Wednesday, 22 August 2007

#26 Greenhoods get the nod

The colour green is a rarity amongst flowers, which makes greenhoods rather special. But not only are these charming little native orchids green, they have an odd, almost comical 'alien' appearance, along with fascinating lives hidden amongst grasses and leaf litter.

Nodding Greenhood, Pterostylis nutans. Notice the pollen packages visible through the translucent hood - resembling frosted glass with green veins.

Late winter has been productive for my orchid hunting in the Hunter Valley, but rather than post several of my recent finds, I'm going to share some of the details of these remarkable natives and their secret lives, along with my images of the cute Nodding Greenhood, Pterostylis nutans.

My previous orchid biology entry gives a general and simple outline of orchid structure, but greenhoods defy the 'normal' petal and sepal structure (if indeed anything amongst orchids is 'normal' or constant).

Greenhood flower structure

Orchid flowers have three sepals and three petals, although at a casual glance, greenhoods don't appear to have this configuration.

Height is variable amongst Nodding Greenhoods. I have visited the colony below on Mount Royal Road east of Singleton over a couple of seasons, and found the plants were all from 4 to 6 cm high. They were growing in moss and lichen over rock. Nodding Greenhoods I've seen in the Lower Hunter growing amongst grasses in sandy soil have been much taller - up to 18cm. [I am presuming the yellow mass in the background is a slime mould]

As their common name suggests, greenhoods have a hood-shaped flower. The two lateral (side) petals and the dorsal (back) sepal have united to form a predominantly green, hood-like structure which protects the reproductive organs.

The two lateral sepals are fused together at the base to form the front of the flower, and protrude to form 'points' or 'ears'. These points generally extend above or to the side of the 'hood'. The third petal is the labellum (or tongue) which is hidden inside the hood.

The ovary of this successfully fertilised greenhood has swollen as seed develops.

In orchids, the male and female reproductive organs are highly modified and fused together, unlike most other plant families where the sex organs are separate.

The remarkable sex lives of Greenhoods

Generally, but not always, greenhood leaves take the form of a rosette of ovate ground-hugging green leaves. All greenhoods are deciduous. After seeds have been released, the above ground parts of the plant die and the plant exists as an underground tuber.

The the seed capsule dries and splits vertically to expose the dust-like seed to the breeze for dispersal. Notice at this stage, the stem has stiffened but is not yet dead - it holds the seed capsule erect in the breeze.

Pollinia (packages of pollen grains) are attached to the tip of the anther (male reproductive organ). The stigma (female reproductive organ) is located just below the anther, and receives the packet of pollen from a visiting insect, which is then transferred to the ovary where fertilisation occurs.

Greenhood flowers are usually pollinated by gnats, and sometimes mosquitos. I didn't know what gnats were, so I checked them out: gnats are common names for a large number of small, non-biting fly or mosquito-like insects in the order Diptera of the genus Mycomya. These tiny flies do not feed. They only live long enough to mate, lay eggs, and die. Eggs are laid in masses in the water or on aquatic vegetation, and larvae feed on living and dead plant matter.

Male gnats are enticed to greenhood flowers by a scent which imitates pheromones (chemical substances) emitted by females of particular gnat species.

The labellum is extremely sensitive to touch, and when triggered by a visiting insect, flips inwards towards the column, temporarily trapping the insect. A pollen package is either collected or received as the insect struggles to escape.

The greenhood flower does not offer nectar to the insect as a reward. This is sexual deception at its best. So the poor young gnat that only has a fleeting life in which to find and fertilise a mate, is duped by this clever little plant.

From a front-on angle, the Nodding Greenhood, Pterostylis nutans, has a sharp down-turned 'beak' and lateral sepals that form points facing the ground.

Cross pollination between different species is largely prevented by using different pollinators, or by placing the pollinium on a different part of the pollinator's body.

Survival strategies of the greenhood

This group includes some of the most drought tolerant orchids in Australia. Survival strategies include the large tuberoids which store moisture; the overlapping rosette of leaves which trap moisture and direct it to the root zone; and the tendency to grow in sites of plant litter accumulation and near rocks where run-off is concentrated.

A rosette of leaves of the Nodding Greenhood

I understand that the fleshy tubers are replaced annually. Dense colonies can also be formed by asexual (vegetative) reproduction, therefore allowing plants to produce offspring without sex.

Asexual reproduction can be made at any time, even before the plant is mature enough to produce seed via sexual reproduction in which insects or other external agents play a part. Vegetative offspring are 'clones' or carbon copies of the parent. The advantage with asexual reproduction is not having to rely on chance, however, the resulting limited genetic variation could be a disadvantage in a longterm evolutionary sense.

Some greenhood species flower more profusely after fire, but if fire destroys plants before seed is set, some species will be disadvantaged.

A colony of Pterostylis nutans, Nodding Greenhoods, amongst leaf litter in open forest in the Watagans National Park.

A taxonomic review of the greenhoods has split Pterostylis into several new genera, but the Nodding Greenhood has a distinctive appearance that is easily recognised. When next you see these charming hooded ground orchids or their many cousins, perhaps you will recall their kinky sex lives or their specialised survival characteristics. They might be small, but there is nothing 'ordinary' about greenhoods.

Greenhood Links

Michorrizal fungal association of greenhoods

Denis Wilson's greenhood observations

Greenhood key - PlantNET

Greenhood biology


Blogger Bill said...

Made my day!!

Gaye from the Hunter said...

I'm pleased, Bill, that these delicate little orchids and my account of their interesting lives has brightened your day :)


Denis Wilson said...

Hi Gaye

What a nice job you have done of explaining the life of a Greenhood. Well done, Gaye. Great photos, as per normal.

The links you come up with constantly surprise me - which is great.

Thanks for the cross-linking to my Greenhood posts. Linking to the "label" works really well - you pick up them all that way. Very nice. I have never done that. Neat trick. Thanks.


Gaye from the Hunter said...

hello Denis,

Thank you. Research, cross checking and searching for interesting and appropriate links is proving a great learning tool for me. I am becoming more experienced at finding reliable information the more I practice.

I am heading off to check out small colonies of Maroonhoods and Tall Greenhoods today, hoping they are still in their prime. I am particularly going to observe the extremely sensitive labellum of the Tall Greenhood, Bunochilus longifolius, after discovering that my previous photographs had captured the labellum withdrawn rather than set. Trouble is, I only found a few plants two weeks ago, so hoping I can relocate them and that some new plants have flowered.


Denis Wilson said...

See what you get into - the more you learn, the more you realise what you didn't notice before.
I have done the same with this same species, and then checked back in earlier photos and found them with tongues down and then tongues clicked up inside. Fascinating.
I look forward to your "findings" on a future blog.