Bribie Island Orchid Society

SPECIES

 

Orchids are the largest and most successful plant group on earth. Estimates of their numbers range from 20 000 to more than 35 000 in 750-850 genera. These numbers mean that nearly 10% of the world’s flowering plants are orchids.

 

 

TERRESTRIAL

 

Habitats:

Terrestrial orchids in Australia reach their optimum development in the sclerophyll forests of the south. Here the open forests and woodlands in some areas may be carpeted with an array of interesting and colorful forms, especially noticeable in the spring when the majority of species flower. Certain species favour particular niches within the forest, thus some are colonisers of the slopes and ridges whereas others are found predominately in sheltered gullies and along watercourses, often in quite shady locations.

 

Terrestrial orchids are often common in coastal districts. Coastal forests stabilised coastal dunes and heathlands are suitable habitats and may support a wide diversity of types. Soils are usually sandy, although peaty barns may predominate in some areas. Coastal swamps often contain interesting species. In such habitats, water is present in winter and spring and the orchid plants can be partially submerged. Terrestrial orchids decrease in abundance with increasing distance from the coast. Suitable inland habitats include mallee communities. sandhills and sandplain vegetation (especially in Western Australia). Some grow along the snow line areas of some high ranges.

 

THE STRUCTURE OF TERRESTRIAL ORCHIDS

 

Roots:

Roots of terrestrial orchids are generally fleshy and very brittle. Perennials such as Calanthe and Phajus produce a mass of thick roots, whereas most deciduous terrestrial have a few, short roots which are often irregularly shaped.

 

Storage Roots:

The roots of many deciduous terrestrial orchids form fleshy organs for storage or resting purposes. Commonly known as tubers, these structures are correctly termed tuberoids, because they are formed on roots and not stems on which true tubers arise. Such structures are very common in Australian terrestrials and they allow the orchids to survive (by avoidance), the extremes of dryness and heat which occur in late spring and summer. Tuberoids contain an apical bud and new roots are produced from the base as it elongates into a new stem. Some species produce a single tuberoid each year which replaces the parent tuberoid. This is known as a replacement tuberoid and it is usually produced at the end of a short root termed a dropper or sinker. Those terrestrials which grow in colonies can produce more than one tuberoid annually, each at the end of a long, fleshy root. Thus these species increase in numbers by vegetative means and the extra tuberoids are termed daughtertuberoids. A few terrestrials such as species of Spiranihes and Cryptosis, have swollen fleshy roots that can act as storage organs.

 

 

Leaf Shape:

Leaf shapes encountered in Australian terrestrial orchids include linear (Diuns sp.), cylindrical (Thelyrnitera), cordate (Acianihus exertus), lobed (Aciaiithus arnplexicailis), some are hollow (Microtis and Prasophyllum).

 

 

Leaf Thickness, Vestiture and Colouration:

Many terrestrial orchids have leaves which are very thin-textured (Corybas, Pierostyllis), whereas others are fleshy (Calochilus robertsotiii), or tough and leathery (Crpiostyh’s subulata). Most are smooth but a few are hairy or have unusual bristle like structures (Caladenia gemmata). In the majority the colouration of both surfaces is similar but in a few terrestrials the under surface of the leaf can be purplish (Acianthus exsertus) or appear as if frosted (Corybas). Many species which grow in very shaded habitats have thin textured, pleated leaves (Calanthe triplicata).

 

Leaf Arrangements:

The leaves of deciduous terrestrial orchids are mainly basal and may be solitary (Acianthus, Thelmitra), in pairs (Chiloglottis), or form a spreading rosette (Pterostylis). Some groups have leaves scattered up the stem in a loose spiral (Habenaria, Spiranthes); others are arranged along the stems in two alternate rows. This later arrangement is termed distichous and is exemplified in Pterostylis longijolia.

 

Inflorescence:

The point where the inflorescence arises is of interest. In deciduous terrestrials the inflorescence is terminal on the stem, whereas in many of the evergreen ty~es such as Calanthe triplicata and Phaius tankervilleae, the inflorescence arises from a nearly basal node.  Many native orchids have one-flowered inflorescences (Chiloglottis. Corybas, Pterostylis). Flower arrangements on an inflorescence can be spiral (Spiranthes sinensis), alternately in two ranks.

 

Effects of Fire:

Fire is a major environmental factor in Australia and the most important habitats for terrestrial orchids are burnt regularly. Members of the Australian flora, including some orchids, have developed remarkable adaptations to cope with fire. Many terrestrial orchids not only survive summer bushfires but have become so adapted to their occurrence, that they have become an integral part of their life cycle. Thus a number of species require fire to induce flowering (that is they will only flower after fires) and many other species flower much more profusely in the season following a burn.

 

About sixteen species of Australian terrestrial orchids are dependant on the stimulus provided by a summer fire before they will flower. Clearly such orchids have evolved during the long history of burning. It is interesting to note that fifteen of the sixteen fire dependent species occur naturally in south-western Western Australia, thirteen of these being endemic to this region. In eastern Australia there are only three species which can be said to be fire dependent and only one of these is endemic to the region.

 

Terrestrials dependent/stimulated by fire. Caladenia, Diuris, Chiloglottis. Corybas, Lyperanthus Aficrotis, Prasophyllum, Glossodia, Thelyrnitra.

 

VEGETATIVE REPRODUCTION

 

Terrestrial:

Many terrestrial orchids which grow in colonies increase by producing more than one new tuberoid each season. Thus the colonies increase in size and spread into new territory. The extra daughter tuberoids are produced at the end of long, stolon-like roots and are of similar shape and size to the parent tuberoid. By this technique some terrestrial orchids can increase two to four fold in numbers each year. Species which exhibit this type of vegetative reproduction can be found in the genera Actanthus, Corybas, Pierostylis, Lyperanthus. To name a few.