The Australian sundew, a member of the diverse Drosera genus, is a carnivorous plant that has evolved a specialized method for capturing and digesting small insects. Australia is a global center for sundew diversity, hosting well over half of the approximately 200 known species worldwide. These plants supplement their diet by trapping small animals, primarily arthropods. This unique adaptation allows them to thrive in harsh environments where other plants struggle, using a “flypaper” mechanism to secure their meals.
The Anatomy of Capture
The capture mechanism begins with the plant’s leaves, which are covered in hundreds of specialized, hair-like structures known as glandular tentacles or trichomes. These tentacles are highly flexible and vary in length, with those near the leaf margins often being much longer than those in the center. Each tentacle consists of a stalk topped by a multicellular glandular head.
The glandular head produces and secretes a clear, highly viscous substance called mucilage. This mucilage forms a glistening, bead-like droplet at the tip of each tentacle, giving the plant its common name, sundew, because the droplets resemble morning dew. This sticky secretion is primarily composed of acidic polysaccharides and contains various ions like calcium and magnesium.
The Physical Trapping Mechanism
The process of catching an insect begins with the lure. The leaves, often brightly colored red or pink, combined with the glistening mucilage, attract small flying or crawling insects. Some species may also emit a faint, sweet scent from the glandular heads, further enticing prey toward the sticky surface.
Once an insect makes contact with the leaf, the mucilage acts as a powerful adhesive, immediately ensnaring the prey. The sticky glue is remarkably elastic and effectively prevents the insect from pulling free, even as it struggles. The more the insect struggles, the more it becomes coated in the thick, polymer-rich secretion, which physically immobilizes it and can lead to suffocation.
The physical contact and chemical stimulation from the struggling insect trigger a rapid response in the surrounding tentacles. This movement, known as thigmonasty, causes the tentacles to bend inward toward the prey and the center of the leaf. This envelopment maximizes the number of glandular heads touching the insect, ensuring the prey is securely held and fully coated in the digestive fluid. The speed of this movement varies greatly among species; some outer tentacles can bend in a matter of seconds, while others take minutes or even hours to completely wrap the prey.
Nutrient Extraction and Digestion
After the insect is securely trapped, the plant shifts from physical capture to chemical breakdown and nutrient extraction. The glandular heads, now in close contact with the insect, release a cocktail of digestive enzymes onto the prey. These enzymes function similarly to those in an animal’s stomach, dissolving the soft internal tissues of the captured insect.
The primary enzymes released include proteases, which break down proteins, and chitinases, which degrade the insect’s hard exoskeleton made of chitin. Other enzymes like lipases and nucleases are also present to break down fats and nucleic acids, ensuring a comprehensive breakdown of the prey. The result is a nutrient-rich “soup” that the plant can absorb.
The nutrients, particularly nitrogen and phosphorus, are then absorbed directly through specialized sessile glands or the tentacle heads themselves. Nitrogen is especially important for the plant to build proteins and DNA. Once digestion and absorption are complete—a process that can take several days—the tentacles slowly uncurl and the leaf reopens, leaving behind only the desiccated, indigestible exoskeleton.
Why Sundews Developed Carnivory
The evolution of carnivory in Drosera is a remarkable adaptation to the challenging environments where they grow. Sundews naturally inhabit ecological niches defined by extremely poor soil quality, such as acidic bogs, peaty swamps, and sandy areas. These habitats are often waterlogged and lack adequate concentrations of essential macronutrients.
The soil in these areas is severely deficient in nitrogen and phosphorus, two elements necessary for plant growth. Carnivory is a survival strategy that allows the plant to bypass the nutrient-scarce soil by obtaining these missing elements directly from animal prey. Some pygmy sundew species, for example, have even lost the ability to use soil-bound nitrate because they lack the necessary enzymes. Supplementing their diet with insects allows sundews to flourish where other plants, which rely solely on root absorption, cannot compete.