In my opinion, the smaller a plant is, the more character it has. Wall rue (Asplenium ruta-muraria) is a wonderful demonstration of this. The genus of ferns to which it belongs, Asplenium, is rather large, containing somewhere along the lines of 700 species worldwide. 

Wall rue can be found growing both in North America and Europe. Its distribution is a reminder of the great land bridges that once connected the continents back when ocean levels were much lower than they are today. The specific epithet "ruta-muraria" roughly translates to "bitter herb of walls." Along with its common name, these seem to hint at where this tiny fern likes to grow. Indeed, at least in Europe, this is a fern of stone walls, growing among the myriad cracks and crevices where microclimates are favorable for its spores to germinate. 

In North America, however, wall rue seems to be a bit more picky. Wall rue is a calciphile meaning it can really only be found in abundance on natural limestone outcroppings. As a result, it is considered a threatened or endangered species throughout most of the continent. The aspect of its habitat I find most interesting is that the limestone it relies upon is the result of an ancient sea that covered parts of North America during the Silurian Period some 443.8–419.2 million years ago. If it were not for the solidified remains of ancient marine organisms, wall rue and many other plant lineages would not be here, at least not in the way in which we know them. 

Another interesting aspect of wall rue biology is that this little fern is helping paleontologists in Europe discover potential glacial refugia - ice free areas where plants and animals were able to survive during the height of glaciation. Refugia were likely epicenters of biodiversity, which expanded to recolonize the continents once the ice sheets receded. 

Wall rue, as well as other rock ferns in the genus Asplenium occur in two forms in nature - a diploid form with two sets of chromosomes and a polyploid form containing multiple sets of chromosomes. Polyploids arise from mutated diploids and can be found growing over a wider range than their more restricted diploid parents. By studying the relatedness of different diploid populations, researchers are able to deduce where some glacial refugia may have been located. In this way, these tiny little ferns are offering a rare but clear window into the Earth's long gone past. 

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

There's Water In Them There Rocks!



Plants go to great lengths to obtain the necessities of survival. Nowhere is this more apparent than in the desert regions around the world. Amazingly, myriads of plants have adapted to the harsh conditions that deserts offer up. Needless to say, water is a major limiting resource in these climates and many of the adaptations we see in desert plant species have to do with obtaining and holding on to as much water as possible. Some species get around the issue by going dormant whereas others stick it out using deep taproots that plug into the groundwater. A select few others hit the rocks.

Rocks? Well, gypsum to be precise. This interesting mineral is quite common in arid regions throughout the world. What is more interesting is that 20.8% of a gypsum crystal is water. Because of this, it has been suspected that gypsum in the soil could be a potential source of water for plants growing in these regions and a team of researchers out of Spain may have found just that.

Meet Helianthemum squamatum. This distant relative of hibiscus grows throughout the gypsum hills of the Mediterranean region. Unlike other desert plants, it is shallowly rooted. Unlike other shallowly rooted species, H. squamatum doesn't go dormant during the dry summer months. The physiology of this species in the context of the dry environments that it grows offers up quite a conundrum. How does this plant get the water it needs to grow through the hottest, driest months of the year?

By analyzing the isotopic composition of the water within the plant and comparing it to background sources, the team found that 90% of the plants water intake during the dry summer months comes from the crystallization water in gypsum! How is this possible? How does a plant get water from a mineral?

The actual physiological processes involved are not yet understood but there are some running hypotheses. The first has to do with temperature. When gypsum is exposed to temperatures above 40 degrees C, water can be released from the crystalline matrix. It would then be available to the plants via passive uptake. 40 degrees C is not unheard of in these environments. Any water that isn't taken up by the plants could be reincorporated back into gypsum when things cool down at night. Another possibility is that H. squamatum grows its roots into and around the gypsum. Using root exudates, it is possible that the plant is able to dissolve gypsum to some degree, thus unleashing the water within. This may rely on the microbial community associated with the roots. Until further research can be done on this, the jury is still out.

The most exciting aspect of this research is the doors it has now opened in our search for extraterrestrial life. Life as we know it depends on water. Our search for this molecule has us looking for planets in a sweet spot where water can be found in a liquid state. Knowing now that at least some life on our planet is able to obtain water from gypsum broadens the kinds of places we can look. Mars is chock full of gypsum. Just sayin'.

Photo Credit: José María Escolano (

Further Reading:

A Temporary Inland Sea in Northeastern North America


There are many species of small, nondescript spurge out there. All too often they go completely unnoticed, even by plant lovers like myself. As I have come to learn time and time again, every species has an interesting story to tell. That is why I started In Defense of Plants in the first place. The story I want to tell you today came to me from a chance encounter I had while exploring a beach on Lake Erie. I was musing over some tumbleweed I had found when I noticed some small spurge barely poking out of the sand around me. I took some pictures and moved on. Had I realized what I would come to learn from this spurge, I probably would have spent more time admiring it.

Our story begins roughly 18,000 years ago during the height of the last glacial period. Much of northern North America was buried under a massive glacial ice sheet. This was unlike anything we can witness on the continent today. In some spots the ice was well over a mile thick. The weight of that much ice on the land caused the bedrock underneath to compress, not unlike a mattress compresses under the weight of a human body. This compression pushed much of northeastern North America lower than sea level. Unlike a mattress, however, rock can take a very long time to rebound after the weight has been lifted. Around 13,000 years ago when the glaciers began to retreat, the land was still compressed below sea level. 


With the ice gone, the ocean quickly rushed in to fill what is now the St. Lawrence and Ottawa River valleys as well as Lake Champlain. A salty inland lake coined the Champlain Sea was the result of this influx of ocean water. For some time, the Champlain Sea provided seemingly out of place maritime habitat until isostatic rebound caused the land to rise enough to drain it some 10,000 years ago. During this period, the Champlain Sea was home to animals typically seen in the northern Atlantic today including whales, whose fossils have been found in parts of Montreal and Ottawa. Coastal plant communities formed along the shores of the Champlain Sea, which brings me back to my little spurge friend. 

Inland beach pea ( Lathyrus japonicus )

Inland beach pea (Lathyrus japonicus)

Sea rocket ( Cakile edentula )

Sea rocket (Cakile edentula)

The species in question is Chamaesyce polygonifolia, the seaside spurge. By no means rare, this obscure little plant is more typically found along the coast of the Atlantic. Along with other species like the inland beach pea (Lathyrus japonicus) and sea rocket (Cakile edentula), these plants followed the shores of the Champlain Sea and remained here in sandy, disturbed habitats ever since. These species are echoes of a brief period of time when North America was going through a lot of changes. Again, had I known this at the time, I don't know if I would have left the beach so quickly that day. I love to be reminded of how small we really are, how fleeting our existence really is. I love meeting species that are players in a much bigger story and Chamaesyce polygonifolia and company are just that. 

Photo Credits: [1] [2]

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