Orchidaceae – The Orchid family.
There are about 28,000 accepted species in around 763 genera plus more than 150,000 hybrids and cultivars.
Classification.
Cassification is constantly changing but orchids are subdivided into the subfamilies Apostasioideae,
Cypripedioideae, Epidendroideae, Orchidoideae and Vanilloidea.
1. Subfamily Apostasioideae flowers have 3 (2) fertile anthers.
These are thought to be the oldest or basal orchids. 2 genera with 15 species.
2. Subfamily Cypripedioideae flowers have 2 fertile anthers. Contains the Lady’s slipper orchids which
have the medial petal (labellum) shaped like a pouch.
3. Subfamily Orchidoideae flowers have 1 fertile anther and there are around 3,500 species.
4. Subfamily Vanilloidea flowers have 1 fertile anther. There are 15 genera and about 180 species.
Vanilla is one of the few orchids that grow as long vines.
5. Subfamily Epidendroideae is divided into 16 tribes including:
- Malaxideae with Dendrobium orchids,
- Cymbidieae which has Cymbidium and Oncidium orchids,
- Epidendreae which includes the genera Cattleya, Epidendrum and Laelia,
- Vandeae including the genera Phalaenopsis and Vanda.
Orchid features.
A characteristic is that the stamens, stigma and style fused into a central column above the inferior ovary.
They are also bilaterally symmetric with 3 sepals and 3 petals, often with resupinate (upside down) flowers, very
small seeds and nearly always a medial petal which is enlarged and modified (a labellum).
Orchids are perennial herbs and grow in almost every habitat.
Most are epiphytic in tropical or subtropical areas, others are terrestrial and prefer temperate areas.
Aerial roots can be metres long and the roots of terrestrial orchids may be rhizomatous or form tubers or corms.
Orchids exhibit two types of growth – monopodial and sympodial.
Monopodial orchids produce one upright stem from a bulb, have alternate leaves
and the flowers grow from the leaf axils near the top of the plant.
An example is Vanda orchids that can have stems up to 2 metres or more high.
Sympodial orchids spread laterally with successive upright stems forming along a creeping rhizome.
As one flowering stem dies another one forms further along the rhizome.
The flowering stems have a variously shaped swollen base (pseudobulb) which stores
water and nutrients and has one or several leaves on the top.
Leaves.
The majority of orchids have simple leaves with smooth edges and parallel veins (a few have a network).
They can be minute or up to metre long. They have no stalks and their bases usually sheath the branch.
Leaf arrangement is commonly in a spiral or 2 ranks, rarely whorled.
They can be ovate, lanceolate, round or folded lengthwise; thin or fleshy; wide or narrow; annual or
perennial and some have a waxy coating to help preserve water.
Most are shades of green but they can be greyish or bluish or other colours and some are patterned.
Inflorescences.
Flowers may be solitary or there can be many along a flower stalk.
They can grow from the base of the plant, the tip of the stem, or in the leaf axils.
Flowers.
There is a huge range of shapes, sizes and colours of both the sepals and petals.
Sepals and petals are sometimes difficult to distinguish and are then collectively called tepals.
The three sepals may all be of equal size, one may be larger, and some or all may be fused.
There are three petals and the medial one is always modified and enlarged and is called the labellum or lip.
The other two are the laterals.
The labellum is often the most prominent and ornate part of the flower.
In flowers with 2 stamens it is usually shaped like a pouch and acts as a landing pad for pollinators.
In flowers with only 1 stamen the labellum may be flat, lobed or a tube.
During development most orchid flowers undergo resupination where the individual flower stalks or ovaries rotate
through 180 degrees so the originally upper medial petal (the lip) becomes the lower medial one.
J.F.
