Drosera

Drosera, which is commonly known as the sundews, is one of the largest genera of carnivorous plants, with at least 194 species. These members of the family Droseraceae lure, capture, and digest insects using stalked mucilaginous glands covering their leaf surfaces. The insects are used to supplement the poor mineral nutrition of the soil in which the plants grow. Various species, which vary greatly in size and form, are native to every continent except Antarctica.
Charles Darwin performed much of the early research into Drosera, engaging in a long series of experiments with Drosera rotundifolia, which were the first to confirm carnivory in plants. In an 1860 letter, Darwin wrote, "...at the present moment, I care more about Drosera than the origin of all the species in the world".

Etymology

The botanical name from the Greek δρόσος drosos "dew, dewdrops" refer to the glistening drops of mucilage at the tip of the glandular trichomes that resemble drops of morning dew. The English common name sundew also describes this, derived from Latin ros solis meaning "dew of the sun".
Drosera was first published in Carl Linnaeus' Species Plantarum in Volume I page 281.
The Principia Botanica, published in 1787, states "Sun-dew derives its name from small drops of a liquor-like dew, hanging on its fringed leaves, and continuing in the hottest part of the day, exposed to the sun".

Taxonomy

The unrooted cladogram to the right shows the relationship between various subgenera and classes as defined by the analysis of Rivadavia et al. The monotypic section Meristocaulis was not included in the study, so its place in this system is unclear. More recent studies have placed this group near section Bryastrum, so it is placed there below. Also of note, the placement of the section Regiae in relation to Aldrovanda and Dionaea is uncertain. Since the section Drosera is polyphyletic, it shows up multiple times in the cladogram .
This phylogenetic study has made the need for a revision of the genus even clearer.

Description

Sundews are perennial herbaceous plants, typically forming prostrate or upright rosettes between in height, depending on the species. Climbing species form scrambling stems which can reach much longer lengths, up to in the case of D. erythrogyne. Sundews have been shown to be able to achieve a lifespan of 50 years. The genus is specialised for nutrient uptake through its carnivorous behaviour, for example the pygmy sundew is missing the enzymes that plants normally use for the uptake of earth-bound nitrates.

Growth form

The genus can be divided into several habits, or growth forms:

Temperate sundews: These species form a tight cluster of unfurled leaves called a hibernaculum in a winter dormancy period. All of the northern Asian, European, and North American species belong to this group. Drosera arcturi from Australia and New Zealand is another temperate species that dies back to a horn-shaped hibernaculum.
Subtropical sundews: These species maintain vegetative growth year-round under uniform or nearly uniform climatic conditions.
Pygmy sundews: A group of roughly 40 Australian species, they are distinguished by miniature growth, the formation of gemmae for asexual reproduction, and dense formation of hairs in the crown centre. These hairs serve to protect the plants from Australia's intense summer sun. Pygmy sundews form the subgenus Bryastrum.
Tuberous sundews: These nearly 50 Australian species form an underground tuber to survive the extremely dry summers of their habitat, re-emerging in the autumn. These so-called tuberous sundews can be further divided into two groups, those that form rosettes and those that form climbing or scrambling stems. Tuberous sundews comprise the subgenus Ergaleium.
The Drosera petiolaris complex: A group of tropical Australian species, they live in constantly warm but sometimes wet conditions. Several of the 14 species that comprise this group have developed special strategies to cope with the alternately drier conditions. Many species, for example, have petioles densely covered in trichomes, which maintain a sufficiently humid environment and serve as an increased condensation surface for morning dew. The D. petiolaris complex comprises the section Lasiocephala.

Although they do not form a single strictly defined growth form, a number of species are often put together in a further group:

Queensland sundews: A small group of three species, all are native to highly humid habitats in the dim understories of the Australian rainforest.
Leaves and carnivory

Sundews are characterised by the glandular tentacles, topped with sticky secretions, that cover their leaves. The trapping and digestion mechanism usually employs two types of glands: stalked glands that secrete sweet mucilage to attract and ensnare insects and enzymes to digest them, and sessile glands that absorb the resulting nutrient soup. Small prey, mainly consisting of insects, are attracted by the sweet secretions of the peduncular glands. Upon touching these, the prey become entrapped by sticky mucilage which prevents their progress or escape. Eventually, the prey either succumb to death through exhaustion or through asphyxiation as the mucilage envelops them and clogs their spiracles. Death usually occurs within 15 minutes. The plant meanwhile secretes esterase, peroxidase, phosphatase and protease enzymes. These enzymes dissolve the insect and free the nutrients contained within it. This nutrient mixture is then absorbed through the leaf surfaces to be used by the rest of the plant.

All species of sundew are able to move their tentacles in response to contact with edible prey. The tentacles are extremely sensitive and will bend toward the centre of the leaf to bring the insect into contact with as many stalked glands as possible. According to Charles Darwin, the contact of the legs of a small gnat with a single tentacle is enough to induce this response. This response to touch is known as thigmonasty, and is quite rapid in some species. The outer tentacles of D. burmanni and D. sessilifolia can bend inwards toward prey in a matter of seconds after contact, while D. glanduligera is known to bend these tentacles in toward prey in tenths of a second. In addition to tentacle movement, some species are able to bend their leaves to various degrees to maximise contact with the prey. Of these, D. capensis exhibits what is probably the most dramatic movement, curling its leaf completely around prey in 30 minutes. Some species, such as D. filiformis, are unable to bend their leaves in response to prey.
A further type of leaf coloration has recently been discovered in a few Australian species. Their function is not known yet, although they may help in attracting prey.
The leaf morphology of the species within the genus is extremely varied, ranging from the sessile ovate leaves of D. erythrorhiza to the bipinnately divided acicular leaves of D. binata. The leaves of some Western Australian species are very small, down to a leafblade or lamina of just 1 mm diameter in the instance of Drosera occidentalis.
While the exact physiological mechanism of the sundew's carnivorous response is not yet known, some studies have begun to shed light on how the plant is able to move in response to mechanical and chemical stimulation to envelop and digest prey. Individual tentacles, when mechanically stimulated, fire action potentials that terminate near the base of the tentacle, resulting in rapid movement of the tentacle towards the centre of the leaf. This response is more prominent when marginal tentacles further away from the leaf centre are stimulated. The tentacle movement response is achieved through auxin-mediated acid growth. When action potentials reach their target cells, the plant hormone auxin causes protons to be pumped out of the plasma membrane into the cell wall, thereby reducing the pH and making the cell wall more acidic. The resulting reduction in pH causes the relaxation of the cell wall protein, expansin, and allows for an increase in cell volume via osmosis and turgor. As a result of differential cell growth rates, the sundew tentacles are able to achieve movement towards prey and the leaf centre through the bending caused by expanding cells. Among some Drosera species, a second bending response occurs in which non-local, distant tentacles bend towards prey as well as the bending of the entire leaf blade to maximise contact with prey. While mechanical stimulation is sufficient to achieve a localised tentacle bend response, both mechanical and chemical stimuli are required for the secondary bending response to occur.

Flowers and fruit

The flowers of sundews, as with nearly all carnivorous plants, are held far above the leaves by a long stem. This physical isolation of the flower from the traps is commonly thought to be an adaptation meant to avoid trapping potential pollinators. The mostly unforked inflorescences are spikes, whose flowers open one at a time and usually only remain open for a short period. Flowers open in response to light intensity, and the entire inflorescence is also heliotropic, moving in response to the sun's position in the sky.
The radially symmetrical flowers are always perfect and have five parts. Most of the species have small flowers. A few species, however, such as D. regia and D. cistiflora, have flowers or more in diameter. In general, the flowers are white or pink. Australian species display a wider range of colours, including orange, red, yellow or metallic violet.
The ovary is superior and develops into a dehiscent seed capsule bearing numerous tiny seeds. The pollen grain type is compound, which means four microspores are stuck together with a protein called callose.