Droseraceae

The sundew family, Droseraceae, is a group of carnivorous flowering plants. It contains about 180 species spread across three living genera, with the vast majority falling under the sundew genus *Drosera*. The family also includes the famous Venus flytrap (*Dionaea muscipula*) and the lesser-known waterwheel plant (*Aldrovanda vesiculosa*), each being the sole surviving species in its own genus. These plants grow on every continent except Antarctica.

These are herbaceous, carnivorous plants that can be annual or perennial. Their leaves grow in an alternating pattern and curl inward from the tip, with at least one leaf surface bearing hairs tipped with glue-producing glands. The flowers are bisexual, typically with three carpels and five each of sepals, petals, and stamens. Their pollen grains have three or more pores and are released in groups of four. Even though they eat insects, the flowers rely on insects for pollination and are usually white to purple, closing at night. The small seeds spread by wind and water.

Most Droseraceae species belong to *Drosera*, the sundews. *Dionaea* and *Aldrovanda* each have just one living species. Sundews catch prey using a sticky substance oozing from hairs on their leaves. *Dionaea* and *Aldrovanda* both use snap-traps that shut quickly when the leaves are touched. *Dionaea* lives on land, while *Aldrovanda* is fully aquatic. Like other carnivorous plants, Droseraceae supplement their nutrient intake—especially nitrogen—by trapping and digesting small animals like insects. This lets them thrive in nutrient-poor places such as sphagnum bogs.

*Drosera* is one of the largest genera of carnivorous plants, and its species vary widely in shape. All share highly modified leaves lined with tentacle-like glandular hairs. At the tip of each hair, a bead of very sticky mucilage is secreted, looking like a drop of dew. This mucilage is mostly water mixed with acidic polysaccharides, making it so sticky that a single drop can be stretched up to a meter and cover a million times its original area. Insects are drawn to its smell and get stuck. These are called flypaper traps, but calling sundew traps "passive" is misleading. They are quite active and sensitive: disturbing one or a few hairs triggers an electrical signal that makes other hairs bend toward the prey, and the leaf curls inward to wrap around the catch for digestion. Today, four subgenera of *Drosera* are recognized: *Regiae* and *Arcturia* each contain only one species (*D. regia* and *D. arcturi*), while the rest are split into two clades, *Ergaleium* and *Drosera*.

*Dionaea muscipula*, the Venus flytrap, is world-famous. Charles Darwin called it "one of the most wonderful in the world." Its leaves are highly modified into a snap-trap that shuts quickly when triggered. On the inner surface of the trap are three large trigger hairs. For the trap to close, two of these hairs must be touched within a certain time. The trap snaps shut by flipping its lobes from a concave to a convex shape. This movement can start as fast as 0.4 seconds after stimulation and finish in about one second.

*Aldrovanda vesiculosa*, the waterwheel plant, is a free-floating, rootless aquatic plant. It is less famous than the Venus flytrap but has a similar trap structure. In 1875, Darwin described it as "a miniature aquatic *Dionaea*." Its trap is aquatic, smaller, and faster than that of *Dionaea*. While *Dionaea* needs two touches to close, *Aldrovanda* only needs one, and its trap shuts about ten times faster.

The family's type genus, *Drosera*, was named by Linnaeus in 1753. The name comes from the Greek word "droseros," meaning "dewy" or "drops of water." A 1787 book, *Principia Botanica*, notes that the sundew gets its name from small, dew-like drops on its fringed leaves that remain even in the hottest sun.

In 1867, Bentham and Hooker placed six genera in Droseraceae: *Dionaea*, *Aldrovanda*, *Drosera*, *Drosophyllum*, *Byblis*, and *Roridula*. They grouped them based on insect traps that seemed to share a common origin, despite major differences in leaf and flower form. In 1922, *Byblis* and *Roridula* were moved to a new family, Byblidaceae (and later *Roridula* was split off into Roridulaceae). In the 1990s, evidence from both morphology and DNA began to show that *Drosophyllum*—another single-species genus (*Drosophyllum lusitanicum*)—differed from the rest of Droseraceae. *Drosophyllum* has a flypaper trap like *Drosera*, but it does not actively curl its leaves to envelop prey. This key difference, along with other traits like pollen structure, hair anatomy, and a woody stem with a deep taproot, led researchers to question its place in the family.