The natural world is full of remarkable adaptations, and one that often sparks curiosity is the phenomenon of plants being sticky. From leaves that feel tacky to the touch to seeds that cling to clothing, this characteristic serves a surprising array of purposes in the plant kingdom. While it might seem like a simple trait, the stickiness of plants is a complex result of specialized biological processes and chemical compounds.
How Plants Become Sticky
Plants achieve stickiness through several biological and physical mechanisms, involving specialized structures and the compounds they produce. Glandular hairs, known as trichomes, are epidermal outgrowths that secrete sticky substances. Trichomes have a head containing secretory cells that produce and store various compounds. For example, tomato plants have type IV glandular trichomes that secrete sticky substances when disturbed.
Mucilage is another widespread sticky substance produced by plants. This thick, gel-like material is primarily composed of polysaccharides, which are long chains of sugar molecules. Mucilage can also contain proteins, lipids, minerals, and water, with its specific composition varying by plant species. This slime is released by cells to form a sticky layer or droplets on plant parts like stems, leaves, flower buds, or seeds.
Resins also contribute to plant stickiness, particularly in trees. These complex mixtures of organic compounds are secreted in specialized surface glands or internal ducts. Sap, the fluid circulating within a plant’s vascular system, can also be sticky, though it is less viscous than resin. While sap primarily carries water, nutrients, and hormones, it can ooze from damaged areas and become sticky upon exposure to air.
The Many Purposes of Plant Stickiness
Plant stickiness serves diverse evolutionary and ecological functions, helping plants survive and thrive in their environments. One notable purpose is insect trapping, especially for carnivorous plants. These plants, like sundews, use sticky mucilage on their leaves to ensnare prey, which they then digest to acquire nutrients. The sticky secretions can act like a glue, preventing insects from escaping and often leading to their eventual suffocation.
Stickiness also acts as a defense mechanism against herbivores and pathogens. Sticky substances can physically trap or deter insects and other small arthropods, immobilizing them on the plant surface. Some sticky compounds contain toxic substances, which can harm pests that come into contact with or ingest them. For instance, wild tobacco plants produce sticky glandular trichomes that can trap insects, deterring herbivory and even attracting beneficial predatory insects that feed on the trapped pests.
Seed dispersal is another important function of plant stickiness. Many plants have evolved sticky seeds or fruits that attach to the fur, feathers, or clothing of passing animals. This allows the seeds to be carried away from the parent plant and dispersed to new locations. These adhesive structures feature hooks, barbs, or a sticky coating to ensure effective attachment.
In some cases, sticky coatings can also aid in water retention and protection against environmental stressors. Waxy, sticky cuticles on plant surfaces can reduce water loss, particularly in harsh or dry conditions. This protective layer forms a barrier that helps retain moisture within plant cells. While not always overtly sticky, some plant exudates, including mucilages and resins, can help seal wounds and prevent excessive water evaporation from damaged tissues.
Diverse Examples of Sticky Plants
Sundews (genus Drosera) are an example of carnivorous plants utilizing stickiness for nutrient acquisition. Their leaves are covered with numerous glandular tentacles, each tipped with a glistening droplet of sticky mucilage. When an insect lands on a sundew leaf, it becomes entrapped by this adhesive fluid, and the tentacles slowly bend inward to further secure and digest the prey. The mucilage also contains enzymes that break down the insect’s body for absorption.
Tobacco plants, such as Nicotiana attenuata, demonstrate stickiness as a defense strategy. These plants possess glandular trichomes that secrete a sticky resin, physically trapping small insects like flies, gnats, and aphids. This stickiness not only directly deters herbivores but also attracts predatory insects, such as the spined stilt bug, which feed on the trapped pests, providing an indirect defense for the plant. This mutualistic relationship reduces damage to the tobacco plant.
Burdock (Arctium species) provides an example of sticky seeds for dispersal. The plant produces burrs, which are the dried flower heads containing seeds. These burrs are covered with numerous tiny, inward-curving hooks that readily cling to animal fur and clothing. This attachment mechanism allows burdock seeds to be carried over long distances, facilitating their spread. The effectiveness of burdock’s sticky burrs even inspired the invention of Velcro.