Primula’s little secret

“The earthly manifestations of God’s world began with the realm of plants, as a kind of direct communication from it. (…) Plants were bound for good or ill to their places. They expressed not only the beauty but also the thoughts of God’s world”

Carl Jung

I have heard from many people that plants are boring creatures because of their incapacity of movement, but on the other hand this is an incredible challenge they have to deal with all the time. They have to answer to with certain problems even though they cannot move or see to, for example, choose the visitors of their flowers. They need to be selective enough to attract pollinators, avoiding nectar robbers, herbivores and their own pollen (self-pollination); they are to me one of the most incredible examples of the mysterious abilities of Nature. There are many fascinating examples on pollination strategies to avoid inbreeding in plants and Primula seems to be quite talented on finding the right strategy on this matter, avoiding selfing like no other. 

Primula acaulis. Source: Flora-on

To avoid selfing, Primula flowers managed to find a way to create hermaphrodite flowers of two different types, each type can only pollinate the other type, and each individual only forms one of the morphotypes. Pin plants (morphotype 1) have a long style and short anthers; thrum plants (morphotype 2) have a short style and long anthers. Pollen of pin plants can only grow down on the style of thrum plants and the pollen of thrum plants only fertilizes pin plants. For this reason, flowers of one type cannot be fertilized by pollen of their own type, and since one individual can’t produce both types inbreeding is hardly possible to happen. 

Morphotype 1 cannot fertilize morphotype 2 and vice-versa, as the pollen is only "fertile" with the opposite morphotype

The secret recipe for this special strategy can be easily explained genetically – pin plants are heterozygous (Aa) or homozygous-dominant (AA) and thrum plants are homozygous-recessive (aa). Because crossing happens only between both morphotypes, there are only two possibilities of crossing, Aa x aa or AA x aa. 

Crossing between both morphotypes can be explained in two ways, depending on the genotype of the pin plants. If they are homozygous (AA, example on the left), all offspring will also be of the pin morphotype because their genotype will certainly be heterozygous (Aa)

However, it is not as simple as this, as there is not only one locus responsible for this character, but a group of three loci tightly linked together (linkage), forming a complex locus or a super gene. Because these genes are so closed together, a cross-over that separates them is highly improbable to happen, however it may happen in rare situations and only in this rare situations self-pollination might occur. When crossing-over takes place within those loci, two complementary kinds of homostyle are formed, with the stigma characteristic of one type and the pollen characteristic of the other, allowing self-fertilization to happen. These homostyles are though naturally eliminated from most populations because of the selective disadvantage that inbreeding brings along. 

Primula veris thrum mophotype (aa genotype). Source: Flickr (padikeo)

Primula veris pin morphotype (Aa or AA genotype). Souce: Flickr (Viveka)

So now, whenever you find a Primula flower, take a look at the reproductive structures, and you will be able to tell if they have an AA/Aa genotype or an aa genotype!

The involving aroma of seductive Oman

Walking in the charming markets of Oman is not only about what we see, it is much about the scents. Unlike most places I’ve visited in Europe and Asia, in Oman even the Market (Suq in Arabic) has a lovely aroma, leaving a comfortable warm feeling in the air. I could feel this scent everywhere – in the streets, markets, houses, clothes and it is about the best memories I have from this land. Therefore I still keep a wee wooden box filled with Omani frankincense in my room, and whenever I open this box the aroma makes me fly back to Arabia, so I do it several times. And I do also keep a little scented stone on my wallet, just because it’s nice.

Frankincense from Oman

Frankincense is very special and sacred since ancient times, and in the past it was equated to gold in the market. It was used as an offering to gods, and as a medicinal fumigant for diseases and against evil odors. Egyptians also believed that frankincense was the sweat of Gods. Frankincense is also mentioned several times in the Bible, and it has been used to make holy incense used for worship since the time of Moses. In Christianity it is very much known as one of the three gifts to baby Jesus, along with myrrh and gold, representing the divinity of Christ, as it was used as an offering to God.

All that made sense to me, and I do understand the status of divinity associated with this odoriferous resin – frankincense has a unique and magical aroma, leading you to a deep involvement with the place. And it is not to be surprised that it brings so many feelings as it comes from the plant’s own “blood”, carrying along all the purity of its nature. It is much as you have the whole soul of the desert transformed in the sense of smell.

Sap from wild Boswellia sacra. Photo by: Khalid Al Farsi

Frankincense stones are nothing but the dried sap (resin) of Boswellia sacra (Burseraceae), a shrubby tree native to Oman, Yemen and Somalia, growing also in Ethiopia.

Natural habitat of Boswellia sacra, in Omani lands. Photo by: Khalid Al Farsi

Burseraceae is a family belonging to Sapindales, having Anacardiaceae as a sister family. The flowers are small but gorgeous, organized in racemes. Floral structures are simple, not hard to recognize, and they seem to have what well-behaved Rosid is expected to have: five sepals, five petals, 10 stamens arranged in 2 rows of five, a superior ovary and a beautiful and wide nectary disc. The most characteristic structures here are the white broad petals, and the conspicuous fleshy nectaries.

Flowers of Boswellia sacra. Source: Flickr (Scott Zona)

Notice that the first row of stamens (closer to the petal row) is opposite the sepals, and the second row (closer to the gynoecium) is opposite the petals. Sorry for the low quality of the pictures.

I admit that today the story wasn’t much of a flower story, but I felt like introducing you this very special plant that brings along with it a high spiritual feeling. Unfortunately, it seems that the wild population of these trees is declining due to a lower regeneration caused by the early death of the youngs before flowering and thus, seeding. However, it seems that it is not considered to be threatened, according to the IUCN Red List, and we hope that it remains likewise and that the population of Boswellia can grow happy and healthy, inspiring us all with its warm and exotic sense from South Arabia.

The family from the skies

“Aster” is the greek name for “Star”, which is also the name of the type genus of one of the most diverse families of flowering plants: Asteraceae (also known in Botany under the old name of Compositae).

When I think of the name Asteraceae I cannot avoid thinking about the supreme star that lights and heats the planet we live in. In fact, I think the name Aster is well attributed due to morphological similarities that we might find between an Asteraceae flower and a star, or even the Sun itself. Stars usually have a core and irradiate light through rays of light. In Asteraceae inflorescences do also resemble stars by having the core flowers tightly grouped in the centre and the ray flowers on the edges.

The fact of having this extremely cosmopolitan and diverse family under the name of “Star”, makes me think that they can be found anywhere in the world as easily as finding stars in the dark sky above. So let’s explore a bit the secret recipe for the high success of this family which became the largest among core eudicots. They which are so widespread that can be found naturalized in all continents (except Antartica) and in most environments. Such success must be related with their floral morphology! And if it’s not directly related with the floral morphology it doesn’t matter because I will introduce it anyway.

The inflorescence type is an exclusive of the family, it is very odd and it comes in a pseudanthium-like inflorescence. I bet some of you are now wondering what the pseudanthium is, or scrolled down the window to check on the glossary. Botanists call pseudanthium to some types of inflorescences where flowers are organized in such a way forming a structure that looks like a single flower, even though it’s a group of flowers! We find these pseudanthia in all Asteraceae, and in this family the pseudanthia are special as they are organized in a disc. The botanical name Compositae was also based on the morphology of such inflorescences, which can also be called compound flowers, meaning a flower made out of many flowers, or opposite to single flower.

When it comes to Asteraceae, even some structures have special names as they don’t occur in other plant groups, starting with the inflorescence, the capitulum. Also the calyx usually differentiates into a pappus (a series of hairs or bristles) instead of the typical 5 sepals. Petals are always fused, forming a stamen-petal tube (disc flowers) or forming the monosymmetric corolla we find in peripherical flowers. Finally, the tubular flowers are bisexual but the peripherical ligulate flowers are usually pistillate (lacking male organs) or sterile.

Florets of an Asteraceae cappitulum

One of the most interesting characters of these flowers is known by the secondary pollination mechanism. This mechanism is simple and effective at the same time. The tube formed around the stigma on tubular flowers is the basis of this mechanism, as stamens are organized all very tight around the gynoecium, and when it pops outside the flower, it is covered with pollen on the outer (abaxial) side of the stigma. This mechanism is interestingly effective as it avoids self-pollination (as the pollen is attached to the abaxial side of the stigma and the fertile region is the inner or adaxial side, which only opens when it sticks totally out from the floral tube).

The pollen gets attached to the abaxial side of the stigma, the receptive part of the stigma is the adaxial side. Mature anthers in the left side and mature stigmas in the right side. The ovaries are always bicarpellate and inferior.

The beauty of simplicity

I have been trying for long time to talk about palm tree flowers. Palm trees have a great symbology in some religions, being considered as the tree of life in ancient Egypt, and in the Kabballah. In Judaism it has a symbolic importance as one of the four plants used during the Sukkot, according to the Torah. Also in the Quraan palm trees, especially date palm trees, are mentioned several times, and so a certain significance is given in the Islamic culture. In Christianity, palm leaves are used to celebrate the Palm Sunday, the Sunday before Easter, remembering Jesus’ triumphal entry in Jerusalem. It happens that in the region where all these monotheistic religions were founded, palm trees are probably the taller trees growing in the wild, so it does not surprise me that they are mentioned in the sacred books in many different occasions, giving them this mysticism.

Virgin Mary nurtured by a palm tree, as described in the Quran (Source: Wikipedia)

However, Arecaceae (the family of palm trees) is originally from tropical and sub-tropical regions, where they are much more diverse due to the typical wet and hot conditions. That is probably why palm trees are usually associated with our idea of paradisiacal islands, exotic beaches and holiday’s advertisements. Only in South-East Asia these trees started to gain a different meaning to me. They were part of the landscape everywhere, giving that tropical touch to the surrounding environment. I could find them in the beach, in the mountains, near the rivers, you name it! And even though I have never paid much attention to these plants because I never found them neither pretty nor very attractive, it seemed to me that I was starting to understand their beauty somehow. In Thailand I had the opportunity to look closely at the flowers like never before, and it was like finding a hidden treasure that nobody have ever heard about.

Male inflorescences and flowers of Pinanga sylvestris

Female tree, infructescences and fruits

The flowers are amazing, they are tiny and immaculate, strong and vivacious – a mix between delicacy and will to live. It was like the flowers were confident and ready for the adversities of the world! Yes, I kind of gave them “flowerality” (or the “personality” of the flower). However, I thought that the structure itself was a bit inglorious to talk about, in the sense that they are so simple that it won’t bring anything new, but I will introduce you the palm tree flower structures anyway! Like all monocots they are trimerous, having a bipartite perianth of 3 organs (3+3 tepals), and 2 whorls of 3 stamens. The carpels are also trimerous. The flowers can be both uni- or bi-sexual, depending on the species, but they are usually small and white, organized in panicles or spikes hanging from the tree top.

Bi-sexual flowers of palm tree

How did such a small and simple flower return such a feeling of amazement? Maybe there was nothing complex to show you here, but isn't simplicity the purest sense of beauty? Isn’t simplicity effective? There is no doubt about this! Keeping it simple seems to be the watchword in palm flowers, and that leads to a pollination syndrome which is still not very well defined in many groups only because it is not specific.

It is just simple, effective and beautiful.

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.