A Surprising Realization About Leaf Windows

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I will never forget the first time I laid eyes on a Lithops. These odd little succulents are truly marvels of evolution. The so-called "living stones" really do earn their name as most are exquisitely camouflaged to match the gravelly soils in which they grow. If bizarre color patterns weren't enough, Lithops, as well as many other succulents, live their lives almost completely buried under the soil. All one ever really sees is the very tip of their succulent leaves and the occasional flower.

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It is the tips of those leaves that make people swoon. Lithops belong to a hodgepodge mix of succulent genera and families that produce windowed leaves. Aside from their striking patterns, the tips of their leaves are made up of layers of translucent cells, which allow light to penetrate into the interior of the leaf where the actual photosynthetic machinery is housed. Their semi-translucent leaves, coupled with their nearly subterranean habit, have led to the assumption that the leaf windows allow the plants to continue photosynthesis all the while being mostly buried. Despite the popularity of this assumption, few tests had been performed to see whether or not the windows function as we think. All of that changed back in the year 2000.

As hinted at above, a variety of succulent plants have converged on a similar leaf morphology. This is where things get a bit strange. Not all plants that exhibit the leaf window trait find themselves buried in the soil. Others, such as Peperomia graveolens for example, produce the photosynthetic tissues well above the soil. Examples like this led at least some researchers to second guess the common assumption of windows increasing photosynthesis and the resulting investigations were surprising to say the least. 

Peperomia graveolens

Peperomia graveolens

A duo of researchers decided to test the assumption that leaf windows increase photosynthesis by channeling light directly to the photosynthetic machinery inside. The researchers used tape to cover the leaf windows of a variety of succulent plant species. When they compared photosynthetic rates between the two groups, not a single difference was detected. Plants who had their leaves covered photosynthesized the same amount as plants with uncovered leaves. These data were quite shocking. Because they tested this assumption across a variety of plant species, the results suggested that the function of windowed leaves isn't as straight forward as we thought. These findings raised more questions than they solved.

Subsequent experiments only served to reinforce the original findings. What's more, some even showed that plants with covered windows actually photosynthesized more than plants with uncovered windows. It seems that windowed leaves function in a completely opposite manner than the popular assumption. The key to this patterns may lie in heat exchange. When the researchers took the temperature of the interior of the leaves in each group, they found that internal leaf temperatures were significantly higher in the uncovered group and this has important implications for photosynthesis for these species.

Fenestraria rhopalophylla

Fenestraria rhopalophylla

High leaf temperatures can be extremely damaging to photosynthetic proteins. If too much light filters through, leaf temperatures can actually hit damaging levels. This is one reason that many of these plant species have adopted this bizarre semi-subterranean habit. Plants that experienced such high temperatures throughout the course of a day had permanent damage done to their photosystems. This led to a reduction of fitness over time. Such lethal temperature spikes did not happen to leaves that had been covered.

Haworthia truncata

Haworthia truncata

If you're anything like me, at this point you must be questioning the role of the leaf windows entirely. Why would they be there if they may actually hurt the plants in the long run? Well, this is where knowing something about the habitat of each species comes into play. Not all leaf windows are created equal. The patterns of their windows vary quite a bit depending on where the plants evolved. In 2012, a paper was published that looked at the patterns of Lithops leaf windows in relation to their place of origin. Not all Lithops grow in the same conditions and various species hail from regions with vastly different climates.

What the paper was able to demonstrate was that Lithops native to regions that experience more acerage annual rainfall have much larger window areas on their leaves than Lithops native to drier regions. Again, the underpinnings of this discovery nonetheless have to do with light availability. Wetter areas experience more cloud cover than drier areas so Lithops growing where its cloudy have to cope with a lot less sun than their more xeric-growing cousins. As such, having a larger window allows more diffuse light into the leaf for photosynthesis without having to worry about the damaging temperatures.

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The reverse is true for Lithops from drier climates. They have smaller leaf windows because they experience more days with direct sun. These species tended to have much smaller windows, which reduced the amount of sunlight entering the leaf. This serves to keep internal leaf temperatures within a much safer range, thus protecting the delicate proteins inside. As it turns out, leaf windows seem to represent a trade-off between photosynthesis and overheating. What's more, some window-leaved species seem to be evolving away from the light transmitting function of their cousins living in shadier conditions. If anything, this serves as a reminder that simply because something seems obvious, that doesn't mean its always true. Stay curious, my friends!

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3] [4] [5] [6]

Something Smells...

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Plant nurseries are a dangerous place for me. Well, not really me so much as my wallet. I am always on the lookout for new and interesting plant friends to bring home. I recently visited a local nursery that has 4 hoop houses worth of plants to ogle. As I was walking the crowded alleyways between row after row of botanical treasures, something tucked away in a back corner caught my eye. There was a stark juxtaposition between burgundy and deep green that I simply could not ignore. I tip toed around a variety of succulents, dracaena, and gesneriads to investigate this colorful curiosity. 

As I approached this odd little plant I realized there was a long spike jutting out of the top. Ah, so this was some sort of peperomia. At this point I could see why it was kept among succulents. The leaves of this peperomia are quite succulent. Like fat little canoes, the leaves appeared to have green window-like surfaces that quickly gave way to a red bilge. This was truly unique. I had to have it. 

Despite the fact that it was the only one of its kind, I got it for a steal. It wasn't planted very well so I had to be quite careful getting it home. Mixing up soil can be fun, especially when you know the plant you are catering to. This was not one of those cases. Regardless, the succulent nature of the plant hinted at a need for a well drained mix. Three parts gravel to one part compost should do the trick. Despite its size, the plant had an under-developed root system. This explains why it was so floppy on the ride home. Once it was in its new pot, I had to go about picking out a perfect spot on the shelf. I knew that plants like Crassulas and aloes turn colors under high light so I figured this would be my best bet at preserving the beauty of this specimen. I watered it and sat back to enjoy its beauty among all the other plants in the collection. 

Later that day I began noticing an odd smell. It wasn't necessarily offensive yet it wasn't easily ignored either. It was also restricted to one area near the plant shelf. My nose didn't reveal the source. I put shoes outside and checked the area for anything that may be starting to rot. Nothing. After a while I must have gotten used to it and after a couple hours I forgot about it. Days went by and every once in a while the smell would creep its way into my nose. I was very confused and yet too busy to be serious about locating the source. 

I like to show off my plants so I made sure to draw attention to this new peperomia any time someone dropped by for a visit. It seemed to resonate well with friends. After a series of inquiries into this plants identity I decided to do my homework. Simply referring to it as a mystery peperomia wasn't satisfying enough. Luckily the internet exists. A quick image search for "succulent red peperomia" gave me my answer. 

My beautiful plant friend was none other than Peperomia graveolens, an endemic of mountainous forests in Ecuador. To my surprise, this is not a species that enjoys a lot of sun. The burgundy undersides are thought to assist the plant in soaking up as much sunlight as possible as it ekes out a living under the canopy. I guess I was going to have to move this plant to a lower shelf. The good news is that the soil mixture I made was going to work. There was no need to disturb the meager root system any more than I already had. 

Apparently this species is only known from two wild populations. All of the plants in cultivation are descendants of collections made in 1973 by some German botanists. This is truly a special plant! As I was reading various plant care websites, a recurring theme in the writing caught my attention. The inflorescence of this species is said to have a "mousey odor." I have seen that term before but, even after years of working in pet stores, I couldn't quite picture what a mousey odor would be like. Urine perhaps? Then I realized something. That strange odor was still present in and around the plant shelf. Could this be what I was smelling? I carefully picked up the plant and gave it a sniff. Yep! There is was. I still don't think of mice when I smell it but I can see how such descriptive terms could be applied. Regardless, my introduction to this wonderful little plant has made it all the more interesting. This is one of the main reasons I keep house plants. My collection is my own little botanical garden that I fill with species that capture my imagination. 

Further Reading:

http://www.iucnredlist.org/details/45780/0