The Sundew, a carnivorous plant belonging to the genus Drosera, employs a two-stage mechanism to capture and consume insect prey. This genus, comprising over 190 species, is characterized by leaves covered in glandular tentacles that glisten with a sticky substance, giving the plant its common name. The plant uses this unique leaf structure as an active flypaper trap to supplement its nutritional intake.
The Mechanism of Adhesion: Sticky Mucilage
The primary trapping mechanism relies on the mucilage, a highly viscous substance secreted from the tips of the stalked glands, or tentacles, covering the leaf surface. This adhesive liquid is not simply water but a complex, acidic hydrogel. Chemical analysis reveals its composition is approximately 4% acidic polysaccharides, which are long, complex sugar chains featuring repeating dimers of glucuronic acid and mannose residues.
The mucilage’s high viscosity and elasticity are crucial for trapping, acting as a physical restraint that prevents small insects from escaping. Cations, such as calcium and magnesium, are abundant in the secretion and help cross-link the polysaccharide chains, giving the glue its remarkable stickiness. These glistening droplets, which resemble morning dew, function as a visual lure, enticing insects to land on the leaf surface where they become instantly entrapped. The initial capture is a passive process, relying entirely on the physical properties of this specialized biological adhesive.
The Active Wrapping Response
Once an insect is stuck, its struggles and the presence of chemical compounds trigger an active response from the plant. This movement is known as thigmonasty, a non-directional response to touch, combined with a chemonastic response to chemical stimuli. The initial contact and movement generate electrical signals, or action potentials, that travel from the tentacle head down to its base.
The tentacles in direct contact with the prey begin to bend inward, quickly securing the insect and increasing the surface area touching the adhesive glands. This rapid bending is followed by a slower, more pronounced movement of the surrounding marginal tentacles and, in some species, the entire leaf blade. This secondary movement is largely driven by the recognition of chemical cues, such as nitrogenous compounds or the plant hormone jasmonate, which signal the presence of a viable meal. The purpose of this wrapping is to create a temporary “outer stomach,” maximizing contact between the prey and the digestive glands for efficient nutrient extraction.
Nutrient Digestion and Absorption
With the prey secured and fully enclosed by the tentacles, the plant begins the final stage by releasing a cocktail of digestive enzymes from the glandular heads. The digestive fluid contains enzymes such as proteases, which break down proteins, and phosphatases, which help cleave phosphate groups from organic molecules.
A specific S1-type nuclease enzyme has been identified to primarily break down nucleic acids like RNA, facilitating the uptake of phosphates from the prey. This enzymatic breakdown occurs externally, effectively liquefying the insect’s internal contents. The resulting nutrient-rich “soup,” containing primarily nitrogen and phosphorus, is then absorbed directly through the surface of the tentacle glands and the leaf itself. The indigestible, chitinous exoskeleton of the insect is typically left behind once the digestion process is complete, which can take a few days to several weeks depending on the prey size.
Why Sundews Must Consume Insects
Sundews thrive predominantly in nutrient-poor environments, such as acidic bogs, fens, and swamps. The waterlogged, acidic soil conditions in these areas inhibit the decomposition of organic matter, making essential minerals like nitrogen and phosphorus scarce or unavailable to plant roots.
Carnivory serves as a means of supplementing these specific soil deficiencies. This adaptation allows the plants to maintain healthy growth and flower in places where competition from non-carnivorous plants is low due to the harsh soil conditions.