What is Velamen?
Apart from lianas and parasites, how come you almost only see orchids covering naked bark in the tropics? Why don’t you see airborne daisies, beans, lilies or bananas? I believe the orchid’s evolutionary triumph (over 23000 species worldwide) is due to their very special roots which rendered them a position near the glorious sun among the tallest of trees, yet without spending tonnes of building material, energy and time to get there. The word ‘epiphytic orchid’ comes from Greek ‘epi phuton’ meaning ‘additional plant’.
The epiphytic orchid root is uniquely shaped compared to the roots of most other flowering plants. An epiphytic orchid’s root is generally called ‘aerial root’ but that is not a sufficient description. Lianas and figs make aerial roots too, but they are just ‘roots in the air’, growing straight down trying to find the ground again, and they are surrounded by a corky bark like any other plant root. In the case of the epiphytic orchids, their roots tend to grow upwards and they are totally independent of soil contact. The fantastic tissue adapted to an entirely soil-free life is called velamen (from Latin ‘velare’ meaning ‘to cover’) and is only found in a few other plant groups (e.g. Clivia in the family Amaryllidaceae for instance). The velamen surrounds the orchid root and acts like a sponge, accumulating water, which is transported via special passage cells to the pipeline (stele) inside the root and from there nutrients go to pseudobulbs and leaves.
This is how we normally view orchid roots in nurseries, orchid houses and private collections. They hang down like ordinary aerial roots because they have either been stripped from their original support and then tied to a tree with the roots hanging down, or they were transferred from the laboratory to a series of vessels and baskets before ending up in the tree. Although they serve their function, it is like seeing an imprisoned magician use his wand for digging.
Here is a free magician (Vanda liouvillei, Orchidaceae) doing his magic. Few other flowering plants can grow solely on the bark of a tree. Also note that the roots strive upwards. In this orchid, the top aerial root tip grows 150 cm away from the base of the green shoot. The purpose of the root is adhesion and to accumulate water and mineral nutrients. At least during its young stage, the root is colonised by fungi which are degraded by the orchid as a source of carbohydrates, proteins and vitamins. This parasitic symbiosis is called orchid mycorrhiza.
Other airborne flowering plants either need a thick layer of soil accumulated on a branch in a wet climate (e.g. Rhododendron), or they quickly try to reach the soil (figs) or they begin growing in the soil and then simply use roots along the stem to cling to the bark (numerous lianas) or they penetrate the tree’s bark and parasitize straight from the wood (Dendrophthtoe).
A transect illustration of the upper layer of an epiphytic orchid root, from the eminent plant physiology textbook ‘Plants in Action’ which is freely available on internet. I have modified the text: V velamen – a spongy water absorbing tissue composed of dead cells when mature. E Exodermis – a cell layer protecting the inner living parts of the root from severe drought. C cortex – living cells with chlorophyll for photosynthesis. P Air-filled and water repellent cells for passage of oxygen and carbon dioxide. PC Passage cell for water and mineral nutrients to the stele (the main pipe-line) not seen in the illustration.
It is possible to imagine steps of orchid evolution from the ordinary ground orchids to the intermediate stage of Papilionanthe and Vanilla orchids, which remain in the soil but climb upwards with aerial roots, until fully adapted epiphytic orchids with no connection to the soil. A prerequisite is the tiny dust-like seeds which are easily transported by wind up to the trees.
When you go jungle trekking you will of course notice that different orchids thrive in different forests (evergreen or deciduous, high altitude or low altitude) but different species are also adapted to different sites in the tree (depending on light and humidity) and different bark. It seems smooth (Ficus) and flaking (Lagerstroemia) bark, not surprisingly, are scarcer in orchid species and individuals than cracked and long-lived bark (Shorea, Dipterocarpus, Quercus). The reasons for this difference is likely to be the availability of cracks for the orchid seed to germinate and the water holding ability of the bark. An epiphytic orchid can grow on almost any substrate if tied to it (concrete, stone, wood, bark, plastic etc) but for a natural establishment from a seed without a gardener the choices are limited.
Flaking bark might be a way of the tree to get rid of heavy epiphytic growth (orchids, mosses, ferns) which together with rain water otherwise may cause a weight so heavy a limb may crack. Instead of flaking bark, the Shorea has a very hard wood to carry the extra weight.
Text & Photo: Eric Danell