What is so special about the Orchidaceae?
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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.
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.
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. Example of pseudocopulation mimicry in the Mediterranean orchid Ophrys Summary The Orchidaceae are an incredibly successful and special family and their adaptability has allowed them huge speciation. The main points we covered in this essay regarding what makes them so special are. 1. Their stamen are all on one side of the flower, not symmetrically arranged like most angiosperms and the majority of orchids only have one. 2. The stamen and pistil are completely united into the column. 3. Tropical species are all epiphytic. 4. Their seeds are tiny and numerous. 5. The presence of the velamen. 6. The flower has a modified petal known as the labellum. 7. Resupination occurs in most species. 8. The existence of pollinia 9. Their ability to adapt their flower to attract a wide variety of pollinators. As you can see the reasons why they are so special are numerous, but many orchids are under threat from deforestation and loss of habitat. It is vital if we are to learn more about their diversity that we make every effort to conserve them.
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