The butterworts are a group of carnivorous plants comprising the genus Pinguicula. Members of this genus use sticky, glandular leaves to lure, trap, and digest insects in order to supplement the poor mineral nutrition they obtain from the environments. Of the roughly 80 currently known species, 12 are native to Europe, 9 to North America, and the rest are found in northern Asia, South and Central America and southern Mexico.
The name Pinguicula is derived from a term coined by Conrad Gesner, who in his 1561 work entitled Horti Germaniae commented on the glistening leaves: "propter pinguia et tenera folia…" (lat. pinguis = fat). The common name "butterwort" reflects this characteristic.
Butterworts can be divided roughly into two main groups based on the climate in which they grow; each group is the further subdivided based on morphological characteristics. Although these groups are not cladistically supported by genetic studies, these groupings are nonetheless convenient for horticultural purposes.
Tropical butterworts either form somewhat compact winter rosettes composed of fleshy leaves or retain carnivorous leaves year-round. Temperate species often form tight buds (called hibernacula) composed of scale-like leaves during a winter dormancy period. During this time the roots (with the exception of P. alpina) and carnivorous leaves wither. Temperate species flower when they form their summer rosettes while tropical species flower at each rosette change.
Many butterworts cycle between rosettes composed of carnivorous and non-carnivorous leaves as the seasons change, so these two ecological groupings can be further divided according to their ability to produce different leaves during their growing season. If the growth in the summer is different in size or shape to that in the early spring (for temperate species) or in the winter (tropical species), then plants are considered heterophyllous; whereas uniform growth identifies a homophyllous species.
This results in four groupings:
The leaf blade of a butterwort is smooth, rigid, and succulent, usually bright green or pinkish in colour. Depending on species, the leaves are between 2 and 30 cm.(1-12") long. The leaf shape depends on the species, but is usually roughly obovate, spatulate, or linear.
Like all members of the family Lentibulariaceae, butterworts are carnivorous. In order to catch and digest insects, the leaf of a butterwort uses two specialized glands which are scattered across the leaf surface (usually only on the upper surface, with the exception of P. gigantea and P. longifolia ssp. longifolia). One is termed a peduncular gland, and consists of a few secretory cells on top of a single stalk cell. These cells produce a mucilagenous secretion which forms visible droplets across the leaf surface. This wet appearance probably helps lure prey in search of water (a similar phenomena is observed in the sundews). The droplets secrete only limited enzymes and serve mainly to entrap insects. On contact with an insect, the peduncular glands release additional mucilage from special reservoir cells located at the base of their stalks. The insect will begin to struggle, triggering more glands and encasing itself in mucilage. Some species can bend their leaf edges slightly by thigmotropism, bringing additional glands into contact with the trapped insect. The second type of gland found on butterwort leaves are sessile glands which lie flat on the leaf surface. Once the prey is entraped by the peduncular glands and digestion begins, the initial flow of nitrogen triggers enzyme release by the sessile glands. These enzymes, which include amylase, esterase, phosphatase, protease, and ribonuclease break down the digestible components of the insect body. These fluids are then absorbed back into the leaf surface through cuticular holes, leaving only the chitin exoskeleton of the larger insects on the leaf surface.
The holes in the cuticle which allow for this digestive mechanism also pose a challenge for the plant, since they serve as breaks in the cuticle (waxy layer) that protects the plant from desiccation. As a result, most butterworts live in humid environments.
Butterworts are usually only able to trap small insects and those with large wing surfaces. They can also digest pollen which lands on their leaf surface. The secretory system can only function a single time, so that a particular area of the leaf surface can only be used to digest insects once.
As with almost all carnivorous plants, the flowers of butterworts are held far above the rest of the plant by a long stalk, in order to reduce the probability of trapping potential pollinators. The single, long-lasting flowers are zygomorphic, with two lower lip petals characteristic of the bladderwort family, and a spur extending from the back of the flower. The calyx has five sepals, and the petals are arranged in a two-part lower lip and a three-part upper lip. Most butterwort flowers are blue, violet or white, often suffused with a yellow, greenish or reddish tint. P. laueana and the newly described P. caryophyllacea are unique in having a strikingly red flowers. Butterworts are often cultivated and hybridized primarily for their flowers.
The shape and colors of butterwort flowers are distinguishing characteristics which are used to divide the genus into subgenera and to distinguish individual species from one another.
Butterworts are distributed throughout the northern hemisphere (map). The greatest concentration of species, however, is in humid mountainous regions of Central America (including Mexico) and South America, where populations can be found as far south Tierra del Fuego. Australia is the only continent without any native butterworts.
Butterworts probably originated from modern day Central America, which is also the center of Pinguicula diversity; roughly 50% of butterwort species are found here. From there, the genus likely spread to Europe, before continental drift split the continents apart.
The great majority of Pinguicula species have a very limited distribution. The two butterwort species with the widest distribution - P. alpina and P. vulgaris - are found throughout much of Europe and North America. Other species found in the United States include P. caerulea, P.ionantha, P. lutea, P. macroceras, P. planifolia, P. primuliflora, P. pumila, and P. villosa.
In general, butterworts grow in nutrient poor, alkaline soils. Some species have adapted to other soil types, such as acidic peat bogs (ex. P. vulgaris, P. calyptrata, P. lusitanica), soils composed of pure gypsum (P. gypsicola and other Mexican species), or even vertical rock walls (P. ramosa, P. vallisnerifolia, and most of the Mexican species). A few species are epiphytes (P. casabitoana,P. hemiepiphytica, P. lignicola). Many of the Mexican species commonly grow on mossy banks, rock, and roadsides in oak-pine forests. Pinguicula macroceras ssp. nortensis has even been observed growing on hanging dead grasses. Each of these environments is relatively nutrient poor, allowing butterworts to compete for space.
Butterworts need habitats that are almost constantly moist or wet, at least during their carnivorous growth stage. Many Mexican species lose their carnivorous leaves, and sprout succulent leaves, or die back to onion-like "bulbs" to survive the winter drought, at which point they can survive in bone dry conditions. This moisture can be supplied by either a high groundwater table, or by high humidity or precipitation. Unlike many other carnivorous plants that require sunny locations, many butterworts thrive in part-sun or even shady conditions.
The first mention of butterworts in botanical literature is an entry entitled Zitroch chrawt oder smalz chrawt (trans: "zitroch" herb or lard herb) by Vitus Auslasser in his 1479 work on medicinal herbs entitled Macer de Herbarium. The name "Zittrochkraut" is still used for butterworts in Tirol, Austria.
In 1583, Clusius already distinguished between two forms in his Historia stirpium rariorum per Pannoniam, Austriam: a blue-flowered form (P. vulgaris) and a white-flowered form (Pinguicula alpina). Linnaeus added P. villosa and P. lusitanica when he published his Species Plantarum in 1753. The number of known species rose sharply with the exploration of the new continents in the 19th century; in 1844, 32 species were known.
It was only in the late 19th century that the carnivory of this genus began to be studied in detail. In a letter to Asa Gray dated June 3rd 1874, Charles Darwin mentioned his early observations of the butterwort's digestive process and insectivorous nature. (See Gray Herbarium of Harvard University (103).) Darwin studied these plants extensively. S. J. Casper's large 1966 monograph of the genus included 46 species, a number which has almost doubled since then. Many exciting discoveries have been made in recent years, especially in Mexico. Another important development in the history of butterworts is the formation of the International Pinguicula Study Group, an organization dedicated to furthering the knowledge of this genus and promoting its popularity in cultivation, in the 1990s.
Butterworts also produce a strong bactericide which prevents insects from rotting while they are being digested. According to Linnaeus, this property has long been known by northern Europeans, who applied butterwort leaves to the sores of cattle to promote healing. Additionally, butterwort leaves were used to curdle milk and form a buttermilk-like fermented milk product called filmjölk (Sweden) and tjukkmjølk (Røros, Norway).
A detailed study of the phylogenetics of butterworts by Cieslak et al. (2005) found that all of the currently accepted subgenera and many of the sections were polyphyletic. The diagram below gives a more accurate representation of the correct cladogram. Polyphyletic sections are marked with an *.
┌────Clade I (Sections Temnoceras *, Orcheosanthus *, Longitubus,
│ Heterophyllum *, Agnata *, Isoloba *, Crassifolia)
┌──────┤ └────Clade II (Section Micranthus * = P. alpina)
┌───┤ └────────Clade III (Sections Micranthus *, Nana)
───┤ └───────────────Clade IV (Section Pinguicula)
└───────────────────Clade V (Sections Isoloba *, Ampullipalatum, Cardiophyllum)