
in a deep tray of water with other pots of plants, stolons from the plant will
quickly grow out of the pot and invade the neighboring pots.
This plant enjoys warm temperatures but is not picky,
15-32°C (60-90°F) is fine. If temperatures are too high,
the flower scapes may abort, even if they are more than 30 cm long.
I grow my U. prehensilis in terraria under fluorescent lights
and in the greenhouse under 50% shade cloth.
Its cultural needs are easily met, so the only challenge
the plant offers is when it flowers. Left on its own, the questing
scape will quickly find other nearby plants--including other
U. prehensilis scapes--and will wind around them.
I forgot about this once and when I checked on the plant a few weeks
later, one scape had found my D. regia and the other a large clump
of D. binata dichotoma--when I finished untangling the mess
I was thoroughly slimed. The easiest way to restrain the plant is
to insert a vertical stick into the pot and
let the scapes wind around that. I use chopsticks, and when
they reach the top I train them back to the bottom and let them climb
up again (Figure 2). Since crawling pests such as wingless aphids use
toppled scapes as bridges from one pot to the next, training the scapes
can decrease the occasional insect problems that inevitably
occur in greenhouses.
In the wild, U. prehensilis grows in tropical and South Africa,
and in Madagascar. In this range it grows in bogs and often shallow
water. It typically flowers during the wet season, but in permanently
wet conditions it flowers all year. I keep my plants constantly wet
and they flower year round, but most heavily
during the late winter and spring.
Growing and studying carnivorous plants is fulfilling for many reasons.
We may marvel at their beauty and
form, be fascinated by how they fill difficult ecological
niches, enjoy the challenges posed by growing them, or even take
ghoulish delight in how they devour their prey. But as I watch the
curiously probing scapes of U. prehensilis thrash around, fitfully
searching the air one day
and then sliding against a terrarium wall which offers no
foot-hold (and I use that term uneasily) the next, I know it is the only
carnivorous plant that makes me laugh.