The leaves of some bromeliads capture water and nutrients in a storage tank via hydrophobic leaf surfaces.

Some plants included in the family Bromeliaceae, such as pineapples, have a unique surface on their leaves that enables them to collect water in a central tank where it can be absorbed and utilized. This adaptation does not occur in all bromeliads, but just those that grow in areas where there may be less access to nutrients (such as hanging on trees where they rely on nutrients dissolved in rainwater).  
The leaves of these types of bromeliads have a convex shape, meaning that they form a curved arch away from the surface they grow on. This shape allows water to drip downward into the central tank, pulled by the force of gravity. The interior, concave shape of the leaf also aids in the gathering and shuttling of water. The edges of the leaf bend upward, creating a structure that looks like a miniature half-pipe. This water collection is helpful for the bromeliad because it enables the plant to collect life-sustaining nutrients from the standing water over a longer period of time. 
The leaves of bromeliads are also coated in small surface cells that are raised like bumps, known as trichomes. These “bumpy” cells have tiny hairs that catch water as it drops. The hairs increase in number as the water moves closer toward the base of the leaves, where the tank is formed. The small hairs on the leaves are coated in tiny, hydrophobic (i.e., water-repelling) wax crystals. Because the wax crystals do not absorb any water, the water rolls off of them until it collects in the central tank. The hairs are several millimeters higher than the outer surface of the leaf and thus hold the water above the leaf itself, keeping water from contacting the leaf surface proper. This is important because the surface proper is not necessarily covered in the same hydrophobic wax as the hairs, and thus water could “stick” to this surface if it came in contact with it.
As the hydrophobic properties of the leaves direct water downward to the plant’s center, a pool forms, acting as a water reserve of dissolved nutrients.

This summary was contributed by Ashley Meyers.


“Hydrophobic leaf surfaces of Bromeliaceae possess a highly irregular microrelief, thereby reducing the adhesion and spread of water on the leaf blade. Hydrophobic trichome layers occur on the abaxial leaf blade surfaces of many mesic Type 1 pitcairnioids and, as these species exhibit the putative primitive ecological condition, water repellency appears to have been an important condition in early Bromeliaceae. The trichomes of Type 4 species are specialized for the alternative function of water and nutrient absorption from a water-filled tank, with epicuticular wax powders employed by some species to shed water from the leaf blades. Hydrophobic trichome layers and wax powders could potentially obstruct pathogens and particulates, aid in self-cleaning, and/or maintain gas exchange during wet weather.” (Pierce et al. 2001:1379)

Journal article
Hydrophobic Trichome Layers and Epicuticular Wax Powders in BromeliaceaeAmerican Journal of BotanyFebruary 2, 2007
Simon Pierce, Kate Maxwell, Howard Griffiths, Klaus Winter

“So successful are these techniques for sending seeds up into the canopy that the massive branches of many forest trees are often densely lined with squatters. These are known as epiphytes and among the commonest are bromeliads. They anchor themselves by wrapping their roots around the branch. Their long leaves grow in a tight rosette around their central bud and channel rain water down to it so that the rosette fills and forms a small pond.” (Attenborough 1995:166)

The Private Life of PlantsJanuary 19, 2017
David Attenborough

“Leaves with waxy trichomes are extremely water repellent… The crucial factor of superhydrophobicity in…leaves is given by the hairs, several hundreds of micrometres high, which are superimposed by a layer of small hydrophobic wax crystals… These surfaces are superhydrophobic, but the water droplets do not penetrate between the hairs; thus, small particles from the leaf surface would not be removed by rinsing with water… Such hairy systems may also be extremely useful for underwater systems because they minimize the wetted area of immersed surfaces and therefore may greatly reduce drag, as well as the rate of bio?lm formation, and are of great interest in biomimetics.” (Koch and Barthlott 2009:1496)

Journal article
Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materialsPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering SciencesJanuary 19, 2017
K. Koch, W. Barthlott

“…Although most studied functional changes were not directly associated with the transition from atmospheric to tank form, our results are consistent with the notion that the atmospheric stage is broadly associated with increased drought tolerance, whereas (larger) tanks allow improved access to nutrients.” (Zotz et al. 2004: 1350).

Journal article
Physiological and anatomical changes during the early ontogeny of the heteroblastic bromeliad, Vriesea sanguinolenta, do not concur with the morphological change from atmospheric to tank formPlant, Cell & EnvironmentJanuary 19, 2017