The Apple of Sodom

It was another beautiful sunny day in Israel and I had decided to go outside Jerusalem to see the Dead Sea like all well behaved tourists do. Just before I get there, I have decided to walk around in that inspiring desert environment that surrounds the area, right close to Ein Gedi. After passing by a family of wild gazelles that made me company for few minutes, I stopped by a wonderful tall shrub. I was standing by a stunning member of Apocynaceae. The plant was Calotropis procera and their flowers, as usual, took the breath out of me! They were fleshy, big and colourful and the only thing I wished was to have someone with me so I could share all my thoughts about those flowers. But there was nobody passing by for as long as I stayed in the area, and the gazelles wouldn’t understand my excitement about their food, so I stay just admiring them for while. In such a harsh environment, the last thing I was expecting to find was such showy purple flowers – that shrub was just like an oasis of beauty in the desert.

On the left side: the gazelle happy family; on the right side: the shrub with the Dead Sea on the background

My astonishment was nothing but the proof of my infinite ignorance: these plants are actually quite common, especially the Dead Sea region. They are commonly called the “Apple of Sodom” because of the characters of the fruits, which are as big as apples and because of their dehiscence. The fruits are described as “exploding” or “dissolve into smoke and ashes” when you try to pluck them, possibly remembering the biblical scriptures of the destruction of Sodom and Gomorrah. This happens only because the fruits are hollow, but unfortunately didn’t have the opportunity to see them “dissolving” on my hands as the plants were not fruiting yet. The seeds produce silky strands which can be used as wicks for oil lamps, except in Jewish Shabbat lamps, according to the Mishna. These strands may also be used as natural textile fibers for other purposes. This biblical plant, as a member of Apocynaceae, produces a toxic milky sap, so be careful while handling it!

Fruits of Calotropis procera (Source: Flowersinisrael.com)

Unfortunately I didn’t dissect any flowers, so I don’t have good pictures to show morphological characters, but I will try to make it understandable. In Asclepioideae (old Asclepiadaceae family, today a sub-family of Apocynaceae) there is a structure called gynostegium. The gynostegium is the fusion between stamens and stigma and it is only known to be found in this family, however it resembles another structure appearing in a totally different family: Orchidaceae. The structure described in Orchidaceae is the gynostemium, and is also described as the reproductive structure of Aristolochiaceae. But the difference between a gynostegium and a gynostemium is not easy to detect, it is a matter of timing of fusion during the development of the flowers. While in Orchids and Aristolochiaceae the fusion between stamens and style is congenital, in Apocynaceae the same fusion is postgenital. But the problem now is to understand the difference between congenital and postgenital fusions.

Flower detail of Calotropis procera

These terms might be confusing to explain and understand, so to make it simple I would just say that the real fusion is the postgenital fusion, as we can identify it as a fusion. In postgenital fusion the organs develop independently until a certain point where they start fusing to each other by marginal adhesion. In congenital fusion you cannot see this for the simple reason that the organs are actually growing and developing together since the beginning.

It seems sometimes that botanists like to make up random names just to make other people, but terminology is a useful tool. In this case the names are so similar that they can without a doubt become a misunderstanding problem easily! The truth is that humans try to understand Nature the best they can by inventing names to communicate with other people, share knowledge and think together. But then we have to interpret the words we’ve created to try to understand plants – at the end it seems that botanists just like to play word-games. And it is much more fun to use words hard to pronounce.

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.

The Portuguese Oak

I have decided to honor Portugal with this week’s post for two reasons: to celebrate the National Day of Portugal (which was last Sunday , the 10^th of June), but also to support the Portuguese team at the UEFA Euro 2012. So I have decided to bring you today the Portuguese national plant – Quercus suber, the cork oak.

Quercus is a monoecious anemophilous plant, meaning that the sexual organs are separated in different flowers and are wind-pollinated. Thus the flowers are simple, small and shy, with a rather simple structure, but I thought this is a good opportunity to introduce you the wonders of wind-pollination. Anemophilous plants (wind-pollinated) are mainly found in two orders: Fagales and Poales and this, first of all, give us some clues about the environment shared among wind-pollinated plants. They are either trees (Fagales) or grasses (Poales), possibly due to the fact that flowers bloom high in the canopy, where the pollen is easily reached by the wind, but not as much by small insects. Another reason is possibly the windy and dry conditions found in steppes or desert-like environments where grasses reign. In such environments, the wind is a common and available resource much prevalent and reliable than insects. Possibly, also due to the winds typical of such environments insects are here less abundant comparing with, for example, forest-like environments. For this reason, plants found here have started to invest their energy in other characters, allowing a more efficient pollination and thus the flowers became adapted to wind pollination.

To make it easier for you to understand the main differences between anemophilous and entomophilous (insect-pollinated) plants, I have made this table to share with you:

Comparision between Anemophilous and Entomophilous floral characters

Because anemophilous plants have no need to attract anything, there is no need to invest energy producing big showy flowers, or expensive precious nectar. Instead, these plants produce many small flowers and large quantities of light pollen, which can be easily carried away by the wind. To facilitate the wind to take the pollen away, the petals are much reduced, and sometimes they are completely missing. Likewise, the stamens are hanging outside the flowers, waiting for the breeze.

Male inflorescence of  Quercus suber with the stamens hanging outside the flowers
(Source: www.flora-on.pt)

Also the typical inflorescences found in Fagales are definitely adapted to this pollination syndrome. Catkins of small and light flowers are easily moved and shacked even by a light breeze, and this allows the pollen to be released and spread easily. Because the pollination isn’t specialized like in entomophilous plants, the stigmas also have to be adapted to the wind pollination, so the surface of pollen reception is much bigger. This can be very easily observed in Poales, where the stigmas are long and feather-like, increasing the surface for an effective pollen reception.

Feather-like stigmas typical of grasses. Left: Arrhenatherum album; Right: Ammophila arenaria (Source: www.flora-on.pt)

Also, the position of the inflorescences in the tree isn’t random – male flowers are found in the lower older twigs and female flowers in the upper younger twigs. This is to avoid self-pollination (it is more unlikely that the pollen from below flies up and fertilizes the female flowers, but if female flowers were below, any pollen grain falling from the catkins above could fertilize them).

Flowers and inflorescences of Quercus suber

Wind-pollinated flowers are so shy that they can be unnoticed many times, especially by non-botanists. The truth is that they are not hiding, they are right there and it is fascinating to observe all the structures that seem to be undercover by the green foliage. Finding them is like finding a precious treasure, which is visible for everyone, but unnoticed by most people. That is the reason why every time I show wind-pollinated flowers to non-botanists they become astonished and quite surprised... These flowers might not be the prettiest, but they are pretty cool too.

The Passion flower

What is the first think you think of when someone mentions passion? Perhaps romantic love? Physical or sexual attraction for someone, or maybe just sex? Do you think of the same meaning of passion when the subject is the passion vine? What is the origin of this plant’s name? Is the passion fruit an exotic aphrodisiac? Well, not exactly but maybe if you’re in the right mood anything can work as an aphrodisiac...

Passiflora (Passifloraceae), the genus of the passion vine, gained this name due to the great imagination and devotion of the Portuguese and Spanish Jesuits which associated most morphological characters of the plant to symbols of the Passion of Christ. While exploring the exotic tropical rainforests of South-America they were astonished not only by the beauty and diversity of the plant (Northern-Central Brazil is an important hotspot of diversity for these plants) but also with all the symbolism that they immediately attributed to the plant. For the Jesuits, the flower of the passion vine had all the symbols of the Passion of Christ, and so it was named after this passion, not any other romantic or sensual passion that people might probably think of – after all it is indeed an exotic and extremely sweet fruit.

Flower diversity in Passiflora sp.

So now you are curious about the symbols, and the Jesuits were indeed very attentive – they looked at all the flower organs finding explanations for numbers and shapes of most of them. Let’s see…

The flower is pentamerous, with 5 sepals and 5 petals, representing the 10 faithful apostles (excluding Peter, the denier and Judas, the betrayer); the corona represents the crown of thorns with which roman soldiers crowned him as the “King of the Jews”; the ovary, a chalice shaped structure, represents the Holy Grail; the 3 stigmas represent the 3 nails used in Christ’s crucifixion and the 5 anthers the 5 wounds (1 in each hand and foot and the last one in his chest caused by the Holy Lance). But not only the floral structures were considered to be the symbols of the Passion of Christ. The characteristic leaves with pointed tips resemble the Holy Lance that confirmed Christ’s death and the tendrils represent the whips during the flagellation. The colours have also been attributed a meaning, since many species are white and blue colors, representing heaven and purity, and we must not forget the purple, which was also the colour of the robe that romans covered Jesus after crowning him.

The association between the flower and symbols of the Passion of Christ

Apart of all the symbolism involving the plant, there are also interesting characters related with the ecology of the plant that I would like to share with you. Every single part of it is edible, meaning that this plant is the herbivore heaven! They can eat the leaves, tendrils, flowers and there is plenty of nectar in the floral and extra-floral nectaries. To avoid getting eaten by all the herbivores, Passiflora seem to have made a deal with the ants by feeding them with nectar from extra-floral nectaries (usually present in bracts but also in leaves). Likewise, the ants protect their plant from herbivores and nectar robbers. Also the ants allow true pollinators to access the floral nectaries, allowing the successful reproduction of the plant. It sounds beautiful, but it is not as simple as it might sound – it relates many ant species, so a lot of different behaviours with different Passiflora species and environments. In any case, Passiflora and ants seem to get along very well and it seems to be a fair symbiotic relationship.

Ants feeding on nectar from extra-floral nectaries of the bracts of Passiflora

After attributing so many symbols to this plant, I wonder what Jesuits thought noticing so many ants around the vines getting rid of the hungry predators, protecting their “holy” source in exchange of precious nectar. I will let you meditate on this, wishing you a blessed Thursday of Corpus Christi.

Long wait, but with new stories to come!

Dear readers,

I am very happy to announce that I am finally back home, and so are the tales! I have been traveling since my last publication, and I found out later the impossibility of traveling and getting organized with good stories and pictures to publish. But I am back now and there is a lot to share! During these trips I have been to many different environments and seen a lot of beautiful flowers.

Each flower has a story to tell – the story of their past, and I try to translate them to you. But also because plants have a deep presence in human cultures, people have interpreted plants and flowers in different ways, giving them meanings and telling other stories which I found amazing and would be pleased to share with you!

A new story is ready to be published tomorrow – don’t miss it! J