Pinguicula
Pinguicula, commonly known as butterworts, is a genus of carnivorous flowering plants in the family Lentibulariaceae. They use sticky, glandular leaves to lure, trap, and digest insects in order to supplement the poor mineral nutrition they obtain from the environment. 126 species are currently accepted. 13 are native to Europe, 9 to North America, and some to northern Asia. The largest number of species is in South and Central America.
Etymology
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…". The common name "butterwort" reflects this characteristic.Characteristics
The majority of Pinguicula are perennial plants. The only known annuals are P. sharpii, P. takakii, P. crenatiloba, and P. pumila. All species form stemless rosettes.Habitat
Butterworts can be divided roughly into two main groups based on the climate in which they grow; each group is then 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 form somewhat compact winter rosettes composed of fleshy leaves or retain carnivorous leaves year-round. They are typically located in regions where water is least seasonally plentiful, as too damp soil conditions can lead to rotting. They are found in areas in which nitrogenous resources are known to be in low levels, infrequent or unavailable, due to acidic soil conditions.
Temperate species often form tight buds composed of scale-like leaves during a winter dormancy period. During this time the roots 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 or in the winter, then plants are considered heterophyllous; whereas uniform growth identifies a homophyllous species.
This results in four groupings:
- Tropical butterworts: species which do not undergo a winter dormancy but continue to alternately bloom and form rosettes.
- *Heterophyllous tropical species: species that alternate between rosettes of carnivorous leaves during the warm season and compact rosettes of fleshy non-carnivorous leaves during the cool season. Examples include P. moranensis, P. gypsicola, and P. laxifolia.
- *Homophyllous tropical species: these species produce rosettes of carnivorous leaves of roughly uniform size throughout the year, such as P. gigantea.
- Temperate butterworts: these plants are native to climate zones with cold winters. They produce a winter-resting bud during the winter.
- *Heterophyllous temperate species: species where the vegetative and generative rosettes differ in shape and/or size, as seen in P. lutea and P. lusitanica.
- *Homophyllous temperate species: the vegetative and generative rosettes appear identical, as exhibited by P. alpina, P. grandiflora, and P. vulgaris.
Roots
Leaves and carnivory
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 long. The leaf shape depends on the species, but is usually roughly obovate, spatulate, or linear. They can also appear yellow in color with a soft feel and a greasy consistency to the leaves.Like all members of the family Lentibulariaceae, butterworts are carnivorous. The mechanistic actions that these plants use to lure and capture prey is through a means of sticky or adhesives substances that are produced by mucilage secreted by glands located on the leaf's surface. In order to catch and digest insects, the leaf of a butterwort uses two specialized glands which are scattered across the leaf surface.
One is termed a peduncular gland, and consists of a few secretory cells on top of a single stalk cell. These cells produce a mucilaginous secretion which forms visible droplets across the leaf surface. This wet appearance probably helps lure prey in search of water. The droplets secrete limited amounts of digestive 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 entrapped 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 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.
Unlike many other carnivorous plant species, butterworts do not appear to use jasmonates as a control system to switch on the production of digestive enzymes. Jasmonates are involved in the butterwort's defense against attacking insects, but not in its response to prey. Of the eight enzymes identified in the digestive secretions of butterworts, alpha-amylase appears to be unique when compared to other carnivorous plants. This research suggests that butterwort may have co-opted a different set of genes in its development of carnivory.
Flowers
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.