March Plant Fact


Bromeliads – A Story Based Upon Unusual Leaves!


Bromeliad Bromeliad Bromeliad
Aechemea fasciata
Tillandsia fillifolia
Neoregelia ‘Gazpacho’
Aechmea chantinii 'Harvey's Pride'
Tillandsia cyanea


Tropical plants were always of interest to me - whether it was their bold foliage or brightly colored flowers, they always radiated that come hither appearance!  However, not having access to a greenhouse, nor adequate indoor space, I never devoted too much time or attention to this group.  Two years ago, I started dabbling in earnest with house plants and suddenly found myself learning a whole new plant vocabulary!  Of the many new plants that I discovered, I was drawn to the unusual diversity of foliar and flower colors of those plants that fell under the umbrella of Bromeliad.  Within one family, plants ranged from those that could seemingly live on air without any soil to those whose leaves literally maintained their own small pond.  As far as leaves and habitat were concerned, this was a family with a very interesting story to tell!   (Pictured at right, Aechmea chantinii 'Harvey's Pride')      
The Bromeliaciae or Bromeliad family contains 56 genera with over 3,000 species – talk about a large family get-together during the Holidays!  The family is named after the Swedish medical doctor and botanist Olaf Bromel (1639-1705).  Father Charles Plumier (1646-1704), a Franciscan brother, plant explorer and the botanist to King Louis XIV bestowed this honor on Dr. Bromel during the 1690’s.  Dr. Bromel was well known within the botanical world and Father Plumier thought it fitting to honor him with a new plant he found during one of his numerous travels to the West Indies.  At the time no one, including Father Plumier or Carl Linnaeus (1707-1778) who in 1753 recognized Bromelia as the accepted name, could have surmised the ultimate size of the family.

The original Bromeliad plants began to appear around 70 million years ago (MYA), in the Guayana Shield or Higlands of what is now Southern Venezuela and Guyana.  The Guayana Shield is geologically called a craton.  A craton is a stable and internal portion of a tectonic plate that has remained relatively undisturbed through the movement of the plates.  The Guayana Shield is located within the South American plate and consists of some of the oldest terrain in the world, with stone dating back 1.7 billion years.  In addition to being old, the soil is also nutrient poor, a plight which initiated a series of creative and ‘nontraditional’ adaptations for Bromeliads in their quest to obtain nutrients.  However, the search for nutrients is only part of the story that led to the diversity within this group.   The remaining chapters revolve around the incredible changes within the geography of South America.  Seventy million years ago, South America appeared vastly different than today.  The Andes had yet to rise, North and South America were not connected via Panama and the Amazon flowed not into the Atlantic, but the Pacific!  Around 20-25 MYA, the Andes Mountains began to lift, a process that continued to around 6 MYA and the two American continents became connected!  Coincidently, the diversification of Bromeliads began at roughly the same time, as did the gradual movement of the group into the Southern reaches of North America and even into the western portions of Africa.  Throughout this diversification, the family began to adapt, not only to the environs of high altitude rainforests of the rising Andes, but also to drier and colder desert conditions.  All the while, most species remained challenged to find nutrients and sources of water – a challenge that was resolved through the foliage.
The original ‘family’ that appeared in the Guayana Shield are represented today by the genus Brocchinia.  They are considered as a basal genus, that is, a genus that represents one of the original groups from which multiple new genera developed.  They were most likely terrestrial dwelling plants, depending upon the nutrients and water they obtained through their root system. Since the soil was low in nutrition, the species developed an ingenious ‘tactic’ which was repeatedly employed by many other genera in the family – the creation of a water tank called a phytotelma (plural phytotelmata)!  In most Bromeliads, the strap-like leaves originate at a central crown and radiate outward, much like the spokes of an umbrella.  The bases of the leaves overlap tightly, forming a vessel or tank that enables the plant to hold water.  (Pictured above is a phytotelma for Aechemea fasciata.)    The leaves assist in the filling of the tank, since they are keel shaped and direct rainwater or even heavy dews down the leaf and into the tank.  Aside from the phytotelma, another leaf adaptation that has aided survival is the modified trichomes or hairs on the leaves.  Gardeners mostly associate trichomes with pubescent or fuzzy leaves, much like that of Lamb’s Ears (Stachys byzantina) or the undersides of a Yak Rhododendron (Rhododendron yakushimanum) leaf; the function of the trichomes in these cases is to prevent desiccation by reducing the drying effects of the sun and/or wind.  However, there are numerous other types of trichomes with different shapes and function.  In Stinging Nettle (Urtica dioica), the hollow trichomes act to inject pain inflicting chemicals into the skin of passerby’s as a defense mechanism.  In Bromeliads, the leaves have peltate or shield-shaped trichomes, which to the naked eye resemble a leaf scale.  Under a microscope these ‘shields’ vary in appearance between genera but regardless, their primary function is for the absorption of water.  The cells that compose the shield are typically attached to a stack of cells called a stalk, which penetrates through the leaf epidermis or outer covering and provides a point of entrance for the water.
Why is the development of a phytotelma and the peltate trichomes so ingenious? The reasons are manifold!  First the obvious – it is a source of water!  Although helpful for the terrestrial dwellers, this function became especially beneficial for those species that became epiphytic. An epiphytic plant is one that lives not on the ground, but high in the canopy of a tree, cactus, cliff or some other structure that provides an elevated habitat. An epiphytic plant does not infer that it is parasitic, as they do not extract nutrients from the host plant, but rather use the host merely for support and the roots serve only as mechanisms of attachment.   Over the course of time, nearly half of the Bromeliad family have become epiphytic.  The recourse to seek higher ground was not a development that happened once over the past 70 million years, but happened on 3 separate occasions with the development of new species, each separated by millions of years!  The pressure for an elevated habitat had several advantages:  it eliminated the competition for water and nutrients on the forest floor; elevated the plant to a brighter location within the forest, enhancing the plants photosynthetic capabilities; and it allowed the flowers to be more easily seen and approached by pollinators.  The challenge for these arboreal dwellers then focused upon securing a source of water and nutrients. The development of a ‘water tank’ by the overlapping leaf bases clearly resolved the issue of a water source – providing that the tank can be frequently refilled.  Interestingly, even with a ready supply of water, the plants have proven to be frugal drinkers.  In Vriesea gigantean, an epiphytic dweller from Brazil, a study was conducted comparing the leaf bases, which serve to form the tank, to the remainder of the leaf.  The trichomes are both more numerous and larger in size at the leaf base as compared to the tips, allowing enhanced water transfer into the leaf.  However, enhance absorption does not equate to water usage.  The leaves limit the transfer of water throughout the upper portion of the leaf by reducing both the size and number of the tracheids and veins – the vascular system within the leaves – thus reducing water consumption.  Thus, during those chance occurrences when rainfall or heavy dews were lacking, the plant was able to stretch its water resources.  The challenge of securing nutrients also became a function of the phytotelma.   The water tank became a habitat for a number of insects, larvae, amphibians and other animals, many of whom spend their entire life in a ‘Bromeliad Pool’.  In exchange for the use of the community pool, the animals and insects provide nutrients in the form of urea and through the decay of detritus, all of which are absorbed into the leaf via the trichomes.  Not only are the leaves creating sugars through photosynthesis, they have now assumed the roles traditionally held by the roots!

Some Bromeliads with phytotelma took the process one step further and became carnivorous.  Referring back to the basal genus Brocchinia, B. reducta has a modified foliar arrangement whereby the leaves no longer radiate outward, but stand nearly vertical.  Many insects are attracted to ultraviolet light, a wavelength that is reflected by many flowers.  As a lure for insects, the leaves of B. reducta also reflect ultraviolet light and the phytotelma emits a sweet fragrance, serving as an attractant to ants.  The trichomes on the leaves are waxy and slippery, providing poor footing for a visiting insect.  Unable to secure a grip, the insect falls into the tank and drowns.  The liquid in the tank is very acidic with a pH near 3 and includes the enzyme phosphatase, which aid in dissolving the carcass and the ultimate release of nutrients for absorption via the trichomes. Devilish!

As the Andes continued to rise and this group of plants spread northward into Mexico and Southwestern North America, dryer and more desert-like regions were encountered.  Epiphytic Bromeliads started to appear in these drier regions around 15 MYA and another well recognized yet small subfamily took form:  the Tillandsioideae.  With only 9 genera, this family is probably best represented by the genus Tillandsia, which we know as Air Plants (Tillandsia fillifolia pictured on right).  One of the best known examples in the SE United States is Tillandsia usneoides or Spanish Moss.  Carol Linnaeus initially penned the genus name in honor of Elias Tillandz (1640–1693), a Swedish born doctor and botanist in Finland who would treat his patients with plants, based upon his botanical knowledge!  It is initially difficult to understand how ‘Air Plants’ and ‘Tank Plants’ – two plant groups that appear and sound as divergent as is worldly possible – are related.  Yet, related they are!

Tillandsioideae no longer relied on a tank for water supply, since the air is drier and the water would simply evaporate more quickly than it could accumulate.  Rather, this subfamily relies totally upon the peltate trichomes to gather water from dews or atmospheric moisture and to channel that moisture into the leaf!  The trichomes literally cover every portion of the leaf and, when dry, provide an attractive silver color to the foliage.  This silver coloring, along with the extensive degree to which they cover the leaf, also allows the trichomes to reflect much of the sunlight, allowing the leaf to remain cooler in hot, sunny locations. Air plants also rely upon the trichomes for nutrition.  In arid environments, Tillandsioideae will often be seen attached to cactus and in those environments, the only nutrition that avails itself are windblown particles or dust, which the trichomes capture.  In areas that receive more rainfall, Tillandsia have the advantage of capturing some of the rainwater as it runs down tree trunks or stone cliffs, which are often enriched with nutrients from various forms of decaying detritus.  Tillandsia can also have very attractive flowers too, as witness in Tillandsia cyanea (picture above on the left), often called the Quill Plant.

The story of Bromeliads is indeed fascinating, but I have yet to reflect upon one of the reasons why I was initially drawn to Bromeliads – the unusual and brightly colored foliage! (Pictured on right, Neoregelia ‘Gazpacho’)  Many of the leaves are awash with rich reds, purple and even yellows!  Great for the Garden or for enhancing a windowsill, but its purpose extends beyond simply keeping myself and other gardeners entertained.  Bromeliad flowers are often relatively small – especially when compared to the vast scale of the surrounding rainforest – or they are often nearly immersed in water within the center of the phytotelma, as is true of Neoregelia ‘Gazpacho’.  Bromeliads are primarily pollinated by Hummingbirds, but in order for the flower to be seen, the target needed to be made larger and more impressive.  Hence, the colored foliage!  Water supply, creation of carbohydrates through photosynthesis, nutrient capture and absorption, and pollinator attractor – indeed, a fascinating story that revolves around the diverse function of leaves!