Orchidaceae
What are orchids?
Orchids are a family of monocotyledons. They have, like other monocots, a single seedling leaf and their floral parts in threes. Orchids are one of the largest families of flowering plants, challenging the daisy or sunflower family, the Asteraceae (Compositae), for the title of the largest of all families. Currently there are known to be 25,000 species of orchids and estimates, based on the current rate at which new species are being discovered and described, suggest that the number may reach 30,000.
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| Anguloa clowesii |
Orchids are a cosmopolitan family found all the way
from within the Arctic Circle to Tierra del Fuego and the islands south
of Australia. They are absent only from open water and from true deserts.
The smallest orchid is thought to be Bulbophyllum
minutissimum which is 3-4 mm tall, but many other orchids approach
it in size. The largest orchids are probably vanillas
which are lianas that reach into the crowns of rain-forests trees and
may be 20m or more in length. Some tropical orchids also form very large
clumps on rocks or in trees. Grammatophyllum speciosum plants
of several hundred kg have been reported from Southeast Asia.
Some orchids live on the ground while others grow perched
on trees or rocks. Vanilla and some of its allies scramble up
shrubs and trees or are lianas that grow up forest trees, using their
roots for support.
What then unites these diverse plants into the family called
the Orchidaceae?
All orchids have protocorms, and these are not found
in any other family. A protocorm is the structure formed after the germination
of the seed and before the development of the seedling plant. The protocorm
has no radicle and instead has mycotrophic tissue (and hence differs from
other flowering plant seedlings).
Other important characters shared by most orchids include:
* The fusion of the male and female organs into a single structure, called
the column.
* A large number of small seeds per ovary.
* Stamens are found on the abaxial side of the flower (the side facing
downwards/away from the stem).
* The lip/labellum (a modified petal) occurs opposite the fertile stamen(s).
* Flowers are often resupinate (have twisted through 180° during development).
* Pollen is usually bound together to form large masses (called pollinia).
