Epiphytic plants need to find a way to compensate for what they do not have — roots that draw nutrients from the soil. Bromeliads like the green wild pine (Tillandsia utriculata) pictured above with vase-shaped shoots — or ‘tanks’ — compensate by collecting moisture and accumulations of decomposing litter (leaves, insects, …). Foliar trichomes (pubescences) on these leaves ‘absorb’ nutrients from the accumulated debris which often remains moist. These bromeliads sometimes are referred to as ‘ponds in the sky’, as their ‘water tanks’ create a micro habit for frogs, insects induing specialized mosquito-species (Wyeomyia, and other critters.
By contract, bromeliads that lack tanks — atmospheric bromeliads — like the Spanish moss (Tillandsia usneoides) pictured above do not have a continuous ‘tank’ of nutrients on which to draw. Atmospheric bromeliads must capture their nutrients sporadically during storms or shortly thereafter before their leaf surfaces dry. Look closely and you will see the very specialized hygroscopic (capable of absorbing moisture from air) trichomes that quickly ‘open’ in response to moisture and quickly close to prevent dehydration during dry conditions.