Orchids

  • About Orchids

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
  2. How orchids grow
    • Growth forms (terrestrial, epiphyte, lithophyte)
    • Growth pattern (sympodial, monopodial)
  3. Ecology
    • Mycorrhizal association
    • Pollinator specificity
    • Endemism and rarity
  • Orchid list: A species list of orchids from the Vumba Mountains.
  • More info: Zimbabwe Flora

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)

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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.