Plants 101: Epiphytes and Air Plants

The Sill seeks to educate and inspire all plant lovers. In our plant novice series, Plants 101, we introduce some of our favorite plant types, explore where they come from, and gently advise how to take care.

What is an Epiphyte?

For as long as there have been vascular plants, there have been epiphytes. Some of the earliest epiphytes were “Club Mosses”- primitive vascular plants like Selaginella. Those evolved about 400 MYA. Throughout the ages, other plants have evolved to be epiphytic as well. There are many families of plants, each with unique adaptations to fit an epiphytic lifestyle. One thing in common, though, is the need for high humidity or frequent showers.

The term epiphyte is translated from the Greek (epi = on top of; phyte = plant). Epiphytes or ‘air plants’ are plants that grow on top of other plants (typically trees) co-existing in the most harmonious, harmless way. They derive their nutrients and other vitals from the air, water, dust, and debris around them. Air plants grow without soil, but may grow in some substrate such as leaf debris or moss caught in between tree branches. To acquire nutrients without soil is a tough struggle for epiphytes, but because they do not interact with soil, many epiphytes have different adaptations for surviving their nutrient-poor conditions.

No Soil, No Problem 

Epiphytes like Tillandsia get nutrients through dust and debris that get caught in their trichomes (hairs). Others like Asplenium (Bird’s Nest Ferns) cull nutrients from leached water. Debris, animal droppings, and even old leaves leach nutrients into water — sort of like steeping tea. This water drips down the tree during a rainstorm and the roots of the Asplenium absorb the nutrient ‘tea’ as it runs over the roots.

Ever industrious, some epiphytes solve their nutrition woes by farming bacteria and fungi. In various orchid roots, for example, symbiotic fungi and bacteria are farmed by the plant to fix nitrogen from the air into amino acids. The orchid then takes the amino acids from the fungi or harvests the bacteria. Ever seen fungi growing along a tree? That fungi takes minerals from the bark and the orchid then taps into that fungi to get its fix of trace minerals, all in exchange for sugar — a symbiotic buffet, indeed. Still, other epiphytes like several species of bromeliads, can take it one step further and become...carnivorous.

Aside from of how epiphytes acquire their nutrients, they require humid, moist environments with frequent rains to live their best lives. Without soil, there is no way for epiphytes to acquire water other than through rainfall. The water makes contact for a brief few seconds then drains off down to the ground. Along with frequent rains to keep them moist, high humidity environments keep epiphytes from drying out. But of course, there is a catch. Although it may rain a lot in these humid environs, the sun is strong and dries these plants out good and fast. Because of this, many epiphytes have evolved succulent traits.

For example, some orchids have evolved pseudobulbs — thickened above-ground stems that look like bulb— to store water to help them survive too much tropical sun exposure. Others like bromeliads have evolved thick, waxy leaves, and CAM photosynthesis — an alternative form of photosynthesis where they can close their pores in the daytime to save water.

Like succulents, epiphytes are not necessarily related to one another and many unrelated plants are epiphytes. Remember, “epiphyte” refers to the growth form, not the relatedness of these plants.

It’s interesting to note that because, though epiphytes are constantly salt-deprived in their native environments, they are extra good at quick salt absorption. Rain water is extra pure, and often salt-less. Therefore, these plants have to grab a salt when it passes by in the runoff. What this means is that if you water them with regular tap water, which is full of salts, they will overload themselves with salts. It’s best to use distilled, warm water.

The function of the roots in epiphytic lineages is somewhat unclear. What we can say is they are obviously used as an anchor and holdfast. It is debatable whether or not the roots retain the ability to absorb water. Suberin— a waxy, rubbery compound that is used to repel or contain water when used in the lining of plant cells— is produced in the roots, as seen in many epiphytic lineages. It makes little sense, as this likely reduces the absorptive capabilities of the roots. However, in other monocotyledonous lineages, such as the family Orchidaceae (orchid family), suberinization of the roots actually benefits some epiphytic species, as they are able to not only absorb water better, but also retain water and use their roots as storage. What we can conclude about suberin is that, it appears if used in a certain way, suberin may not actually interfere with the absorptive capabilities of the roots, but actually improve it.

We are Family

Let’s take a look at the families that live under the epiphyte roof:

The Bromeliad Family (Bromeliaceae)

Bromeliaceae is currently placed in the larger plant order, Poales, the order of grassy monocots. This economically important family includes Spanish Moss (not a true moss but Tillandsia usneoides), pineapples, Tillandsia and others. Plants in Bromeliaceae are neotropical (native to the the Americas). Only one species, Pitcairnia feliciana, is native to Africa, although it is theorized that birds may have carried that one over to Africa where it has since speciated. This is analogous to how (unrelated to Bromeliaceae) Cactaceae are all native to the Americas, but one species of Rhipsalis is native to Africa. Members of Bromeliaceae (bromeliads) are herbaceous and evergreen, rarely shrubby, perennials. Bromeliads are adapted to the scrub/dry-forest biome, with high humidity and periods of drought. 

Bromeliad evolution has trended towards epiphytism with many species possessing reduced, suberin-fortified roots, and a wide array of diversity in trichome structure, which affects water absorption by the plant. Most plants in Bromeliaceae use CAM photosynthesis, opening their stomata at night. For the epiphytic species, they cannot be picky about when water is available, and thus have specialized trichomes on their leaf surfaces that can absorb water at anytime. They can be terrestrial or epiphytic, with leaves spirally arranged (although in a few species are distichous) around a central stem. Leaves can be colorful ranging from all green to all different colors and variegations, with entire, serrate, or serrulate margins that always possess peltate trichomes (hairs) for absorption of nutrients. Inflorescences (flowering structures) terminal, usually with sessile, sometimes fragrant flowers sheathed by colorful bracts. Many species’ inflorescences are long-lasting.

Many of the terrestrial bromeliads have evolved watertight cups in the center of their rosette to catch rainwater, and others have evolved more specialized trichomes to capture mist and rain water without evolving a cup structure. These adaptations, coupled with several CAM photosynthesis-evolution events, pave the way for the trend of epiphytism within the family. In cup-bearing bromeliads, such as Guzmania lingulata and Vriesea splendens, the leaf petioles (stalks) have either fused or grow so tightly that water can pool in the plant. The peltate trichomes not only swell, but uplift exposing more surface area of the plant to the water, maximizing absorption. For some species, the roots have degenerated so much that this may be the only means of water absorption. Relationships with poison-dart frogs spawning in these cups help keep the plants safe from herbivory. In other cup-bearing lineages, only three species of bromeliads ever have gone to the extreme of being carnivorous: Brocchinia reducta, Brocchinia hechtioides and Catopsis berteroniana. Although it is argued that these species are not truly carnivorous, as they do not produce digestive enzymes, a recent study has found that they do produce phosphatase, a digestive enzyme, even though the bulk of digestion is done by the bacteria that live in the cup of the bromeliad.

The evolution of carnivory is due to the nutrient poor environment that these bromeliads have evolved in—the tall and towering tepuis of South America. These extremely tall plateaus jet into the clouds, and are referred to as tabletop mountains, rising abruptly into the air. These geologic formations of Precambrian quartz and sandstone are so tall that the climate on the top of the plateau is different from the surrounding lowlands. The incessant rainfall washes the nutrients off of the tepuis and it is cooler and windier than in the lowlands. To survive, these plants have evolved to absorb nutrients from whatever falls into their cups, including insects.

In epiphytic, non-cup-forming lineages, such as Tillandsia, the entire leaf is covered in absorptive trichomes. These trichomes are usually peltate (shield-like) and upon contact with water, they undergo a shape change. The shields, which are normally upright and perpendicular to the leaf surface, will fold to become parallel to the leaf surface, with a space in between the trichomes and the leaf surface. Not only will water collect in the in-between space, but the trichomes will also absorb the water. Laws of osmosis dictate that water follows solutes, so the air plant will actively transport the solutes from the water into itself, which draws the water in with it. This process collects solutes and particulates floating in the air, and actually makes air plants an indicator of air quality. Tillandsia usneoides, Spanish moss, has been used as an air quality indicator, and it has been measured to successfully capture all of the toxins and particulates in the air.

The trichomes do serve purpose when they are upright and the plant is dry. The upright nature around the plant coupled with increased trichome density in epiphytic lineages helps to reduce air flow around stomatal pores. The trichomes are also usually white and dampen and scatter light to reduce the drying effects of too much light, as well as reduce UV damage. They also help to trap dust particulates, which is the currently accepted theory as to how they acquire nutrients. Another theory is that the roots of epiphytic bromeliads will make associations with nitrogen-fixing bacteria and use those bacteria to harvest nitrogen from the air. This has been found to be true in orchids and other epiphytes, and is likely true for Tillandsia too, but due to a lack of funding, there is no further research.

There is one major food-important bromeliad, and that is the pineapple, Ananas comosus. The pineapple is itself an even more fascinating plant, as its inflorescence does not terminate. Instead, as the fruit is forming on the stalk, the top of the pineapple keeps growing and has the ability to grow into a completely new plant. Bromeliads all contain some level of Bromelain, a proteolytic (protein-degrading) enzyme. Bromelain is also the reason why pineapples can tenderize meat. Many allergic reactions to raw pineapples are actually from the effects of the pineapple literally digesting you. Bromelain can easily be destroyed by salting the pineapple or cooking it. B.B.Q. anyone?

The Orchid Family (Orchidaceae)

Orchidaceae is the second-most diverse family of Angiosperms (flowering plants), second only to Asteraceae (the composite, or sunflower family). Like many orchids — and monocots for that matter — there are three sepals, and three petals, arranged in a triangle, and an inverse triangle, respectively. The lower petal, referred to as the lip or labellum, is usually the most modified part of an orchid. Many orchids have evolved modified flower structures in order to form complex symbiotic relationships with their pollinators. The three traits that make orchids unique are the modified petal, a fused reproductive column and seeds with no food that must parasitize fungi to germinate.

Orchids are found on all seven continents and have diversified to fit every ecological niche imaginable, except for true deserts. The largest subfamily of orchids, Epidendroideae (epi = on top of; dendros = trees) are mostly epiphytic in areas ranging from tropical to semi-arid. These orchids are air plants and include some of the more commonly cultivated gerera such as Phalaenopsis (The classic “moth” orchids), Cattleya (the fragrant, showy corsage orchids), Dendrobium, Oncidium, Encyclia, and Epidendrum. 

Polypodiales & Select Ferns

Although many ferns are terrestrial, some ferns such as Asplenium (Bird’s Nest Ferns) and Platycerium (Staghorn Ferns) are epiphytic and can be grown either terrestrially (in soil) or epiphytically (mounted or soilless). 

BASIC INSTINCTS

Here’s a brief summary on what epiphytes need and some pointers for keeping them healthy.

Sunlight

Thrives in bright, indirect light, but can tolerate bright direct light. Keep smaller air plants out of intense, direct sun. (Learn more about light requirements here)

Water

Soak weekly in a bowl of room temperature water for 15-30 minutes. Shake excess water off afterward. (Learn more about water requirements here)

Humidity

High humidity is ideal.

Temperature

65°F-85°F (18°C-30°C). Not below 60°F (15°C).

Size

These plants tend to vary in size, but grow slowly.

Common Problems

SYMPTOM: Soft, darkened base

CAUSE: Plant rot

SYMPTOM: Curled or shriveled leaves

CAUSE: Low humidity, underwatered

SYMPTOM: Not green when wet

CAUSE: Dead plant

Precautions

No worries for pets. Most epiphytes are non-toxic. But best practice is always to keep houseplants out of reach of small children and pets. (Learn more about plant toxicity here)