Drosera

Sundews, the common name for the genus Drosera, are among the largest groups of carnivorous plants, with at least 194 species. They belong to the family Droseraceae and use sticky, stalked glands on their leaves to attract, trap, and digest insects. This diet makes up for the lack of minerals in the poor soil where they live. The plants come in many sizes and forms and are found on every continent except Antarctica.

Early research on Drosera was largely carried out by Charles Darwin, who ran many experiments on Drosera rotundifolia. These were the first to prove that plants could be carnivorous. In an 1860 letter, Darwin wrote that he cared more about Drosera than about the origin of all the species in the world.

Etymology

The name Drosera comes from the Greek word drosos, meaning "dew," referring to the shiny drops of mucilage at the tips of the glandular hairs, which look like morning dew. The English name "sundew" has the same idea, from the Latin ros solis, or "dew of the sun." Carl Linnaeus first published the genus in his Species Plantarum in 1753. A 1787 book, Principia Botanica, notes that the plant gets its name from small, dew-like drops that hang on its fringed leaves and stay even in the hottest part of the day under the sun.

Taxonomy

The cladogram on the right, based on a study by Rivadavia and colleagues, shows relationships among subgenera and sections. The single-species section Meristocaulis was not included in that study, so its position is uncertain. More recent work places it near section Bryastrum.

The position of section Regiae relative to the genera Aldrovanda and Dionaea is also unclear. Because section Drosera is polyphyletic, it appears multiple times in the cladogram. This study highlights the need for a revision of the genus.

Description

Sundews are usually perennial, rarely annual, herbaceous plants. They form prostrate or upright rosettes from 1 to 100 cm tall, depending on the species.

Climbing species have scrambling stems that can reach up to 3 m, as in D. erythrogyne. Some sundews can live for 50 years. The genus is specialized for getting nutrients through carnivory; for example, pygmy sundews lack the enzyme nitrate reductase, which plants normally use to take up nitrates from the soil.

The genus can be divided into several growth forms. Temperate sundews form a tight cluster of unfurled leaves called a hibernaculum during winter dormancy. All northern Asian, European, and North American species belong to this group. Drosera arcturi from Australia, Tasmania, and New Zealand is another temperate species that dies back to a horn-shaped hibernaculum.

Subtropical sundews grow year-round under steady conditions. Pygmy sundews, about 40 Australian species, are tiny, reproduce asexually via gemmae, and have dense hairs in the crown center that protect them from intense sun. They form the subgenus Bryastrum.

Tuberous sundews, nearly 50 Australian species, survive dry summers by forming underground tubers and re-emerge in autumn. They can be rosette-forming or climbing, and make up the subgenus Ergaleium. The Drosera petiolaris complex includes 14 tropical Australian species that live in warm, sometimes wet conditions.

Some have petioles covered in hairs that keep humidity high and collect morning dew. This complex belongs to section Lasiocephala. A small group of three species—D. adelae, D. schizandra, and D. prolifera—are often called Queensland sundews. They live in humid, dim understories of Australian rainforests.

Leaves and carnivory

Sundews are known for the glandular tentacles on their leaves, which are topped with sticky secretions. The trapping and digestion process usually uses two types of glands: stalked glands that produce sweet mucilage to attract and trap insects, and enzymes to digest them, and sessile glands that absorb the nutrient mixture. Some species, like D. erythrorhiza, lack the sessile glands.

Small prey, mostly insects, are drawn to the sweet secretions. When they touch the mucilage, they become stuck and cannot escape. They usually die within 15 minutes, either from exhaustion or because the mucilage clogs their breathing tubes.

The plant then releases enzymes—esterase, peroxidase, phosphatase, and protease—that dissolve the insect and free its nutrients. These nutrients are absorbed through the leaf surfaces and used by the plant. All sundew species can move their tentacles.