In Flanders' Fields the Poppies Blow

Poppies have always give me good childhood memories of fields in late spring, playing with dolls made of poppy flowers is also part of those days spent in the field. Even my mother used to call me “Mary Poppy” when I was a child making of poppies definitely very much familiar and dear flowers to me.

My grandfather taught me how to make poppy dolls like this one...
Source: www.wildplantforager.com

They are, however, associated with heavier and darker topics, such as death and sleep since ancient times and by different cultures, but also more recently with war and the victims of World War I, due to the famous poem “In Flanders Fields”, by John McCrae. Red poppies, Papaver rhoeas, are therefore symbol of wartime remembrance, and are still used in some countries to commemorate those who died in WWI.

 

In Flanders Fields by Willy Werner

But it doesn’t end here, poppy fields have another negative connotation, aside the Flanders fields – and I’m not talking about Papaver rhoeas anymore. I’m talking about Papaver somniferum, also known as the opium poppy. Since ancient times (and by ancient times I mean the Neolithic age!) poppies have been cultivated and used for food, medical and recreational purposes due to its pain relief properties. Oh yes, ancient people from Mesopotamia to Egypt, from India to Persia, they already used this powerful drug. It allowed advances in medicine, with especial emphasis to surgeries, due to the morphine found in the flowers, a powerful anesthetic compound. So, because of its properties, these flowers became associated with sleep, death, drugs and war. Not the greatest subjects to be associated with.

Afghan farmers collecting raw opium in a poppy field
Photo: Rahmat Gul

I’m not going to talk about crops of poppies in Afghanistan and in the Golden Triangle for opium production– enough of such topic! I would like to give you a botanical perspective of this flower too. Papaveraceae – the poppies’ family; is belongs to basal eudicots (order Ranunculales) and are known for having dimerous and dissymmetric flowers, even though exceptions are found in the family, it is quite a stable character. However, Papaveraceae is a somewhat diverse family with 44 genera and 760 species, divided in 2 subfamilies, so I will focus for today on the genus Papaver only.

As you have probably noticed, they do have 4 petals, but if you look carefully you will see that they are inserted in two different whorls – two whorls of two petals. In fact, the calyx is also made out of two sepals, however they tend to be deciduous (falling off after blooming), so it is of no surprise if you don’t find the sepals in a poppy flower. The dissymmetric nature of these flowers is also reflected in other organs, the stamens, the carpels and even the ovules arise in two rows.

But the most interesting structure of the genus Papaver is the gynoecium. The stigmas are laterally confluent, and since they usually don’t have styles (meaning the stigmas are sessile), a stigmatic disc is formed on the top of the ovary – the pollen receptive surface. When ripe, the unilocular but multicarpelate ovary forms a dried fruit, a capsule, together with the stigmatic disc. As a dehiscent fruit, the capsule has valves or pores to allow the seeds to be released from the capsule when dried and ripe.

Left: Morphology of the flower, Papaver rhoeas (Photo: Dennis Heidrich)
Right: Capsule of Papaver somniferum (Photo: http://www.flickr.com/photos/f4/)

Papaver is also a genus know to be bud-pollinated – this means that sexual organs may be ripe before blooming and that pollination may occur even before the flower is open to pollinators! This means that even though poppies cannot control their sexual impulses before flower blooming allowing self-pollination to occur, they found a strategy – they are not self-fertilized due to other mechanisms, a mechanism that even though does not work 100% it can help to avoid selfing! So this means that the fact that self-pollination may occur before blooming doesn’t mean that this flowers don’t need the little flying helpers for cross-pollination, this is why they keep their attractive attributes!

Ranunculaceae and the origin of petals

There are events that change the course of evolution. In my opinion one of the most important events that have changed the course of evolution in flowering plants was the transition between spiral to cyclic flowers. This episode was obviously gradual as everything in Nature is, but it is still incredible to observe this transition in living plants and there is a group telling us this episode better then any other.

Early diverging eudicots from APG III tree

Early diverging eudicots is the transitional group between basal angiosperms and core eudicots. In fact, plants belonging to this group are inbetweeners, they share characters from both groups, bringing an extreme floral diversity to the plant world. They were the authors of many innovations, such as nectaries, zygomorphy, perianth differentiation in two different whorls (calyx and corolla) and many other novelties. One of the best examples of this diversity is Ranunculaceae, and today I have brought one of the most stunning creations of this family in the form of Aquilegia. In fact, most members of Ranunculaceae have incredible structures where you can almost interpret the gradual formation of cyclic flowers, many of them with extravagant nectaries acting also as petals. Aquilegia is just another example, and the flowers are pretty much incredible!

Side view (left) and front view (right) of Aquilegia flower

In Aquilegia, pentamery is already established, and we can easily find the bipartite perianth with 5 sepals and 5 petals. The petals, however have a very particular character – they are spurred, producing nectar to attract pollinators. This is the reason why many authors prefer using the term “nectar leaves” instead of “petals” in Ranunculaceae.

Left: View of the back of Aquilegia flower; Right: Detail of Nectar leaf of Aquilegia

These nectar leaves are of extreme importance on the evolution of a bipartite perianth, and in Ranunculaceae this transitional situation is still visible. They are not true petals yet, but structurally that’s how we interpret these structures. The tepals, structurally acting as sepals in this Aquilegia are the true perianth members of the flower. The nectar leaves are nothing more than stamens that lost their original reproductive function, becoming staminodes (or sterile stamens) and latter gaining a new function to attract pollinators. This is how true petals were born, they are nothing but modified staminodes that evolved in the direction of pollinator attraction. On the other hand, sepals evolved directly from tepals (leaves) gaining the function of organ protection (the reproductive organs – stamens and carpels).

Origin of petals in Aquilegia. The outer whorl of stamens loosing the reproductive function, and gaining later the function of pollinator attraction, becoming showy  and colourful (like petals) and producing nectar. Tepals at the same time gain the function of flower protection, like sepals.

However, some researchers believe that the origin of petals is not entirely the same for all members of Ranunculaceae. Some members of the family might indeed have their bipartite perianth as a result of the ascension of the bracts to the base of the receptacle, originating the sepals. Likewise, the true perianth (the tepals) gave origin to the petals. Ranunculaceae seems to be a family of flower structure experiments, trying which structure is the best to have. In any case, it seems that after this group pentamery and bipartite perianth were successfully achieved characters, well established in the rest of Eudicots.