Orchids are relatively ancient plants whose earliest forms can be traced back some 200 million years. During their long existence, there have been several major climatic transformations such as interglacial cycles. Plants were forced to adapt to survive; otherwise, they simply disappeared.
Epiphytism for Tree-Clinging Survival
One of the most remarkable adaptations is the transition from terrestrial growth (i.e. plant rooted in the soil) to epiphytic growth (i.e. plant clinging to tree branches). This plant metamorphosis is truly astounding.
Author’s note: For a humorous reminder of epiphytic growth, I suggest you read the column by the sparkling Audrey Martel.
Recent studies on the genome of certain orchids indicate that the transition from terrestrial growth to epiphytism may have occurred at various times in plant life on Earth, in different places and for a variety of reasons.
In some cases, plants became “epiphytic” to benefit from more sunlight at a time when vegetation cover was so dense that it prevented light from reaching the ground. It was discovered that some orchids probably became epiphytic to escape water-saturated soil during periods when their habitat was submerged by torrential rains. Some plants were thus forced to leave the ground to survive, clinging to trees or raised rocks.
Decoding the plant genome – a monk’s job!
It is through the analysis of plant DNA structures (i.e. phylogenetic) that the evolutionary history of plants has recently been deciphered. In fact, the genome contains not only the “current” morphological information of plants, but also that of their ancestors, at least in part. Thanks to the use of various highly complex analysis models, it is possible to obtain (in large part) the story of plant evolution recorded in its genome!
Of course, each plant species has its own story to tell, since it has been subjected to environmental constraints specific to its habitat. However, genetic analysis of a large number of orchids indicates that epiphytism appeared at least three times in orchids: 39 million years ago, 11.5 million years ago and some 4.1 million years before our era.
Plant Conformities Predisposing to Epiphytism
Whatever the geographical location and time of these transformations, the path to epiphytism was marked by comparable evolutionary milestones.
For example, thickening of the foliage is a common feature of epiphytes, enabling the creation of small water reserves that are often beneficial to soil-free plants. « Foliar succulence » (as scientists call it) may have developed before or after migration to tree branches, but it remains a real adaptive asset for orchids perched high up.
Another very useful adaptation of the epiphytic orchid is “stem succulence”. Here, it’s the stems that support the leaves, which then thicken to store water and nutrients. Over time, the stems are typically transformed into chubby pseudobulbs to provide the plant with even more reserves, as can be seen in the photo below. The succulence of pseudobulbs is thought to have preceded the ascent of orchids to trees by around 4 million years. It is therefore a morphological transformation that predates epiphytism in the orchids studied.
Most Spectacular
The most spectacular morphological change, however, is to be found in the plant’s roots: clinging to the trees, they no longer have soil to provide a reserve of water and nutrients. The plant, perched high up, has had to develop a completely new structure for capturing and retaining water: the “velamen”. It’s a kind of plant velvet that surrounds the roots, soaking up water at the slightest rainfall. Velamen is thought to have appeared in several parts of the world during different climatic upheavals.
In a future column, we’ll take a closer look at the different forms of velamen, which is of the utmost importance to the orchid collector. A poor understanding of how roots work has proved to be the main cause of failure for novice collectors. Velamen roots need to be watered and fertilized very differently from standard terrestrial roots.
Climatic Tango
One of the shocking revelations of phylogenetic research has been the eloquent and unexpected demonstration of a certain reversibility of epiphytism. Indeed, we now have proof that several epiphytic orchids have become terrestrial again when climatic conditions have allowed. In other words, their “genetic memory” directly helped them to switch from an aerial to a terrestrial state, and vice versa, when the environment so dictated.
I like to think of these plants as “dancing to the climatic tune”, taking a “giant leap” towards the treetops and a step back towards the ground, depending on the prevailing conditions in the local environment. It’s a truly impressive example of biological adaptation, where the richness of the genome seems to have played a major role in the survival of several plant species.
Vanilla – the Ambivalent One!
As you may already know, vanilla is an orchid. It is one of the few orchids cultivated on a large scale for human consumption. Its production is particularly laborious and complex, making it one of the most precious of food products.
In fact, vanilla (e.g. Vanilla planifia) is a climbing orchid that grows like a creeper, reaching heights of some 10 metres if it finds suitable support. Its growth normally starts on the ground, but as it develops, it will regularly produce aerial roots and anchor roots that will attach themselves to the support. These same roots will enable it to take root quickly if the stem is cut and put back in contact with the ground. It seems that vanilla’s genetic make-up has maintained this terrestrial/aerial ambivalence, giving it a formidable capacity for adaptation and resilience to bad weather.
Acknowledgements
The author wishes to acknowledge the impressive research work carried out at the Muséum National d’Histoire Naturelle (Sorbonne University) by Géromine Collobert and other researchers on Orchid Phylogenesis. Her work is unequalled and extraordinarily valuable for the horticultural world.
Editor’s note
Mr. Charpentier’s orchid columns are published on the Lazy Gardener website in a legal “Copyleft” model. Republication of his columns is therefore permitted, provided the following notice is added:
This article was originally published on the JardinierParesseux website (in French) and in English on the LaidBackGardener blog.
What complexity on all levels!