There are about
80 species of butterworts worldwide. Many can be found in Mexico and in Europe. They grow on
wet, dripping, moss-covered rocks in the mountains, on moist, grass-covered
sandy ditches in savannas, and in other similar conditions, sometimes in company of other carnivorous plants. Of six species found in the southeastern
United States, five are indigenous to the region. Three other species grow in
northern U.S. and Alaska. (List of U.S. Species)
Butterworts are rosette of thin leaves, typically lying
prostrate, with white, fibrous roots. The upper surface of the leaf is covered
with short stalked glands that secrete clear sticky mucilage to trap small
animal prey. The leaf feels greasy to the touch because of this secretions. This
gave the plant its common name. The genus name Pinguicula is derived
from the Latin word pinguis, meaning fat.
Many butterwort rosettes have close resemblance to each
other, and it is often difficult to distinguish one species from another when
the plants are out of flower. The leaf is yellowish-green in most species but is
a distinctive reddish color in some species (including one U.S. species). The diameter of the rosette
varies anywhere from 20mm to 150mm depending on the species.
Prey Capture and Digestion
Most of the prey is small insects such as ants and
mosquitoes. When an insect lands or crawls on the upper surface of the
butterwort's leaf, the
insect is mired down to the surface by the sticky substance secreted by the
stalked glands, as in sundews. Unlike sundew tentacles, however, these short
stalked glands offer no movement. The stalked gland consists of 16 radiating
glandular cells supported on a single-celled stalk.
Scattered over the entire leaf surface are another kind of
glands with a similar structure. These are sessile (stalkless) glands consisting
of 8 radiating glandular cells and are responsible for the secretions of
digestive fluids and the subsequent absorption of the digested material. Studies
have shown that nitrogenous compounds precipitate the secretions of enzyme from
sessile glands in a matter of minutes.
When a small insect -- or any nitrogenous substance such
as a piece of meat -- is placed on the leaf surface, there is copious secretions of digestive fluid from the sessile glands. Often the trapped insect sinks down
to the leaf surface -- completely submerged in the digestive fluids -- and
comes into direct and tight contact with sessile glands on the leaf surface. The
secretions are also shown to contain an antiseptic substance which effectively
prevents bacterial activities during the course of the digestive process if the
prey is small enough which, in nature, is usually the case.
The resultant digested material is absorbed through the
sessile glands. Studies using radioactive isotope (carbon 14) show that products
of digestion are rapidly taken into the leaf in a matter of a few hours and then
are carried to the other parts of the plants.
Often the leaf movement is observed in association with
the capture of prey. When a small prey is placed on the leaf surface near the
leaf margin, the leaf begins to curl up. It is a slow motion taking several
hours and possibly lasting several days. Also, the amount of curling is rather
limited. It is unlikely, therefore, that the leaf movement contributes to the
entrapment of the prey in the same degree as observed in the leaf-folding in
some sundew species. It is also noted that the contact of an insect with the
leaf (away from the margin) results in the dishing of the leaf area below the
All leaf movements are known to be caused by the
stimulation to the leaf surface and are attributed to a growth phenomenon. Although
not as active as tentacle bending or leaf folding in sundews, the leaf movement
in butterworts is a matter of common observation. How much significance should
we assign to it is debatable. It is generally interpreted that the upwardly
curled leaf margin -- and localized dishing of the leaf by
stimulation -- helps to hold the secretions in place
during the digestion process.
All butterworts produce a showy display of colorful
flowers which are borne at the tip of a slender, often glandular, pubescent
scape. Early spring is the flowering season for most U.S. species. The
zygomorphic (laterally symmetric) flower has a sympetalous (united petals) corolla forming a
cylindrical tube which divides into five lobes on the front and terminates in a
slender spur on the back of the flower. The spur is a nectar container, a
structure commonly seen in insect-pollinating flowers. Near the lower entrance
of the corolla tube projects a hairy structure known as a palate. The
microscopic structure of the palate hairs, along with the color and shape of the
entire corolla, is characteristic to each species and is used for the
identification of the species. On the upper side of the inner corolla tube are
located two stamens and a stigma lobe. A wide stigma lobe covers anthers for incoming pollinators, thus providing a mechanism to discourage self-pollination.
After the flower, the scape grows further as it
straightens itself. The seeds mature in a month or so and
the seed capsule contains hundreds of tiny, powder-like seeds. Seeds have a
distinctive surface pattern for each species.
Apart from the carnivorous nature of the butterwort
plants, people in Scandinavian countries have mixed the leaf extracts with fresh
milk to make it curdle.