The orchid family (Orchidaceae) is one of the largest families of flowering plants, estimated at 22000 to 25000 species. In Southern Africa 466 species occur, they are divided over 52 genera. In Zimbabwe 340 species are found of which 80 % grow in the Eastern Districts.
1. FLOWER STRUCTURE
Orchids are well known for their complex flowers, which differ from all other plant families. They belong to the monocotyledones which differ among other things from the dicotyledones by having their floral parts arranged in numbers of three. Another difference with other plants is that the fertile parts of orchids are combined in one organ (the column)
The flower has six perianth lobes, arranged in two whorls of three. The outer three are called sepals, the inner ones petals (P). The sepals are green or brightly colored and are unlobed. The two lateral sepals (LS) are often different from the median dorsal sepal (DS). The median petal (lip (L) or labellum) is always different from the lateral petals. It is usually larger and might have an entirely different colour and form. The often extreme modification of the lip is connected with the pollination of the flowers: it lures the pollinator to the centre of the flower where the sexual parts are located.
The lip is normally the lowest part of the flower, this makes it easy for the pollinator to land. In order to achieve this the flower stalk will rotate 180 ° when the bud opens. When the lip is at the lowermost part of the flower and the dorsal sepal is at the top, the flower is called resupinate. Most southern African flowers are resupinate. Some orchids have the lip at the top and the median sepal below, their flowers are not rotated, these are called non-resupinate .
The male (stamens with pollen bearing anther) and the female parts (pistil consisting of an ovary and stigma) are united into a single structure called column, which is the centre of the flower. The anther which has two pollen sacs (thecae) is situated at the top of the column. The pollen sacs are sometimes covered by a lid. The pollen is not loose, it forms firm masses called pollinia. The pollinia are attached by stalks (stipites or caudicles) to sticky discs (viscidia) which themselves are attached to the rostellum . Orchids are pollinated by insects. When an insect visits the flower, the viscidia with the attached pollinia sticks to its head or body. When the insects lands on the next flower, these pollinia are transferred to the stigma of this flower, thus fertilizing it. The receptive stigma is very diverse in structure. The rostellum separates the anther from the stigma, which prevents self-pollination. The taxonomic division in the orchid family is often based on the structure of the column.
The flowers of orchids are symmetrical on only one axis and are called zygomorph flowers.
Flowers of orchids are either single or arranged in an inflorescence. An inflorescence has two parts, the stalk (peduncle) and the flower bearing part (rachis). The inflorescence may be unbranched with stalked individual flowers (raceme) or branched with each branch bearing several flowers (panicle)
2. HOW ORCHIDS GROW
2.1 Growth forms
Orchids are perennials, occurring in a variety of growth forms :
Terrestrial or ground orchids are rooted in soil; they occur in different types of habitat ( forest floor, grassland, woodland, ..) and at altitudes from sea-level up to 3000m. Many of them are deciduous which means that the plants loose their leaves and have a dormant period in which no growth occurs. The underground parts (tuberoids) store food reserves to ensure the development of new shoots at the beginning of the next growing season. Other species are evergreen.
Some terrestrial orchids do not have any chlorophyll, they get their nutrients from dead and rotting material, these are called saprophytes.
Epiphytic orchids live on trunks and branches of shrubs and trees. It is a widespread misconception that orchids are parasites : orchids cause no harm to their host and use them for support only. Epiphytes receive nutrients from the air and from small bits of detritus left by birds and dead plant material. They depend on rain and mist for their water supply. The main advantages for epiphytes are less competition with other organisms, a better light supply and better protection from parasites, diseases and fire. To overcome water-stress most epiphytes have developed adjustments, enabling them to store water, such as succulent pseudobulbs or fleshy leaves. There are many different host plants, they are mainly chosen because of their structural and chemical bark features. Many species show particular preferences as to the type of host and often are very localized in their occurrence.
Lithophytic orchids grow on rocks. The surface of rocks seems to provide a good place for germination and attachment of seedlings. Only a few orchids are exclusively lithophytic, most will also be able to develop as epiphytes or terrestrials .
Due to their specific requirements orchids are good indicators of the ecological state of an environment. They can only grow well in unspoiled places and die quickly if their environmental conditions change. Epiphytic species are mostly confined to tropical regions while terrestrials are more widespread in cooler areas.
The Vumba with its temperate climate and its high rainfall & mist forms an ideal area for epiphytes, a great variety is found here. Epiphytes occur mainly in forests and woodlands. Forest epiphytes such as Tridactyle tridactylites, T. bicaudata, Polystachya subumbellatum, Diaphenanthe rutila grow high up in the tree canopies where they are still able to receive sufficient light. The greatest abundance in epiphytes however is found in the Mist belt Brachystegia woodlands. Whole trees are covered in mosses, lichens, ferns and orchid species such as Cytorchis ringens, Stolzia repens, Ypsilopus erectus, Tridactyle tricuspis and Polystachya spp. In the lower altitudes the species variety is lower and one might find Cytorchis praetermissa, Microcoelia exilis and several species of Bulbophyllum. Some terrestrial species of Cynorchis and Disperis are found on the forest floor. The main group of terrestrial orchids however can be found in grasslands. Species of Disa, Satyrium, Eulophia and Habernaria are common.
2.2 Growth patterns
Sympodial growth occurs in terrestrial and many epiphytic species. Orchids grow from a horizontal stem, each stem develops in a single growing season and produces a flower or inflorescence. In the next season a new shoot appears from the base of the previous one and takes over growth. The stems may swell and form bulb-like structures called pseudo bulbs, in which water is stored.
Monopodial growth is restricted to epiphytic orchids. The main stem of the plant has an continuous growth of leaves from one growing point, they produce flowers at different intervals in time. The stem can be single or branched. Both leaves and roots have been adapted so that optimum storage of food & water occurs.
3.1 Mycorrhizal association
Growing orchids from seeds is rather difficult. Reason for this is that orchids depend on mycorrhizal fungi for their germination and for the growth of young plants. These fungi live in the roots of the plants and supply them with nutrients, which they obtain from decaying organic matter. After the seedling phase most orchids – especially if they have green leaves and thus chlorophyll – do not depend on the fungus any more. Both orchid and fungus benefit from this relation : the orchid receives nutrients, the fungus a place to live. This kind of relationship is called symbiosis.
3.2 Pollinator specificity
There is a close relationship between the morphology of the orchid flower and the pollinator that it uses. Cross pollination between different species is largely prevented by using different pollinators or by placing the pollinaria on different parts of the pollinators body. Orchids are thus species specific : one species of orchid often attracts one species of pollinator.
Moths and hawk moths are attracted to flowers that give off a scent at dusk or during the night. They are rewarded with nectar. Bees and butterflies not only go for a scent, they also are attracted to the usually bright flowers. They receive nectar and sometimes even oil. Sunbirds are also attracted to bright flowers and receive nectar. Flies, which are looking for a place to lay their eggs, are seduced by pungent smells and fleshy looking flowers. Not all orchids produce nectar or oils and these species have to deceive their pollinators. Some species may mimic nectar-producing plants. Others use sexual deception. The flowers looks like the female of a specific insect and this way fools the males.
Some orchids may be self-pollinating, this usually happens in areas were natural pollinators are scares. A few Southern African orchids use vegetative reproduction through fragmentation or by means of the formation of tubers or stolons.
3.3 Endemism and rarity
The minute and light seeds of orchids are scattered around by the wind in all directions. This way the seeds do not always fall into suitable areas and many of them are not able to germinate, this does not cause problems since thousands of seeds are produced. However this does not mean that individual orchid species are spread over large areas. The specific relationship between orchids and fungi & pollinators greatly limits the occurence of orchids to places were these fungi & the right pollinator will be found.
The population size of orchid species vary greatly, from extensive colonies of 30-50 plants to a few scattered individuals. A species is called rare when only a few plants are known from a small number of localities. With the specific environmental conditions that orchids require, they can easily become endangered or extinct. 72 orchid species are listed in the Red data plant list, 9 of these occur in the Vumba (Mapaura & Timberlake, 2002).
A large percentage of orchid species are endemic. Zimbabwe knows 23 endemic taxa of which 17 occur in the Eastern district (Mapaura, 2002). The Vumba has less endemics than Chimanimani or Nyanga, it is less isolated and its geology is more common. Yet the one endemic to the Vumba is an orchid and at least 3 other species, occurring elsewhere in Africa, are restricted to the Vumba for Zimbabwe.