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
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
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
2. HOW ORCHIDS GROW
2.1 Growth forms
Orchids are perennial plants, occurring in a variety of growth
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.
terrestrial orchids do not have any chlorophyll, they get their nutrients
from dead and rotting material, these are called saprophytes.
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.
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
2.2 Growth patterns
There are two different growth patterns known in orchids :
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
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 arrive from the stem and have been adapted so
that optimum storage of food and 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
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,
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.