Palo Verde

One of the first plants I noticed upon arriving in the Sonoran Desert were these small spiny trees without any leaves. The reason they caught my eye was that every inch of them was bright green. It was impossible to miss against the rusty brown tones of the surrounding landscape. It didn’t take long to track down the identity of this tree. What I was looking at was none other than the palo verde (Parkinsonia florida).

Palo verde belong to a small genus of leguminous trees. Parkinsonia consists of roughly 12 species scattered about arid regions of Africa and the Americas. The common name of “palo verde” is Spanish for “green stick.” And green they are! Like I said, every inch of this tree gives off a pleasing green hue. Of course, this is a survival strategy to make the most of life in arid climates.

Despite typically being found growing along creek beds, infrequent rainfall limits their access to regular water supplies. As such, these trees have adapted to preserve as much water as possible. One way they do this is via their deciduous habit. Unlike temperate deciduous trees which drop their leaves in response to the changing of the seasons, palo verde drop their leaves in response to drought. And, as one can expect from a denizen of the desert, drought is the norm. Leaves are also a conduit for moisture to move through the body of a plant. Tiny pours on the surface of the leaf called stomata allow water to evaporate out into the environment, which can be quite costly when water is in short supply.

The tiny pinnate leaves and pointy stems of the palo verde. 

Not having leaves for most of the year would be quite a detriment for most plant species. Leaves, after all, are where most of the photosynthesis takes place. That is, unless, you are talking about a palo verde tree. All of that green coloration in the trunk, stems, and branches is due to chlorophyll. In essence, the entire body of a palo verde is capable of performing photosynthesis. In fact, estimates show that even when the tiny pinnate leaves are produced, a majority of the photosynthetic needs of the tree are met by the green woody tissues.

Flowering occurs whenever there is enough water to support their development, which usually means spring. They are small and bright yellow and a tree can quickly be converted into a lovely yellow puff ball seemingly overnight. Bees relish the flowers and the eventual seeds they produce are a boon for wildlife in need of an energy-rich meal.

However, it isn’t just wildlife that benefits from the presence of these trees. Other plants benefit from their presence as well. As you can probably imagine, germination and seedling survival can be a real challenge in any desert. Heat, sun, and drought exact a punishing toll. As such, any advantage, however slight, can be a boon for recruitment. Research has found that palo verde trees act as important nurse trees for plants like the saguaro cactus (Carnegiea gigantea). Seeds that germinate under the canopy of a palo verde receive just enough shade and moisture from the overstory to get them through their first few years of growth.

In total, palo verde are ecologically important trees wherever they are native. What’s more, their ability to tolerate drought coupled with their wonderful green coloration has made them into a popular tree for native landscaping. It is certainly a tree I won’t forget any time soon.

Further Reading: [1] [2]

Hydatellaceae: The Other Basal Angiosperms

Photo by Kevin Thiele licensed under CC BY 2.0

Though rather obscure to most of the world, the genus Trithuria has enjoyed somewhat of a celebrity status in recent years. A paper published in 2007 lifted this tiny group of minuscule aquatic plants out of their spot in Poales and granted them a place among the basal angiosperm lineage Nymphaeales. This was a huge move for such little plants. 

The genus Trithuria contains 12 species, the majority of which reside in Australia, however, two species, T. inconspicua and T. konkanensis, are native to New Zealand and India. They are all aquatic herbs and their diminutive size and inconspicuous appearance make them easy to miss. For quite some time these odd plants were considered to be a group of highly reduced monocots. Their original placement was in the family Centrolepidaceae. All of that changed in 2007.

Close inspection of Trithuria DNA told a much different story. These were not highly reduced monocots after all. Instead, multiple analyses revealed that Trithuria were actually members of the basal angiosperm lineage Nymphaeales. Together with the water lilies (Nymphaeaceae) and the fanworts (Cabombaceae), these plants are living representatives of some of the early days in flowering plant evolution. 

Of course, DNA analysis cannot stand on its own. The results of the new phylogeny had to be corroborated with anatomical evidence. Indeed, closer inspection of the anatomy of Trithuria revealed that these plants are truly distinct from members of Poales based on a series of features including furrowed pollen grains, inverted ovules, and abundant starchy seed storage tissues. Taken together, all of these lines of evidence warranted the construction of a new family - Hydatellaceae.

The 12 species of Trithuria are rather similar in their habits. Many live a largely submerged aquatic lifestyle in shallow estuarine habitats. As you may have guessed, individual plants look like tiny grass-like rosettes. Their small flower size has lent to some of their taxonomic confusion over the years. What was once thought of as individual flowers were revealed to be clusters or heads of highly reduced individual flowers. 

Reproduction for these plants seems like a tricky affair. Some have speculated that water plays a role but close inspections of at least one species revealed that very little pollen transfer takes place in this way. Wind is probably the most common way in which pollen from one plant finds its way to another, however, the reduced size of these flowers and their annual nature means there isn't much time and pollen to go around. It is likely that most of the 12 species of Trithuria are self-pollinated. This is probably quite useful considering the unpredictable nature of their aquatic habitats. It doesn't take much for these tiny aquatic herbs to establish new populations. In total, Trithuria stands as living proof that big things often come in small packages. 

Photo Credits: [1]

Further Reading: [1] [2] [3]

Mighty Magnolias

Magnolias are one of those trees that even the non-botanically minded among us will easily recognize. They are one of the more popular plant groups grown as ornamentals and their symbolism throughout human history is quite interesting. But, for all this attention, few may realize how special magnolias really are. Did you know they they are one of the most ancient flowering plant lineages in existence?

Magnolias first came on to the scene somewhere around 95 million years ago. Although they are not representative of what the earliest flowering plants may have looked like, they do offer us some interesting insights into the evolution of flowers. To start with, the flower bud is enclosed in bracts (modified leaves) instead of more differentiated sepals. The "petals" themselves are not actually petals but tepals, which are also undifferentiated. The most striking aspect of magnolia flower morphology is in the actual reproductive structures themselves.

Magnolias evolved before there were bees. Because of this, the basic structure that makes them unique was in place long before bees could work as a selective pressure in pollination. Beetles are the real pollinators of magnolia flowers. The flowers have a hardened carpel to avoid damage by their gnawing mandibles as the feed. The beetles are after the protein-rich pollen. Because the beetles are interesting in pollen and pollen alone, the flowers mature in a way that ensures cross pollination. The male parts mature first and offer said pollen. The female parts of the flower are second to mature. They produce no reward for the beetles but are instead believed to mimic the male parts, ensuring that the beetles will spend some time exploring and thus effectively pollinating the flowers.

It is pretty neat to think that you don't necessarily have to track down a dawn redwood or a gingko to see a plant that has survived major extinction events. You can find magnolias very close to home with a keen eye. Looking at one, knowing that this is a piece of biology that has worked for millennia, is quite astounding in my opinion.

Further Reading: [1] [2] [3] [4]

How North America Lost Its Asters

It's that time of year in northern North America where many of the most famous and easily recognized species come into flower, the asters. Some of my favorite plants once resided in this genus, but did you know that referring to our North American representatives as "asters" is no longer taxonomically accurate?

Since the time of Linnaeus, plants and animals have been categorized based on morphological similarities. With recent advances made in the understanding and sequencing of DNA, a new and more refined method of classifying the relationships of living organisms has been added to the mix. Much of what has been taken for granted for the last few decades is being changed. One group that has been drastically overhauled are the North American asters. At one time there were roughly 180 species of North American flowering plants that found themselves in the genus Aster. Today, there is only one, Aster alpinus, which enjoys a circumboreal distribution. 

Because the concept of "Aster" was developed using an Old World species (Aster amellus), New World asters were not granted that distinction. The New World species have shown to have their own unique evolutionary history and thus new genera were either assigned or created. By far, the largest New World genus that came out of this revisions is Symphyotrichum. This houses many of our most familiar species including the New England aster (Symphyotrichum novae-angliae). Some of the other genera that absorbed New World aster include Baccharis, Archibaccharis, Ericameria, Solidago, and Machaeranthera, just to name a few.

Taxonomy is often a difficult concept to wrap your head around. It is constantly changing as we come up with better ways of defining organisms. Even the concept of a species is something biologists have a hard time agreeing on. Surely, genetic analyses offer some of the best methods we have to date, a fact that the Angiosperm Phylogeny Group is constantly refining.

For some, this is all a bunch of silly name changes but for others this is the most important and dynamic form of natural science on the planet. Having a standard for naming organisms is a crucial component of understanding biodiversity. With a name, you can take the next step in getting to know and understand a beloved species. One thing to consider is that, as species are split and regrouped, often times what was thought to be one species turns out to be many. In the case of organisms which are threatened or endangered, a split like that can unveil a disastrous elevation into a far more dismal ranking.

Further Reading: [1] [2]

Meeting Amborella trichopoda

When I found out I would be seeing a living Amborella, a lump formed in my throat. There I was standing in one of the tropical houses at the Atlanta Botanical Garden trying to keep my cool. No amount of patience was ample enough to quell my excitement. How was I going to react? How big were these plants? Would I see flowers? Could I touch them? What were they growing in? My curiosity was through the roof.

Naturally this sort of excitement is reserved for those of us familiar with Amborella trichopoda. This strange shrub is not something that would readily stand out against a backdrop of tropical flora. However, if life history and ecology were to be translated into outward appearances, Amborella would likely be one of the most gaudy plants on this planet. What I was about the lay eyes on is the only member of the sole genus belonging to the family Amborellaceae, which is the sole member of the order Amborellales.

Even more exciting is its position on the angiosperm family tree. As flowering plants go, Amborella is thought to be the oldest alive today. Okay, so maybe this shrub isn't the oldest flowering plant in the world. It is likely that at one time, many millions of years ago, there were more representatives of Amborellaceae growing on this planet. Until we turn up more fossil evidence it is nearly impossible to say. Still, Amborella's place in the story of flowering plant evolution is consistently located at the base.

That is not to say that this shrub is by any means primitive. I think the first thing that shocked me about these plants is just how "normal" they appear. Sans flowers, I didn't see much out of the ordinary about them. They certainly look like they belong on our timeline. Without proper training in plant anatomy and physiology, there is little one could deduce about their evolutionary position. Regardless of my ignorance on plant morphology, there is plenty to look at on Amborella.

For starters, Amborella has tracheids but no vessel elements, making its vascular system more like that of a gymnosperm than an angiosperm. Its small flowers are borne in the axils of the evergreen leaves. It has no petals, only bracts arranged into a spiral of tepals. The female flowers consist of 4 to 8 free carpels and do not produce a style. Male flowers look like nothing more than a spiral cluster of stamens borne on short filaments.

If plant anatomy isn't enough to convince you, then the genetic analyses tell a much more compelling story. DNA sequencing consistently places Amborella at the base of the flowering plant family tree. Again, this is not to say that this shrub is by any means "primitive" but rather its lineage diverged long before what we would readily recognize as a flowering plant evolved. As such, Amborella offers us a window into the early days of flowering plants. By comparing traits present in more derived angiosperms to those of Amborella, researchers are able to better understand how the most dominant group of plants found their place in this world.

Another interesting thing happened when researchers looked at the DNA of Amborella. What they found was more than just Amborella genes. Inside the mitochondrial DNA are an unprecedented amount of foreign DNA from algae, lichens and mosses. In fact, an entire chunk of DNA corresponded to an entire mitochondrial genome of a moss! Researchers now believe that this is a case of extreme horizontal gene transfer between Amborella and its neighbors both growing on and around it. Both in the wild and in cultivation, Amborella is covered in a sort of "biofilm." Whether or not such gene transfer has assisted in the conservatism of this lineage over time remains to be seen.

At this point you may be asking how this lineage has persisted for over 130 million years. For the most part, it is probably due to chance. However, there is one aspect of its ecology that really stands out in this debate and that is its geographic distribution. Amborella is endemic to Grande Terre, the main island of New Caledonia. This is a very special place for biodiversity.

New Caledonia is a small fragment of the once great super-continent Gondwana. New Caledonia, which was part of Australia at that time, broke away from Gondwana when the super-continent began to break up some 200-180 million years ago. New Caledonia then broke away from Australia some 66 million years ago and has not been connected to another land mass since. A warm, stable climate has allowed some of the most unique flora and fauna to persist for all that time. Amborella is but one of the myriad endemic plants that call New Caledonia home. For instance, 43 species of tropical conifers that grow on these small islands are found nowhere else in the world. The whole region is a refugia of a long lost world.

Being a biodiversity hot spot has not spared New Caledonia from the threats of modern man. Mining, agriculture, urbanization, and climate change are all threatening to undo much of what makes this place so unique. The loss of a species like Amborella would be a serious blow to biodiversity, conservation, and the world as whole. We cannot allow this species to exist only in cultivation. New Caledonia is one place we must desperately try to conserve. Meeting this species has left a mark on me. Being able to observe living Amborella up close and personal is something I will never forget as my chances of seeing this species in the wild are quite slim. I am so happy to know that places like the Atlanta Botanical Garden are committed to understanding and conserving this species both in the wild and in cultivation. For now Amborella is here to stay. Long may it be that way.

Further Reading:

http://bit.ly/29MuMuw

http://bit.ly/29MuML0

http://bit.ly/29ZKNJS