HomeIn vivo is Latin for 'in life'.
The Orchids belong to the monophyletic group and is one of the three largest families in the plant kingdom. The orchid family is considered to be the second largest family of flowering plants (Arditti, 1992). The family has the name Orchidaceae and is divided into five sub-famelies (Apostasioideae, Cypripedioideae, Vanilloideae, Orchidoideae, Epidendroideae).
Orchids are monocot plants and are cathegorised by their way of growing. They may be epiphytic, terrestrial and litophytic. About 70% of the worlds orchids are epiphytic and/or litophytic, 25% are terrestrial and 5% of the worlds orchids grow in mixed substrates (both litophytic, epiphytic and terrestrial) (Arditti, 1992). Orchids do also occurre as saprophytes.
Epiphytes grow on trees or other objects above ground level. They do not act as parasites. Plants living epiphyticaly in a tropical rain forest face better light conditions than plants living on the ground level. On top of that, the epiphyts are more easily pollinated by flying pollinators. The roots of an epiphyte are developed for a life above soil. They need eg. high air humidity (read more about roots) since they often are forced to take most of their water from the air. Together with epiphytic ferns and bromelias, orchids grow and bloom on tropical trees.
Many orchids growing 'with' a tree have adapted very precisely to the tree's 'behaviour'. When the tree e.g. drops it's leaves to a period of rest, the orchid does the same thing. (Rittershausen, 2002).
Litophytes are alot like epiphytes. They are somewhere in the middle of terrestrial and epiphytic orchids (ie. they seem similar to epiphytes but grow as terrestrials). They often grow harshly and on rocks in poor areas.
Terrestrial orchids grow on the ground level in the soil. Most of these can be found in the cooler parts of our planet (but not all). There is a great number of species even among the terrestrial orchids. Some species of Lady Slipper (Paphiopedilum) have been, and still are, highly valued by collectors and are illegally poached from the nature to be smuggled here and there.
Terrestrial orchids often grow from rhizomes. The habitats vary greatly; some constantly grow in wet soils and has deep roots, others have root-systems closer to the ground surface. Soil that is easily penetrated and containing sand support ideal conditions for orchids with deep growing roots while other orchids grow where there is less soil and but more rocks. Terrestrial orchids often have hairy roots.
Saprophytes are orchids growing in and living from dead material. They may grow very quickly since there is a though competition for dead material in the tropical rain forest (many destruents want a share of the goodies). Saprophytes are often dependant on mykorrhiza (there are orchids without chlorophyll and that live heterotrophic)
Big and soft leaves are signs telling that the orchid can't stand very high light intensities. (Big leaves may mean that the plant tries to fetch as much light from the sun as possible and that it thus grows at lower light intensities). Hard and thin leaves are more adapted for dry conditions with high light intensities.
Succulent plants (and plants having a relatively thick cutikula) are especially adapted to dry conditions and often face periods of drought in nature.
The foliage of orchids should have a pale green colour and in general look fresh. Dark foliage is generally a result of growing in low light intensities. Too high intensities may cause the foilage to shade yellow (drying it out) and burning it to a brownish colour.
Orchids
often have thick and white roots. It is very usual that orchids have aerial
roots (they do not require substrate eg. soil to grow in). When these roots
grow and hit some object, they attach to it and continues to grow on the object
as epiphytes.
Many orchids, as well as other plants able to survive in dry conditions, have fast mechanisms of taking up water when such is at hand. Water is taken up through a white silvery layer called velamen, that surround the roots of the orchid. the velamen is made from dead cells and is very effective in absorbing moist when there is humid but also acts as a water container keeping the water in the roots when the conditions are dry. Very few terrestrial orchids have velamen (Arditti, 1992).
The thickness of the roots can be 'interpreted' in order to feed the plant according to its needs. Thinner roots has a larger surface in relation to root mass than thicker roots and orchids with thin roots are thus able to absorb water and nutrients more efficiently. Such orchids can be given less nutrient (eg. half (½) the concentration) than you would give orchids with thicker roots. Some orchids have hairy roots (e.g.Phapiopedilum). The hair makes the absorbing surface very large. One should feed these orchids with lower nutrient concentrations (about a fourth (25%) of the regular concentration of the nutrient solution).
If roots are exposed to light they probably become really shiny with a silvery velamen. That's to prevent water from diffusing from the root tissues. However, roots may turn green (if it's moist enough). Inside the roots there are parenchyma cells that have photosynthetic capabilities (Arditti, 1992). There are also entirely leafless orchids relying on the photosynthesis from their roots.
The methods of storing and saving water are many. Phalaenopsis has succulent leaves, Dendrobium grow with thick oblong water storing structures (similar to pseudobulbs) and many orchids produce thick pseudobulbs.
A further adoption to minimize the loss of water is to modify the photosynthesis. Cacti and many orchids have the CAM photosynthesis. The stomata (similar to 'vents' for gas release and gas uptake) are tightly closed during the hot day and open first during night. Common plants with regular photosynthesis open their stomata during the day to respire and photosynthesise. All respiration involves water and in hot environments the water is extremely valuable for the plant. Since temperatures are lower during the night, the loss of water will be substantially less if the stomata would open and respire. The orchids (and cacti) are able to 'store' the night-time absorbed carbon dioxide until the next day. During the day, the stomata are closed, and the orchid can process the stored carbon dioxide.
Orchids need a fungi to germinate and sometimes to grow and live. From seed, they develop to protocorms (structures of many more or less undifferentiated cells) which produce roots and leaves and develop to seedlings that can photosynthesise and live as autotrophs (all orchids are not autotrophs and protocorms do have a photosynthesis). The fungi infecting the orchid is often a saprophyte (some are common patogens for plants).
The reason to why orchids can live in some of the most absurd places on Earth (half-deserts, rock cliffs etc.) is the mykorrhiza. The mykorrhiza is especially important for the orchids without chlorophyll, often growing entirely below ground. Two examples are: Neottia nidus-avis and Epipogium aphyllum. The latter is also rootless (does have 'root hairs' from its rhizome) (Attenborough D., 1995).
One 'earth orchid' found in Australia also flowers entirely below ground! (The common flowering of such peculiar orchids is done by producing a flower spike reaching up above the surface.) (Attenborough, 1995)
The orchids are specifically adapted to live with 'its' fungi. Protocorms, orchids, and callistructures probably produce substances inhibiting and killing the fungi at a certain point, not to be victims themselves. These fungicdes are called phytoalexines and were proven already in1899 by Noël Bernard (a French).
Some orchids contain poisons and substances similar to narcotics. The orchids have been used for ages in the herbal medicine, especially in Asia (Dendrobiums and Phapiopedilum).
In grown plants there are differences in how and where the infection occurs. Some species seem only to be infected during certain seasons for limited periods. Some believe that this is correlated to resting stages of the orchid; during the rest, the fungi is limited to the velamen and stays inactive until the orchid starts growing actively again. As the orchid produce new cells (e.g. root tissue) the fungi can start colonising again.
The infection occurres in specific tissue (e.g. in the rhizome and in the roots). The hyfae grow into the cells and live as spiral-like structures, pelotons. (The cell wall of an embryo (in a seed) is not penetrated by the fungi.)
Fungi from parts of orchids have been isolated and seeds have been grown symbiotically in vitro, with the fungi inoculated. It's evident that it's a more specific relationship between fungi and seed (embryo) than between fungi and plant. A grown plant can be infected by several species of fungi whereas the seed only accepts one or a few fungi. If the wrong fungi infects the seed, the embryo might die. Isolated fungi (from plants) have sometimes not been able to germinate seeds symbiotically in vitro.
Organisms nourishing on dead organic material are saprophytes. There is a small number of orchids entirely unpigmented (without chlorophyll) which are unable to photosynthesise. These orchids may live from decaying organic material from around the growing spot, but are mostly affected by their mykorrhiza. Often, there is an ectomykorrhiza; where the orchid receives nutrients and sugars from a fungi which in the first place got the energy and nutrients via a mykorrhiza with a second plant. The second plant has the ability to photosynthesise and produce sugars. The fungi can be a symbiont, returning some nutrients to the plant for its sugar, or it could be a parasite, sucking the sweetness out of that poor plant.
All orchid flowers have a similar design. The variety of shapes and colours is however large since every specie is adapted to the specific requirements needed to attract the right pollinator. Insects and birds are common pollinators. The orchid species are adapted after the pollinator and often the pollinator has adapted to the orchid as well. Sometimes there is a remarkable coevolution evident.
Orchids don't have organs such as stamen and pistil, they have a structure called the column or the gynostemium which is an organ of both pistil and stamen. (Some orchids have two 'pistils', very few have three (Arditti, 1992).)
Large parts of a flower are infertile. They are however important as to luring a certain pollinator (either with smells or with visual appearance).
The 'infertile reproductive' organs of an orchid flower:
Hidden behind a small cap (the anther cap, E), the 'male' organ, pollinia, can be found. The cap can easily be removed (see image) and the pollinia (two yellow grains) are exposed. Too pollinate the flower, pollinia from another flower must somehow reach the 'female' organ, the stigma (F). Phalaenopsis flowers have a small hole beneath or on the back of the position of the pollinia. Two thin strands of hair-like structures may also be visible; these keep the pollinia from falling out. The stigma is also sticky, especially on flowers that are mature enough to be pollinated. A flower can generally not be pollinated with its own pollinia, but there are many exemptions....
Orchids don't have pollen. They rather have pollinia which, in a way, are yellow
lumps of pollen. These have a sticky string attached to them which has the function
to get stuck onto pollinators.
A single capsule of seeds may contain 1500 to 3 million seeds (Arditti, 1992). Orchid seeds are extremely small (0,3–14µm) and contains no (or very little) stored energy (endosperm), only an embryo (Arditti, 1992). To germinate they need the help of a fungi. The fungi supplies the seed with nutrients and sugars through endotrophic mykkorhiza. In nature this just happens. When growing orchids from seed artificially, there are two methods. One is to imitate nature by inoculating a fungi onto a substrate with nutrients and let the fungi germinate the orchid seeds. This is tricky and can go very wrong. A second method is to germinate the seeds asymbiotically on a medium containing sugars and all the nutrients and vitamins needed by the seeds (which would otherwise have been supplied by the fungi).
Since the seeds only contain an embryo, they are incredibly light (3-14µg) (Arditti, 1992). They also contain some air, making them float in water. The product of all of this is that the seeds spread like dust. Another factor making orchids successful colonisers.
The evolution of orchids occured after the split of the continents. This theory is probably correct since we today can see an obvious pattern of the distribution (Arditti, 1992).
An exiting part of the orchid biology is the pollination. The orchids have individualised appearances to attract certain pollinators. Some flowers even have motions, e.g. a vibrating lip (looking like a moving larva).
A South American Oncidium with small flowers growing in clusters from a large flower spike look just like bees. They are also pollinated by bees, attacking the flowers as if they were intruders (Attenborough, 1995).
A very famous Central American orchid, the Coryanthes, has a very smart pollination mechanism. It produces a liquid, filling a beaker-like structure (which is a part of the flower) to a level of a few millimetres. With a sweet scent and bright colours, the flower attracts male bees. The bees land on the flower and are soon messing around in an oily and slippery solution. Suddenly, one of the bees have fallen down into the liquid-filled beaker.
In this tub, there is only a single way out; through a small canal or tunnel. It's only on this spot, in particular, that the bee may find grip and be able to escape. The walls surrounding the container are otherwise extremely slippery, making it impossible for the bee to climb out. On the way out through the escape tunnel, pollinia are fastened onto the back of the bee. If the bee would have had a package of pollinia on it already, the pollinia would have been grasped by the flower on the way out. (Attenborough, 1995)
Orchids with imitating flowers, looking like the pollinator often have a production of pheromon-like substances to lure the males of a certain insect. These insects are deceived (to sex in this case). The fly or the bee wont get out much from the visit, but the flower has succeeded.
There is evidence for that orchids being naturally 'pollinator limited' (they only have a small number of available pollinators) have a longer period of flowering than orchids that usually are pollinated more quickly (Catling et. al, 1991). Observations also suggest that orchids flowering in longer periods of time, keep the ability to spread the pollinia longer than the ability to receive pollinia from another flower (Light et. al, 1998).
Flowers are not only visually adapted, they can also produce pheromone-like substances or other smells from certain cells. This is mostly done during certain hours of the day-cycle (there is no use in trying to attract an inactive pollinator).
Some orchids have a terrible odour, somewhat like rot (as a lur for flies), some have s sweet scent of honey while others smell vanilla or lemon.
There are naturally orchids producing nectars to attract the pollinator. One very well known example is the Star of Madagascar orchid, Angraecum sesquipedale, from Madagaskar. The specie has a long spur with nectars at the end. It's pollinated by a moth (Xanthopan morgani predicta) with an extremely long trunk-like organ. Notice the scientific name of the moth specie, predicta. It was this moth that was predicted to exist by Charles Darwin after his examination of the orchid.
Another (biochemical-) coevolution has probably occurred with orchid and fungi. Some research indicate that an infected orchid produces substances that are taken up by the fungi. The fungi then turn these substances into other metabolites, such as vitamins (Arditti, 1992).
About 3% of all the orchid species are probably able to perform kleistogamy and pollinate themselves. Some of these have the ability to self-pollinate if no pollinator would reach them, it's simply a method of survival. However, there are also orchids with flowers that merely open; they pollinate themselves instead of bothering with sex.
Epipactis helleborine is an example of an orchid that can pollinate itself if no pollinater has been able to do the job. (E. helleborine is a terrestrial orchid growing on the northern hemisphere; in the temperate zone). This orchid rotate its pollinia toward the stigma (Light et. al, 1998).
Arditti J. (1992) Fundamentals of Orchid Biology Wiley and Sons. New York
Attenborough D. (1995) The private life of plants Domino Books Ltd., Jersey.
Catling P.M., Catling V.R. (1991) A synopsis of breeding systems and pollination in North American orchids Lindleyana 6:187-210
Frisk O. (2003) Orkidéernas pollinatörer Svenska Orkidésällskapet 2003:10.
Light M.H.S., MacConaill (1998) Factors affecting germinable seed yield in Cypripedium calceolus var. pubescens (Willd.) Correll and Epipactis helleborine (L.) Crantz (Orchidaceae). The Botanical Journal of the Linnean Society, London.
Reinhammar G.L. (2004) Kort om orkidésystematik Orkidéer, Svenska Orkidésällskapet 2004:5
Stabén K. (2002) Vad orkidébladen kan berätta Svenska Orkidésällskapet 2004:3.
Stabén K. (2002) Ett orkidéblad berättar Svenska Orkidésällskapet 2004:5.
University of Sydney (2004) Fungal biology
Copyrights © Reserved Markus Axelsson, 2005.
