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. 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 Noël 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.
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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 ...

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