THE SECRET LIFE OF ORCHID Roots CAROL SIEGEL There are some four million different kinds of animals and plants in the world. Four million different solutions to the problems of staying alive. -David Attenborough N CHALLENGING ENVIRONMENTS and hostile The Velamen Radicum: A Root Sponge conditions, orchids have a tough time staying alive. Wherever they live, they face critical shortages of In most of the plant kingdom, roots are the hidden I half of the plant, burrowed deep within the soil. With some elements needed for survival. Growing in leaf lit- ter at the shady base of the forest canopy, they rarely get seventy-five percent of orchids living as epiphytes in sufficient light for adequate photosynthesis. Growing tropical rain forests, aerial roots are no longer hidden. precariously on branches at the top of the forest canopy They hang down like long beards, freely exposed to the or sitting exposed on a rock, they get plenty of light but air or sometimes growing along the surfaces to which are desperate for a secure hold, for water, and for es- they are attached. The roots are thick and smooth with sential nutrients. Yet orchids have not only survived a silvery-white color, except for the green growing tip. but also thrived to become a dominant life form, repre- A spongy, multi-layer of dead, air-filled cells, known as senting fully ten percent of all plant species. Against all the velamen radicum, covers the root and gives it its odds, orchids have figured it out. Orchid roots are one distinctive color. During a rain storm, the cells soak up very elegant solution to the orchid’s problem of stay- water like blotting paper, becoming transparent, reveal- ing alive. Their structure is uniquely adapted to quickly ing the green core of the root underneath. Unlike most capture and retain water and nutrients, and their ve- plants, aerial roots have chloroplasts and can perform lamen cleverly endows the root with survival advan- photosynthesis, helping to provide food for the orchid. tages. Moreover, roots have exploited fungi to provide The outer layer, the velamen, borders on the inner cor- the plant with whatever the environment lacks. Some tex with an exodermis and endodermis . The velamen orchid roots even provide a home for ants that supple- commonly has root hairs that increase the surface area ment orchid nourishment. Orchid roots are an amazing for absorption as in Acianthera pubescens (syn. Pleuro- adaptation, stunning in complexity and efficiency. This thallis smithiana) and aid in anchoring the plant to the is the secret world of orchid roots. substrate as in angraecums and phalaenopsis. The next Root transection of Restrepiella ophiocephala (drawn from a free-hand section). 26 Orchid Digest, Jan., Feb., Mar. 2015 only about fifty percent in that time. The velamen acts as a place for temporary water storage, allowing more time for absorption through the passage cells which maximizes intermittent rain fall. When the velamen dries out, empty velamen cells slow water loss. Fibrous caps found above the passage cells also restrict water loss. David H. Benzing and associates at Oberlin Col- lege suggested that differentially thickened cells below aeration points in the root may open and close, expand and contract like leaf stomata, to retain water. These ad- aptations help orchids be quick on the uptake and slow on the release of precious water. The experimenters also explored the role of velamen in nutrient retention and showed that charged particles, like negatively-charged phosphate, were also retained in the velamen, probably due to positive charges in the cell walls. Another study by Martin Trepanier and as- sociates showed that aerial roots of phalaenopsis could absorb large amounts of nitrogen from urea and ammo- nium directly through the velamen. It has, in addition, been suggested that velamen may prevent overheating by reflecting heat with its white color although this needs further study. ©Ron Parsons, grown by Cindy Hill ©Ron Parsons, grown Angraecum viguieri has aerial roots. layer, the exodermis, separates the velamen from the cortex. The exodermis consists of both thick-walled cells and passage cells that shuttle water and nutrients to the cortex of the root. An innermost layer, the endo- dermis, separates the cortex from the central cylinder or stele which is composed of vascular tissue, the phloem and the xylem. The phloem shuttles food, and the xy- lem transports water and minerals to the plant. One of the problems of the epiphytic lifestyle is that water and nutrient supply are precarious and sporadic. Orchid roots penetrate neither into soil nor into the liv- ing tissue of their hosts. Even though they may live in an area of high rainfall, when the rain stops, they grow in a harsh, dry environment with brutally little access to moisture. Gerhard Zota and Uwe Winkler did a series of experiments showing that velamen allows the orchid to capture and retain the first solutions arriving in a rain- storm, which have the most nutrients. They found that dry velamen very quickly absorbed the water. Within 15 seconds, the velamen of 11 species tested was almost fully saturated. Within a minute, it was fully saturated. In contrast, water loss from a different set of ten spe- cies’ velamen was extremely slow, taking several hours. Thicker velamen retained much more water after an hour. Miltonia spectabilis, with the thinnest velamen, took one hour to lose the water stored in the velamen. by Golden Gate Orchids ©Ron Parsons, grown Phalaenopsis Malibu, with the thickest velamen, had lost Miltonia spectabilis var. moreliana Orchid Digest, Jan., Feb., Mar. 2015 27 The presence of velamen is not confined to epiphytic intricate coils. The ectomycorrhizal fungus has threads plants, and there are numerous terrestrial orchids with that form a sheath on the outside of the root, a net that velamen. Benzing, in 1996, felt that terrestrial orchids does not penetrate the cell wall. They are often associ- with velamen could go through repeated drying cy- ated with woody trees like oaks, pines, and eucalyptus cles without damage. A collapsed cortex can kill the and form a tripartite relationship with orchid, fungus root, and the velamen provides mechanical protection and tree. against desiccation, preventing collapse. There are also Fungi are not plants and cannot produce their own some orchids which lack velamen altogether. food through photosynthesis. For food, they break On the other hand, velamentous roots are not unique down dead organic material or attack and live on or to orchids and have also been found in other plant fam- within living plants, animals, and sometimes other fun- ilies such as Araceae, Liliaceae, Dioscoreaceae, Tacca- gi. It is impossible to pick up a handful of dirt and not ceae, Amaryllidaceae, and Commelinaceae. pick up a handful of fungi. They are everywhere. There are more than one million species of fungi, at least six The Dirt on Mycorrhiza: times more than plant species. Since they spend most of The Root Fungus their lives underground, we are often unaware of their ubiquitous presence until we see their fruiting bodies Velamen is not the only orchid root adaptation for (mushrooms or toadstools), eat shiitake mushrooms in survival. The ability to attract and digest mycorrhizal an omelet, or develop athlete’s foot at the gym. Fungi fungi is another vital root adaptation. It allows the can grow to enormous sizes; in the Malheur National orchid access to extra food, nutrients, and water in a Forest in eastern Oregon, it was reported that a fungus difficult environment. It is a hundred years since Noel Armillaria ostoyae (honey mushroom) extended over Bernard and Hans Burgeff realized that orchids are 2200 acres with a mass of 605 tons, thought to be the married to fungi. The fungi that associate with orchids are called mycorrhizae. The very name itself implies a relationship between the Greek “mykos” or fungus and “riza” or roots. Mycorrhizal infection is a universal fea- ture of the Orchidaceae. Orchids specialize in deceiving other life forms both above and below ground. Above ground, orchids often promise pollinators food, sex, and a place to breed for their pollination services; in one-third of cases, they give nothing at all in exchange! Below ground, and some- times above, they lure fungi into their roots and offer them the promise of a free lunch and then eat them up, giving little or nothing in return. In the give and take of life, orchids are most often the takers. Liars and cheats, orchids make a living by exploiting other species. Ninety percent of other plant families involve fungi at some point in their life cycles, the fungus receiving carbon in sugars and the plant receiving water and nutrients. However, orchids seem to be the only large higher plant family that consistently exploits fungi as an alternative food source—effectively and one-sidedly parasitizing the fungus. (Although Cameron and as- sociates recently showed some carbon transfer from adult Goodyera repens to its fungal partner, there is very little research proving that orchids do not consistently take advantage of fungi. Perhaps future research will change this view.) During some life stages, such as the germination and growth of seed, the orchid relies en- tirely on fungus for growth, while at other stages, the orchid makes use of both fungus and light-based nu- trition, sometimes one supplementing the other. Some orchids grow entirely underground, totally dependent on fungus, like Rhizanthella gardneri. These orchids have to tap into the nutritional pipeline of a fungus. Mycorrhizae are divided into ectomycorrhizal and ©Adrian Davies/Minden Pictures endomycorrhizal fungi. The endomycorrhizal fungus has threads that penetrate the root cell wall and grow in Fungal hyphae (mycelia) 28 Orchid Digest, Jan., Feb., Mar.