The Science of Botanical Art

Epiphytes

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 16, Issue 2

 

When we think about plants we see them growing in soil. To that end nature has designed roots in a different way than shoots. With no friction to fight, the apex of a shoot rises up by having its core of meristematic cells divide. The initial cells they form are thrust downward, and for a short span they enlarge and differentiate into various tissue systems. When the cells reach their final size they stop growing, in a sense forming a platform upon which the meristems rise, almost as if they were in the process of building a brick wall, cell by cell.

Roots do this too, in an opposite fashion, throwing their initials upward as they burrow into a hostile substrate of humus, sand, and loam, among other uninviting things. Aside from the different direction, drawn down by gravity in a movement called geotropism, the meristematic cells of a root also divide to form a completely different set of cells. They are loosely grouped, rather mucilaginous, and act as a lubricant, in a constant process of being worn away and replaced, to ease the friction caused by the growing root tips.

For most plants this bipolar growth pattern is typical, but there are exceptions. One group have adapted to a specific niche far above the ground. How is it they live without the sustenance and water provided by roots?

Epiphytes (epi =upon, phyte= plant) grow on other plants, enabling them to reach heights where sunlight is available without sacrificing energy creating the trunks or long vertical stems that their host plants do. They have evolved probably from terrestrial habits by extending their roots. They derive moisture and nutrients from air, rain and accumulations of debris in their aeries. There are some 24,000 epiphytic species, including lichens, mosses, ferns and fungi. The greatest number, by far, are flowering plants. Epiphytes are not parasites and don’t derive any of their nourishment from their hosts. They are not harmful except in the rare case when sheer number of epiphytic plants on a branch might cause a break.

We also have hemi-epiphytic plants in all categories that rely on attaining their nourishment from soil and by aerial roots that have adapted to their roosts. These plants appear to be examples of an evolutionary pathway leading to purely epiphytic habit. Some mutants arose that severed the connection to the earth, and forced other means of existence in this precarious substrate and it obviously worked.

In epiphytes, water storage is critical, and roots developed negative geotropism, growing against gravity to form nest-like structures designed to capture organic matter. The roots also developed an outer covering of spongy, dead tissue made up of highly vacuolated cells called velamen, used to store water. When the velamen is filled with water it is transparent, allowing sunlight to penetrate to the photosynthetic roots.

Some epiphytic orchids have succulent leaves, others form thickened stems that are called pseudobulbs for water storage, which is another solution to the problems of life in an environment where rain can be violent and short-lived without a means of containment. Not all epiphytic orchids have this phenomenon, so it requires a bit of research, but it is something to look for and to document.

The simple thick-textured leaves of these plants clasp the stem tightly, thus providing catchments to hold water. This is true not only of orchids but of bromeliads, another family that is well represented in this habit. These pools of water have many purposes, providing drinking sources for the many critters living in the canopy for one. They also create breeding spots for the aquatic life stages of certain insects and frogs. Most important, these pools are areas where the plants can obtain nutrients otherwise not readily available, by their ability to extract vital nutrients from the organic matter that rots in these catchments. Another problem to be solved by these plants is one of intrinsic water loss. Typically most are covered with a waxy layer, and many employ a special form of photosynthesis, where their stomates (the naturally occurring openings in leaves which allow for the transfer of gasses) are open only at night, thus keeping H2O loss to a minimum.

Most tropical orchids are epiphytes, but in the case of slipper orchids some species are terrestrial, so be sure of what you paint. Most orchids you purchase are sold in some rooting medium, which makes them commercially practical, but beware. It is not soil. A closer look reveals it is some form of wood chips or sphagnum. It is used to provide a substrate to hold the plant, but its portrayal in a painting will offend a purist botanical juror, who would prefer to see the velamen coated roots, and might get your lovely painting rejected. Cattleya, Dendrobium, Epidendron, Cymbidium, Phalaenopsis and Vanda are all more easily obtained genera that are epiphytes, no matter how they may be packaged. Even if you are painting a terrestrial orchid, it may pay to check underground. Many have velamen-coated roots and some have rhizomes and tubers that might add another dimension to your portrayal.

  • Dracula vampira, watercolor on vellum, ©Carol Woodin 2010