What is so special about the Orchidaceae
Orchids have captured man’s interest since the dawn of time. They display diversity unparalleled within the plant kingdom and are regarded as the most complex plants, both in structure and function. Orchids are spread throughout the whole world, from the Arctic to the Antarctic and every habitat in-between. There are about 35,000 species over 500-600 genera, making them the largest family of the angiosperms, in fact, 1/7th of the total number of angiosperm species belong to this family.
The name “orchid” comes from the Greek philosopher Theophrastus, the father of botany, who commented on how the unusual paired roots of a plant looked like a pair of testicles or in Greek “orchis”. Interestingly it was thought that the flower was able to “invoke Venus” and allow the parents to choose the sex of their child, a practice that continued into the middle ages. The characteristics and relative importance may not be apparent at first, they are herbaceous perennials, like many other plants, but their diversity comes not from one specific characteristic but from a whole range that integrates into the plants form and function.
Orchids, as stated before, are found all over the world in many different habitats. The temperate zone orchids, like the ones found in the UK are all terrestrial, but the Tropical and Sub-tropical species are nearly all epiphytic. The term epiphytic means that the plant isn’t bound to the substrate, it actually lives on another plant. It is not to be confused with parasitism though as the host plant does not come to harm through this relationship. Orchids living on plants, mainly trees, obtain all their nutrients from rainwater or the detritus washed down with it.
There are however many advantages to being an epiphyte that the orchids have exploited. Firstly it gets you off the ground, thus avoiding herbaceous predators such as slugs and snails. Also in tropical habitats the canopy shades the ground and so you are able to get more exposure to sunlight, and the air currents help prevent burning from the sun’s rays. The air currents also provide an important medium for the effective dispersal of pollen and proximity to wind pollinators. The orchids display a very unique form of germination.
Back in the 17th century when botanist’s interest in these wonderful organisms was at it’s peak their cultivation was agreed to be neigh on impossible. No matter how hard they tried botanists were unable to grow orchids from seed and many thought that they would never be able to. The seed of an orchid is like no other; it is very small 1mm long by 0. 5mm wide on average and contains no endosperm unlike the other angiosperms. This means that the seed didn’t seem to have an adequate food store for germination. It wasn’t until 1904 when a French botanist by the name of Noi?? Bernard was walking in the woods and stumbled a mass of germinating orchid seeds. He noted that all of them had a fungal infection of some kind and through further investigation discovered that the orchids needed those fungi in order to germinate. When the parent produces a seed it is essentially a mass of undifferentiated cells, this is dissimilar to angiosperms, which show distinct differentiation in their seeds. They are also produced en-mass; due to their size there can be anything from 1330 to 4 million per capsule.
This is a clever trick as it ensures that maximum dispersal is achieved and as the seed doesn’t have a large food store very little energy is put into their creation. They are wrapped up in a transparent coat and then dispersed by wind. Once they land in a suitable situation and the conditions are right they start to germinate. Several structural changes occur, it swells and a few cell divisions occur, it adopts a green colour, and develops a tiny structure that resembles a corm, or underground stem with root like functions. This structure is called a protocorm.
However it waits before continuing with its development. What it is waiting for is a fungal infection. If infected it resumes growth and forms a larger dislike protocorm. This soon develops an apical meristem. The meristem gives rise to one leaf and later on to more leaves. The appearance of roots completes formation of the miniature orchid plant. The fungus that penetrates the seed is key to this whole process; it produces enzymes that break down complex starch into simple sugars, which the orchid can use for growth, vitamins, minerals and possibly even hormones.
Once established the fungus is moved to the root system of the plant where it is known as an endomycorrrhiza, where the fungus actually penetrates the inside of the root cells. This symbiotic relationship is found in almost all plants but the fact that a fungus is needed for germination is unique to the orchids. Example of a fungal infection in an orchid root The roots of some terrestrial orchids are bulbous, but most tropical epiphytic species have elongated greenish or white roots, which adhere tenaciously to surfaces, often forming a thick mat.
This may not seem very special, they have a mychorriza which is common to most plants, are all secondary roots arising from the stem but what they do have is a special zone of cells surrounding the root known as the velamen. This is a multilayered structure (multiple epidermis) of dead cells, ranging from 2 to 18 cells wide, with special thickenings in the cell walls. The special thickenings seem to prevent cellular collapse and provide the root with some protection from mechanical injury. When the root is wet, the velamen fills passively with water, aided by perforations or tears in the walls from when the roots went through a drying cycle.
While dry, the velamen may provide a barrier to water loss via transpiration from the wet, internal cells of the root. Also the perforations provide the perfect home for nitrogen fixing fungi that are found in many species of orchid, a possible reason for the structure of the velamen. TEM Section through the velamen of Clowesia russelliana It is quite a trick to have developed this velamen as it allows the plant to survive a lot more effectively in dryer conditions, such as those experienced by epiphytes where water is scarce and hard to retain.
Finally in several saprophytic orchids the entire plant is mainly red, purple or yellow with roots that produce a flowering stem at appropriate times. The fact that the plant is saprophytic is of particular interest as this means it doesn’t obtain it’s organic substances through photosynthesis; it relies on a fungus to create them for it. This exploits the advantages of an orchids symbiosis with fungi even further. This is a very specialised form of feeding seen in only a few plants. Unlike angiosperms in general the orchid family shows two types of stem growth depending on the species. One type is referred to as sympodial.
This is where the growth of the main stem ceases at the end of one season and doesn’t resume in the next, instead a new stem develops. The old stem may become thick, short and fleshy, forming a structure known as a pseudobulb. The other form of growth is known as monopodial. Here the main stem grows indefinitely and no pseudobulbs are formed. As we continue moving up the plant we see the next adaptation that makes the orchids so special is the development of the flower. When in bud form the labellum, the modified petal, is above the other two petals, yet when the flower opens it is below them.
The bud actually twists on its axis in a process known as resupination. The flowers align their labellum according to gravity so the twist may not always be a whole 180o, for example, the flowers of the genus Cymbidium all align so their labellum is facing downwards, no matter what the orientation of the flower originally was on the branch. The evolutionary reason for this odd process of twisting is unsure, but it is thought that in some flowers this facilitates the use of the labellum as a “landing pad” for insect pollinators.
In fact most species of orchid have ultra violet markings on their labellum to act as landing lights for insects that can see into the UV spectrum. Although not all species of orchid rotate their labellum at one point in time they did and it is considered a basic feature of the family that has been lost by some species. Those species may not resupinate at all or some may in fact hyper resupinate, where selection originally favoured the non-resupinate flowers and they compensated for this by extending their twist to a full turn. This type of behaviour is seen mainly in some species of Malaxis.
However the characteristic that makes all orchids so special is their flower. It displays a huge variety and level of adaptation that is very pollinator specific. It still contains the basic components of all monocotyledonous flowers, but they are heavily modified and in some cases beyond all recognition. They range in size from the flowers of Platystele ornata immdia, which are only 1mm across to the large flowers of the Cattleya subfamily, which can easily reach 30cm in diameter. All orchid flowers are bilaterally symmetrical and being monocots all have petals and sepals in groups of three or in some cases multiples of three.
The sepals of an orchid can be of any colour except black and are can be a different colour to the petals. One of the petals is modified in comparison to the others and is usually the source of the huge visual variation in orchids. This petal becomes the labellum, or lip, and is used in many different ways of attracting pollinators, the least being its’ bright colours and patternation. Orchids have also separated themselves from the angiosperms even further by the adaptation of their reproductive organs into one single column. These are known as gynandrous orchids.
The stigma (part of the pistil) stamen, style and the anthers are all united and highly modified. The pollen grains can occur separately or, more commonly compressed into masses known as pollinia Plants can be either unisexual or, in most cases, bisexual. TEM of a pollen grain from Epipactis microphylla The reason for all this adaptation is quite simply pollination, the main function of the flower. Orchids have become the most successful family of plants in terms of species not by chance but by the fact that they are able to adapt to exploit almost all types of pollinators including bees and other insects, birds, bats, snails and even frogs.
This has moved to the extremes where only one specific species is able to pollinate the orchid. The white orchid of Madagascar holds nectar at the end of a foot long spur, the only insect with a proboscis long enough to pollinate this flower is a night flying moth Xanthopan morgani praedicta. The orchids use a wide variety of pollinators but perhaps what makes them so special is the mechanisms they use to ensure pollination. Pseudopollen and Wax Insects, probably due to the formation of pollinia, do not use orchid pollen but on orchids of the polystachya species there is pseudopollen to be found on the labellum.
This is a mealy-like foodstuff that attracts bees all the same to the flower. A few species of the maxillaria species produce a wax that is collected and used in nest construction. Scent Although some species of orchid do not even need to produce pseudopollen, instead they give off the scent of nectar and by the time the bee has found out that there is no reward it has already had pollen deposited on it. An example of this is the Guatemalan tiger orchid. Other species of orchid such as the scorpion orchid and the genus Dracula emit pungent odour that mimics rotting flesh or a fungus.
Insects are attracted to this smell and attempt to lay eggs or feed. Moveable parts Many orchids such as those in Bulbophylum and Peristeria have hinged lips that move easily and are tipped toward the column by the weight of the pollinator. Others including species of the aforementioned Bulbophylum, and Pleurothallis have hairs or appendages that attract flies when they move in the wind. However these are passive movements. A certain degree of active movement can occur, although technically it is a release of tension.
In the orchid Poroglossum there is a sensitive lip that, when stimulated by an insect, flips up enclosing the insect between the lip and the column. The only exit for the insect now is through the back of the flower and on the way it picks up/deposits pollinia. The Cynoches when stimulated by the insect Cetris fasciata “throws” a lump of sticky pollinia at its’ guest, another example of a release of tension. Trap Flowers A heavy modification of the labellum has produced trap flowers.
There are many different types; in fact the Poroglossum mentioned earlier is a form of trap flower. Other types include the genus Gongora. This has a slippery labellum and column. When the bee lands in the labellum it falls off and is guided by the other petals to fall down the column in a slide-type fashion. When it reaches the end of its run the bees strikes the viscidium, which adheres to the insect’s abdomen, so that when it flies off so does the pollinia. If the insect falls for the same trick twice the pollinia is deposited at the end of the run at the stigma.
Another type of trap is the bucket trap, exhibited by the Peruvian bucket orchid. The labellum has evolved into a reservoir for a fluid produced by glands at the top of the flower. If an insect falls into this trap the only way to avoid drowning is to escape via a small exit between the column and the labellum. In so doing pollinia is deposited onto the back of the insect. Mimicry This is perhaps the adaptation that really makes orchids so special. Their level of mimicry is incredibly complex and are able to mimic a wide variety of things.
Firstly they can mimic other plants, Epidendrum ardens closely resembles the flowers of an Ericaceous shrub, Gaultheria, that is abundant in the same region. They can therefore “hijack” the pollinators of that plant for their own devices. However using another pollinator is not as good as having your own, the pollen you deposit has a good chance of finding its’ way into another species. The best types of mimics in the orchid world therefore are ones that resemble insects. These orchids not only look like insects but also emit pheromones that make them smell like insect, completing the illusion.
The flowers of Oncidium, when moved by the breeze, resemble a male Centris bee flying. These bees are highly territorial and another male will swoop in and sting it, the flower then deposits its’ pollen on it as it fly’s away. Another form of insect mimicry is called pseudocopulation, where the flower looks like a female insect. As the male tries to mate with it pollination occurs. What is remarkable about this is that the mimicry can be so exact even the surface texture is reproduced, and that it has evolved separately in orchids in completely different parts of the world.