Read an Excerpt

CHAPTER 1

ORCHID EVOLUTION

Orchids evolved during the Late Cretaceous period, roughly 76 to 105 million years ago. This is much earlier than botanists once thought and makes Orchidaceae one of the 15 oldest angiosperm families, of which there are 416 in total. Few orchid fossils older than 20 to 30 million years have been found, and it was thought that orchids evolved relatively recently compared to many other groups of flowering plants. That they have a poor fossil record is not surprising because most orchids are herbs, which generally do not fossilize well, and their highly modified pollinia are difficult to recognize in the fossil record.

DINOSAUR DEPENDENCE

All five orchid subfamilies evolved before the end of the Cretaceous period, which means that orchids and dinosaurs overlapped. Considering the great diversity of orchid pollinators, we can only wonder if orchids managed to adapt to pollination by dinosaurs before the latter became extinct 65 million years ago. Vertebrates in general are uncommon orchid pollinators, and nearly all of those recorded are birds — direct descendants of the dinosaurs. There were many small species of dinosaurs, so it is possible that some visited flowers to collect nectar and, like many animals today, were deceived into pollinating orchids. Any orchids adapted to dinosaur pollination would have become extinct with their pollinator, and so are now lost to us.

DISTRIBUTION

The discovery that orchids were much older than previously thought was a result of the widespread sequencing of DNA that only became possible in the mid-1990s. This greater age makes a good deal of sense when it comes to understanding the geographic distribution of orchids. It was long assumed that orchids could have reached their current worldwide distribution relatively recently by long-distance dispersal of their small, almost microscopic seeds. Due to their dependence for food and minerals on the fungi with which they associate, orchids do not include food reserves or minerals in their seeds, unlike, for example, a bean in which the stored food and minerals make up the bulk of its much larger seed. Orchid seeds are, therefore, light and easily distributed by the wind, which theoretically could propel them over long distances. However, the longer an orchid seed remains aloft, the more the small embryo dries out, making most orchid seeds inviable before they can travel great distances. So, most orchid species have a limited distribution, even as constrained as a single mountain. Orchids have instead achieved their worldwide distribution by passively riding the continents, which at the time the plants evolved were much closer than they are today.

POLLINATION

Orchids are well known for elaborate pollination mechanisms that have evolved to achieve the mating of different plants, or cross-fertilization. Flowers of most plants, including orchids, contain organs of both sexes, but self-pollination is as generally undesirable in plants as it is in animals. Most plants, and orchids in particular, have evolved methods, often exceedingly complicated, to avoid self-pollination happening. This process has long fascinated scientists, including Charles Darwin, who studied pollination of orchids in detail and was so enthralled by the plants that his first book after publication of On the Origin of Species (1859) was entirely dedicated to orchids. The short title, Fertilization of Orchids, gave little hint of its main hypothesis, unlike its full and explanatory title, On the Various Contrivances By Which British and Foreign Orchids Are Fertilized By Insects, and On the Good Effects of Intercrossing (1862). Among the orchids studied by Darwin were a large number of tropical species provided by the then Director of the Royal Botanic Gardens, Kew, Sir Joseph D. Hooker.

POLLINATOR DECEPTION

Most orchids produce pollen in two to six tight bundles, called pollinia. These are often attached to ancillary structures that together are called a pollinarium, which attaches the pollinia to the pollinator's body, usually in a position that makes it difficult for the animal to remove them. Most orchids look as if they contain a reward for pollinators but few actually offer it. Some even produce long nectar spurs that are devoid of nectar. Rates of visitation by pollinating insects to such deceptive flowers are, understandably, low. Insects learn quickly to avoid these rewardless flowers, but they make the mistake often enough for it to be effective in a system in which a single visit can result in deposition of thousands of pollen grains, each fertilizing one of the thousands of orchid ovules produced by each flower. A rare mistake by a deceived pollinator is enough for the orchid to produce large numbers of seeds.

Darwin himself came to the conclusion that outcrossing, or pollination between unrelated plants, is so advantageous for most orchids that deceit and corresponding low rates of visitation are the general rule. Apparently, setting seeds in only a few flowers but guaranteeing that these are of high quality (due to cross-fertilization involving flowers on different plants) makes deceit a successful strategy. In this case, the cheating orchids have prospered, despite the fact that they so badly treat the insects upon which they depend. There is no mutual benefit for the orchid and its pollinators as there is in pollination systems with rewarding plants; the deceiving orchid could go extinct and the animal would only experience a slight improvement in its condition due to fewer floral visits without a reward. However, if the animal pollinating a deceitful orchid species becomes extinct, then the orchid also disappears or develops a method by which to self-pollinate its flowers, which has been known to evolve when an orchid species reaches an island without its pollinator accompanying it.

SEED PRODUCTION

The combination of delivery of whole pollinaria on a single visit and fertilization of a correspondingly large number of ovules in the ovary means that from a single pollinator visit a massive number of seeds can be produced. That many orchids, such as some species of Dendrobium, Epidendrum, and Oncidium, bear large inflorescences with hundreds of flowers may seem like an extreme waste of energy, but production of mature ovules ready for fertilization is delayed until pollination takes place, thus reducing energy inputs associated with these large numbers of flowers.

MIMICRY AND DECEIT

Deceit involving mimicry of other local plants that produce a reward for their pollinator is another common habit for orchids. Although not offering a reward itself, the orchid benefits from pollinators that fail to distinguish between a cheating orchid and the rewarding species, and so the former obtains a degree of pollinator service that drops dramatically if the latter is not present. In other cases, a deceitful orchid species is not mimicking a single reward-offering species in the immediate neighborhood, but rather is using a suite of the traits associated by pollinators with the presence of a reward. These include fragrance, color, "nectar guides" to direct a pollinator to the center of the flower, and a nectarless cavity or spur of the correct shape and size to suggest that nectar is present. A quick look at the species illustrated on this page demonstrates many of these features in what is termed "general" or "non-specific" deceit.

In many groups of orchids, a much more specific type of deceit, involving sexual attraction, has evolved. Darwin was unaware of this phenomenon, although he speculated on what might be happening with native British bee and fly orchids (genus Ophrys). The details would probably have shocked him and many other botanists of that time. It is thought that mimicry of the female of a species of bee, wasp, or fly begins as some other more general type of deceit and subsequently becomes more complicated and specific. For example, the orchid Anacamptis papilionacea appears not to be mimicking any specific nectar-producing species in its habitat and is instead just a general reward-flower mimic. However, there are more males than females among the insects it attracts, so it appears that some sort of sexual attraction is operating, which could lead to further change on the part of the orchid to enhance this aspect of the deceit.

Many orchids using visual sexual mimicry also produce floral fragrances that are identical to the sex pheromones produced by the female of the insect species to attract a male. This at first sounds wholly preposterous: how can a flower evolve to produce something so alien to a plant as an animal sex pheromone? However, once it became known how the biochemical pathways operate by which such animal hormones are produced, it also became clear that plants share these same general pathways and often produce minor amounts of such compounds as part of their general bouquet of scents. Thus, the assembly of a highly specific sexual pheromone starts out with production of small amounts of similar compounds that become predominant when an increased presence in the mixture generates higher rates of male visitation, such as that observed in A. papilionacea. When combined with visual cues, such fragrance compounds reinforce the "message" being sent to male insects, and sexual mimicry is the result. Orchids in many distantly related groups have independently evolved this sexual mimicry syndrome, which, now that we know the genetic and biochemical details, is not as surprising as it first appeared.

SYMBIOTIC RELATIONSHIPS

Orchids have a symbiotic relationship with soil fungi that enables germination of their seeds and sustains them in early phases of their development, when they are unable to be photosynthetic and make their own food. These fungi are so-called "wood-rot fungi" that break down dead wood in the soil and form masses of fungal tissue, known as pelotons, inside the cells of the orchid embryo. The exchange that occurs in the early stages of germination is entirely one-way in favor of the orchid, and it is not clear why the fungi participate in this process. There is no obvious benefit to the fungal partner; the embryonic orchid prospers, but there are only costs for the fungus. Once the orchid seedling forms its own leaves, then sugars that are produced by the orchid are exchanged for minerals from the fungus, which is much better at retrieving minerals from the soil than the plants. However, some orchids continue throughout their life to be a drain on the food reserves of their fungal partner.

FUNGAL PARTNER SWAP

Some ground orchids are known to switch fungal partners as they grow older and associate instead with "ectomychorrhizal" fungi, which regularly exchange minerals for sugars with forest trees. Orchids associating with ectomychorrhizal fungi have been found to contain sugars produced by the trees, the sugars recognized as distinct from those produced by the orchids as they leave a clear chemical fingerprint created by the fungus as they pass through it. These orchids abandon the wood-rot fungi that helped them germinate, without ever giving those fungi a reward for this service, and then switch to a fungal relationship that provides them with sugar produced by the trees in their habitat. We do not yet know how orchids manage these complicated relationships nor why the fungi involved should participate in such a decidedly one-sided relationship.

FUNGAL RELIANCE

A number of ground orchids carry the parasitic relationship one step further and forego ever carrying out photosynthesis — a phenomenon termed "holomycotrophy" or, more literally, "totally fungus eating." These orchids, such as the Bird's Nest Orchid (Neottia nidus-avis) of Eurasia, also switch to an ectomycorrhizal fungus as described above and obtain all their sugar indirectly from the neighboring trees. The underground orchids from Australia, genus Rhizanthella, not only produce none of their own food but also avoid raising their flowers above the soil surface. Unsurprisingly, the subterranean pollinator of Rhizanthella species is unknown.

Holomycotrophy is not confined among plants to orchids — for example, some members of the rhododendron and blueberry family, Ericaceae, form similar parasitic relationships with ectomycorrhizal fungi. All holomycotrophic plants, including orchids, that get their food entirely from fungi have in the past been classified as "saprophytes," meaning plants that live off decomposing material in the soil. This, however, is not an appropriate term because such plants are fungal parasites and not directly living off decaying material. Moreover, the food these orchids are stealing comes not from the fungi involved with rotting of wood in the soil but rather from fungi that are living in symbiosis with nearby forest trees.

THREATS TO WILD ORCHIDS

CONSERVATION

Plant conservation has long been the poor relation of animal conservation. It is much easier to get the public's attention if the plea for money involves the so-called "charismatic megafauna" such as elephants, tigers, pandas, rhinoceros, and cheetahs. Few plants have the same potential, but orchids come close. In a horticultural context, orchids grab the attention of the public, with many thousands drawn to orchid exhibitions.

Orchid conservation has had a few successes. For example, the Yellow Lady's Slipper, Cypripedium calceolus, has been the focus of a long-term restoration project funded by English Nature, the conservation arm of the United Kingdom Government. There are now flowering plants in several wild areas that were reintroduced as cultivated seedlings a decade ago. So far, none of these plants has produced seeds, but conservation is a slow process, even at its speediest. It takes a long time to overcome the problems created by our forebears, just as it will take a long time for our children to overcome the damage caused by the present generation.

MEETING HORTICULTURAL DEMAND

Other efforts to restore seedlings produced in cultivation to their natural habitats have failed dismally. Poachers removed a tropical Asian slipper orchid species, the spectacular Paphiopedilum rothschildianum, within months of it being replanted in forest reserves on Mount Kinabalu, in Sabah state, Malaysia — a UN World Heritage Site. For many showy orchid species, collection for the horticultural trade is a major threat, one that has proven almost impossible to surmount.

The outcomes for the two lady's slipper species mentioned above were entirely different, but largely because the sites for C. calceolus were kept completely secret and guarded 24 hours a day while the plants were in flower. Also, horticultural demand for plants of Cypripedium is minimal as they have a partly justifiable reputation for being difficult to cultivate, in contrast to the high demand for the easily cultivated P. rothschildianum.

Relative to the number of orchid species in the world, those that are threatened by unsustainable collection for horticulture are a small percentage. For the great majority of orchid species, there is so little demand that concerns over their extinction for this reason can be discarded. There is legislation in place — the Convention on International Trade in Endangered Species of Fauna and Flora (CITES) — that has sought to control the unsustainable harvest of many species, mostly animals, but also of many plants, including all orchid species. For those species that are horticulturally desirable, the CITES provisions have not prevented the commercial exploitation of wild-collected plants, such as P. rothschildianum, and have to be considered a failure. The greatest threats to orchid species in many countries come, in fact, from conversion of their natural habitats for agriculture, mining, and human habitation, about which the CITES regulations can do nothing. The only orchids that have been formally and extensively assessed by the IUCN (International Union for Conservation of Nature) are the slipper orchids (Cypripedioideae).

HUMAN CONSUMPTION

Another major threat is posed by the use of many orchid species as food and medicine. Salep is a kind of starch made from the tubers of many terrestrial orchid species in the eastern Mediterranean and the Middle East, including members of the genera Anacamptis, Orchis, and Ophrys; it is used to make a dessert or a beverage. The tubers are collected unsustainably from the wild, and in many areas of Turkey all terrestrial orchid species are becoming rare as a result. Making a bad situation worse, orchid tubers are now being collected in many of the surrounding countries where salep is not consumed to supply the demand in those where it is.

(Continues…)

---

Excerpted from "The Book of Orchids"
by .
Copyright © 2017 The Ivy Press Limited.
Excerpted by permission of The University of Chicago Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---