Thursday, 5 November 2015

Putting all eggs in one basket

When competition is high, plants have to be creative to explore the diversity of the available potential pollinators. Not all pollinators are attracted to fruity, sweet scents and flavours; some flies, for example, feed and lay their eggs on flesh of rotting animals, thus the smell of fresh carcasses are a temptation for these flies! Flowers, pliant as they are, can imitate this smell, deceiving the hungry flying creatures to the illusive corpse feast. I could name a few species of plants with this kind of strategy, but for now I will focus on one species – the titan arum, a plant from Sumatra (Indonesia) famous for being ridiculous on both size and smell – the kind of eccentricity that like to grow in tropical rainforests.

Two titan arums in Sumatra, early 1900's. The inflorescence (right) can reach over 3 metres in height, whereas the leaf (left) can reach up to 6 metres tall!

 Titan arum, Amorphophallus titanum (Araceae) does have the largest inflorescence described so far. Yes, that is not a flower, but a pseudanthium. A pseudanthium is defined to be an inflorescence that mimics a flower, thus the flowers are usually very small and strategically packed in compact inflorescences. In Araceae, the structure that bears the flowers is called a spadix, which is generally enclosed in a big showy bract – the spathe. Interestingly, the visible part of the spadix is not fertile, and acts as an osmophore, essential for an effective pollination in this family of plants. It is here where this fetid (but magic) pollination story starts: the osmophores (floral fragrance glands) produce the smell, which will attract all sorts of flies and beetles. Of course the smell has to be really intense to reach vast distances in a tropical forest, so the spadix also produces a great deal of heat (up to 40ºC in A. titanum!), which is believed to help convey the scent further. Isn’t it fascinating?!

The inflorescence is formed by the spadix and a big bract called spathe.
Photo: David A. Purvis (New Reekie in bloom at the Royal Botanic Garden Edinburgh, June 2015)

The fertile parts of the spadix (the flowers) are hidden by the spathe in a space called floral chamber. Araceae are known for being really good hosts for theirs pollinators: once the visitors arrive, they sneak into the floral chamber where they can spend a peaceful night, protected from night dangers and feed their bellies with some pollen. Meanwhile, they hang around on flowers, obviously some of that pollen gets attached to their bodies and carried away to the next pseudanthium the following morning.

Entrance to the floral chamber: flowers are already visible at the base the of spadix
Photo: Chlorophil7

Araceae flowers are usually very small and reduced to the essential parts – the sexual organs. So the tepals are absent or highly reduced; male flowers are reduced to a single stamen, and female flowers to a single carpel.  Now, I haven’t been counting them, but it is said that in one single A. titanum pseudanthium the number of female flowers can go up to about 450, condensed in the base of the inflorescence. Male flowers (as are smaller) can be even more, ranging from 500 to 1000. To avoid self-pollination, female flowers are receptive earlier. So, when the male flowers start releasing fertile pollen, the stigmas of the same inflorescences are not receptive anymore, and there is no danger of self-pollination. It is also very common in Araceae the formation of infertile male flowers (also called staminodes, as each flower corresponds to one single stamen), the function of these staminodes is not very clear yet. Titan arum, however, seems to lack such staminodes.

The base of the spadix bears both male flowers (single-stamen flowers; top of the inflorescence) and female flowers (single-pistil flowers; base of the inflorescence).
Left photo: isenbergs2007; Right photo: Brian 

As you can imagine, producing such a big structure, plus all that smell and heat is a great expense of energy resources. Thus, the plant only flowers for about 3 days every thousand days (I have learned this one from Sir David Attenborough).

David Attenborough with a wild Amorphophallus titanum in Sumatra filming for BBC's The Private Life of Plants

Titan arum is quite a fantastic plant, not only it looks unreal but it is incredible how such a rare flowering event is enough to succeed on fruit production and perpetuate the lineage… It seems to be the case where "putting all eggs in one basket" actually works.


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