Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Tuesday, October 21, 2008

Waratah flowers - details

The Waratah flower is actually a complicated structure of paired flowers, joined into a head, which is classed as a "terminal conflorescence" (according to the Botanists). This means, as we can all see, that there is a large flower structure (composed of many individual flowers), at the end of a branch. There is a circle of bracts (modified leaves) which served to protect the flowers as they developed (follow this link to see a photo of the bracts curled up around the flower). The bracts also make the entire flower head more prominent, visually.

What these photos do not show is the bit about the flowers being held "in pairs". That is a botanical distinction which indicates (to botanists) that these flowers are related to members of the Grevillea genus. I shall show some Grevillea flowers in a few days, where that point will be more evident.

Today I wish to concentrate on the intricate flower structure which Waratahs have developed, and discuss the implications for the pollination process of Waratahs.

Where to start? Well, we know that Waratahs are attractive to birds and to bees. That is because they produce copious amounts of nectar. Their red colour also helps make them attractive to birds (for birds "see" red colours very clearly - because of their special optic nerves). In Australia, a whole tribe of birds has evolved along with these Australian members of the Proteaceae group of plants. These plants which are similarly shaped, (nearly all tubular flowers, with nectar). And so there is a mutual dependance arrangement going on, between the Honeyeater tribe of birds (from tiny Spinebills to large Wattlebirds) and this group of related plants. Gardeners and Birdwatchers know this, for they are always talking about planting out "bird-attracting" gardens.

How does it work? This shot shows three flowers from closed bud stage (lowest) to open, and then fully open (top). What it does not show is the "nectary" which is a tiny gland deep within the base of the flower. However, when you look closely at a waratah flower, when it is freshly opened (usually just the first few rows of flowers at the base) you will see tiny gleaming droplets of nectar, inside the curled parts of the opened flower. In some cases, the nectar will leak out, and collect on the red bracts at the base of the flower. In which case, expect ants to be attracted to these flowers.

As the individual flower matures, the style (the long pointed part of the flower) straightens out, and emerges backwards from a longitudinal split in the tubular part flower (which is known as the "perianth"). Then the perianth lobes curl arond on themselves, and shrink back down towards the base of the flower.

From the point of view of pollination, the trick is that the anthers in these flowers are held deep withing the tube (the perianth) and do not protrude at all. That means that unlike many familiar garden plants which present masses of pollen (on long anthers), which makes their pollen readily available to insects, the Waratah and other related plants, use what is known as a "pollen presenter". This is the most remarkable part of their trick, for the "pollen presenter" is in fact the female organ of the flower - the "stigma". But for the first few days after the flower opens, the stigma is not mature (as a female organ) and instead acts to carry the pollen which has been stuck onto it when it was curled up inside the (previously unopened) flower. In the image above, you can see there are four dark dots inside the curled perianth segment. Those are the male anthers, and while the flower was still unopened, and the style was still curled up inside the tube, the stigma (the pad on the inside edge of the style) was in a position to touch the anthers. So, the stigma collects the pollen from the anthers, and as it opens out, it takes the pollen with it.

In the next image, you can clearly see the dark brown coloured pollen on the stigma - the inside of the point of the style (curved rod).

The next stage is less visible. The pollen dries up, and then the face of the stigma (the "inside face" of the curled style then become mature as its female stage, and becomes sticky, ready to receive any pollen from another Waratah flower which might be brushed onto it by a passing Honeyeater, or bee.

The curved shape of these flowers is important, for Honeyeaters have long thin, curved beaks which they use to poke inside the tubular flowers of the Waratah, in search of the reward of some nectar. As they do so, they touch the pollen presenter, and receive a dusting of pollen on top of their forehead (or just around the beak). The bird then goes off to another flower. And if that flower is at the slightly later stage of development, when its female stigma is now mature (sticky), it till be able to receive pollen dusted onto it by the bird's feathers on its forehead.

You will often notice a Honeyeater (of some description) sitting on top of a Waratah flower. As it reaches down toward the base of the flower, to get the nectar, its head will come down from above the flower, making a perfect match between its forehead and the position of the stigma. That is why Waratah flowers are shaped they way they are - to suit the pollinators which have evolved with them.

I noticed today that my blogging colleague who writes "A Snail's Eye View" has written a report on pollination structures in Western Australian Banksias, which are related plants, despite some apparent differences. Their internal flower structure is very similar, and hence the pollination process is nearly identical, except that they rely more on insects, and tiny "Honey Possums" for pollination. Banksias develop a woody cone after pollination, whereas Waratahs develop a pod. Apart from that the pollination system is very similar indeed. You can also clearly see on Snail's Blog that Banksia flowers are also held in "pairs", even though the overall structure is also a "conflorescence" (as is the Waratah head).

In an example of "parallel evolution" many red tubular flowers in Central and South America are pollinated by Hummingbirds, which are not particularly closely relate to our Honeyeaters, but which have adapted to feed on long tubular red flowers. As they do so, Hummingbirds receive dobs of pollen on their foreheads, from the plants, which they then take on to other plants. Sound familiar?

In my opinion, this is just an illustration of the fact that plants have shaped the world to suit their interests.

And we like to kid ourselves that we are the "masters of the universe"?


Robert V. Sobczak said...

One of my favorite natural history authors is Austrailian: Tim Flannery. An interesting point about plants shaping the world ... that makes me pause for thought.

Denis Wilson said...

Ah, yes, I read an interesting book a few years ago where the author argued that Potatoes used us to take over the earth.
It is an interesting perspective on things.
Tim Flannery is top value.

Mosura said...

An interesting read which leads me to the question:

Can Waratah self pollinate from it's own leftover pollen once the stigma matures?

mick said...

Thanks for a very interesting post.

Anonymous said...

I have a waratah that has been in the ground two years. I have been watching 'buds' form very slowly over the year, but not grow much in size. Now suddenly these buds have opened to form leaves October). Is this an early stage of flower development, or is this a growth stage to form the stem for the flower to grow next year?

Denis Wilson said...

Dear Anon.
Your last note is spot on - your plant is preparing for next year.
But the good news is that at least it is growing, and ought flower for you next season.
The flower buds are noticeable larger than leaf buds.
Fat and rounded in shape.
Leaf buds are more pointed.
As far as I know, as long as your plant is healthy, it should flower.
But you cannot "promote" flowering next year, as far as I I know.
Perhaps some blood and bone, but not a commercial fertiliser.
Maybe a small amount of native plant fertilizer.
I do not fertilize mine, but I have good soil.
You do not say where you are, or what soil type you are on.
Best of luck.