#19 Getting to know Orchids

Although I have been enjoying finding and photographing native orchids in their natural habitat, I became aware that I knew little of the plant structure or of its environmental requirements, so I set to work to increase my understanding of these beautiful and fascinating plants. I’ll share my newfound knowledge here in basic and simple terms, which is all my grey matter can fathom at this time.

A Diuris species, possibly D. maculata - near Broke in the Hunter Valley, October

Orchids are one of the largest families of plants on Earth. There are about 1200 species of native orchid in Australia occurring in more than 100 genera. I have probably encountered a couple of dozen species, so I’m not likely to photograph much of that list in my life time, but I’m working on it.

Orchids are categorised by their way of growing: epiphytic, terrestrial and lithophytic. Orchids also occur as saprophytes.

Epiphyte:

Epiphytic could be described simply as ‘aerial’, that is, they grow on trees or other living plants above ground level with their roots exposed to the air. Epiphytes are not parasitic in nature, therefore do not obtain moisture or nutrients from their host, but use the surface of their host as a firm substrate upon which to anchor their roots. All the orchid’s nutrition and moisture is obtained from the atmosphere by its exposed roots.

Note the aerial roots of the epiphytic orchid, Bulbophyllum exiguum, (Mat Orchid), clinging to the tree. Barrington Tops National Park, April.

By anchoring themselves on tree trunks, epiphytes gain access to more light in the dim rainforest environment, as well as placing themselves in a convenient position for flying pollinators.

Terrestrial:

‘Terrestrial’ means ‘of ground’, therefore terrestrial orchids grow at ground level in soil and leaf litter. Approximately two-thirds of Australia’s native orchids are terrestrial, and vary greatly in habits.

Ground-hugging leaves of a terrestrial orchid, Pterostylis nutans (Nodding Greenhood) - Mount Royal, Hunter Valley, June.

Lithophyte:

Lithophytes are epiphytic orchids that anchor themselves and grow on rocks or cliff faces instead of on living trees. They also chiefly obtain their moisture and nutrients from the atmosphere. A good example of a lithophytic orchid is Dendrobium speciosum, the common and showy Rock Lily.

Dendroium speciosum, (Rock Lily), forms large heavy clumps which hang from rocks and cliff faces anchored by the roots clinging to rock surfaces - Watagan National Park, just south of the Hunter Valley, October.

Saprophyte:

Saprophytic orchids grow on, or in, dead organic material. Some saprophytes do not have chlorophyll and therefore are unable to photosynthesise. These plants rely upon their mycorrhizal fungus for their nutrition for their entire life. [Mycorrhizal fungus is explained below]. Some saprophytic orchids live their life completely below the ground, so I guess I’m never going to stumble across these mysterious little plants.

Dipodium punctatum (Blotched Hyacinth Orchid) with no leaves or green pigment, is an example of a saprophtic orchid - Allyn River State Forest, Hunter Valley, November.

Ok, that’s got orchid growth habit categories sorted. That’s the easy part! I was very interested to learn more about the specialised association that exists between orchids and fungi. This association is vital to the survival of orchids.

Mycorrhizas:

The following is a simplified explanation of my understanding of orchid mycorrhizas:

Orchids develop a mutually beneficial relationship (symbiosis) with a microscopic thread-like fungus, a relationship known as a mycorrhiza. Generally, neither the orchid plant nor the fungus parasitises the other, but mutually benefit from one or more aspect of the other’s existence.

The mycorrhiza is said to increase the rate at which the orchid plant can extract nutrients from the environment, especially phosphate, but its main role is to provide the germinating seed, and developing plant, with carbon. Dust-like orchid seed contains virtually no energy reserves of its own, and under natural conditions, will not successfully germinate without infection by the mycorrhizal fungus.

The fungus continues to supply the developing plant with all its organic energy until such times as the plant starts to photosynthesise.

This mycorrhizal association allows some orchid species to remain leafless for seasons, to years, until the aboveground environment is suitable for leaf production, and therefore, photosynthesis. It is unclear what, if anything other than substrate, the fungus gains from the relationship.

The structure of orchid flowers:

Orchid flowers contain both female and male reproductive structures fused together in a ‘column’, usually located in the centre of the flower. Three sepals and three petals surround the column. One of the petals, the labellum (often referred to as the ‘lip’, but resembling a tongue), is greatly modified to form a ‘landing pad’ for insect pollinators.

The image below is labelled to identify the orchid flower parts:

A and D = petals
B and C = lateral (side) sepals
E = dorsal (upper back) sepal
F = labellum (lip) [which is the third petal]

G= column (fused sexual organs)

Flower parts labelled. Petalochilus pictus (White Lady Fingers) - Heatherbrae, Hunter Valley, May.

But orchid flowers come in an amazing array of odd and contorted shapes where these various basic flower parts defy that simple arrangement. Often the dorsal sepal forms a protective hood over the column and labellum giving the impression that other components of the flower are not present. The Helmet Orchids are excellent examples of a hooded dorsal sepal hiding all other flower parts. In the image below, the labellum can be seen under the ‘hood’, but the two remaining sepals and petals are tiny, insignificant and hidden.

Corybas aconitiflorus (Spurred Helmet-orchid), tipped slightly back so that the hidden labellum is visible beneath the large hooded dorsal sepal. Barrington Tops National Park, Hunter Valley, May.

Orchid pollination:

Orchids employ various remarkable means of attracting and duping pollinators. The sex life of orchids is fascinating, and after delving into the subject, I would say that these flowers are far more ingenious and resourceful at snaring a partner than we mere humans.

With the exception of those orchids that are self pollinated, most orchids are pollinated by insects. Some orchids use secret passageways, intricate hinges and temporary traps to hang on to their flying pollinator long enough to pass on, or collect, a precious pollen package (pollinium).

But the complex lures used to attract specific pollinators are truly remarkable. Some orchid flowers emit scents mimicking pheromones produced by specific female wasps or bees to attract males of their species. The aroused male insect seeks out the flower which visually resembles a female mate, and frantically attempts to mate with the flower, thus collecting, or delivering, pollen.

Some orchid flowers have evolved to rely on one specific insect species to perform pollination duties, which emphasises the fragility of ecosystems. If one insect disappears, so too could one plant go extinct.

Orchids and wildfire:
Some terrestrial orchids are unable to flower well, or flower at all, unless they are subjected to wildfire.

An unusual, but common orchid, Spiranthes sinensis, (Pink Spiral Orchid or Ladies' Tresses). Although I took this photo in the New England Region of NSW, it is also a Hunter Valley species - March

My thanks to Denis Wilson of Nature of Robertson Blog for assisting me to understand the structure of the orchid flower. Denis' orchid entries can be found here.

Additional orchid reading:

Orchid mycorrhizas

Pollination by sexual deception

Underground orchids of Australia