The Round Leaved Orchid

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In the northern temperate regions of North America, late June marks the beginning of what I like to call orchid season. If you're lucky you may stumble across one of these rare beauties in full bloom. Their diversity in shape and size are mainly a result of the intricate evolutionary relationships they have formed with their pollinators. I spend much of my time botanizing trying to locate and photograph these botanical curiosities and any time I get to meet a new species is a very special time indeed. 

Take the round leaved orchid (Platanthera orbiculata) for example. For years I have only known this species as two round leaves that are slightly reminiscent of the phaleanopsis orchids you see for sale in nurseries and grocery stores. The leaves can be quite large too. With their glossy appearance, they are the easiest way to locate this plant.

When conditions are right and the plants have enough stored energy they will begin to flower. Rising from the middle of the pair of leaves is a decent sized inflorescence loaded with greenish white flowers. The flowers are interesting structures. Not particularly colorful, they have a long white lip and considerable green nectar spurs. There are said to be two varieties of this species, each being characterized by the length of the nectar spur. Unlike many orchids that offer no reward to pollinators, P. orbiculata produces nectar. The flowers are pollinated by noctuid moths, which is probably why they are white in color. Whereas most lepidopteran pollinated orchid attach their pollinia to the proboscis of the butterfly or moth, P. orbiculata attaches its pollinia to the eyes of visiting moths. 

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If this isn't strange enough, the pollinia themselves have some of their own intriguing adaptations. Visiting moths take a certain amount of time to successfully access the nectar from the nectar spur. If the plant is to avoid wasting precious pollen on itself, then it must find a way to delay this process. The pollinia are the solution to this. When first attached to the eyes, the pollinia stick straight up. This keeps them away from the female parts of the plant as the moth feeds. Only after enough time has elapsed will the stalks of the pollinia begin to bend forward. At this point the moth will hopefully have moved on to the flowers of an unrelated individual. Pointing straight forward, they are now perfectly positioned to transfer pollen. 

Like all orchids, P. orbiculata relies on specialized mycorrhizal fungi for germination and survival. At the beginning of its life, P. orbiculata relies solely on the fungi for sustenance. Once it has enough energy to produce leaves it will repay the fungi by providing carbohydrates. However, the relationship is not over at this point. Every spring, P. orbiculata produces a new set of leaves as well as a whole new root system. The fungi supply a lot of energy for this process and if the plant is disturbed (ie. dug up by greedy poachers) or browsed upon, it is likely that it will not recover from the stress and it will die. The mycorrhizal fungi it relies on live on rotting wood so finding well rotted logs is a good place to start searching for this species. With declining populations throughout much of its range, it is important to remember to enjoy it where it grows. Leave wild orchids in the wild!

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

Green Islands

Autumn is here and all across the northern hemisphere deciduous trees are putting on a show unlike anything else in the natural world. The range of colors are spectacular both from afar and up close. If you're like me then every single leaf is worth investigation. The trees are shedding their leaves in preparation for dormancy. The leaves aren't dying outright. Instead, the trees are reabsorbing the chemicals involved in photosynthesis as a way of getting back some of the energy investment that went in to producing them in the first place. 

If you look closely at some leaves, however, you may notice green spots in an otherwise senescent leaf. Why is it that certain parts of these leaves are still photosynthetically active despite the rest of the photosynthetic machinery shutting down around them? The answer to this question is way cooler than I ever expected. 

These "green islands" as they are called are almost always associated with an insect. If you look closely towards the base of these spots you will usually find a tiny leaf mining larvae of a moth busy munching away at the remaining photosynthetic tissue. The most obvious conclusion at this point would be to say that the moth larvae are the cause of the green islands. However, it is not that simple. 

When researchers raised the moth larvae under sterile conditions, they did not produce the green island effect. This proved to be a bit of a conundrum. Why would this happen in the wild but not under sterile conditions in a lab? The answer is bacteria. 

It would appear that the moth larvae have a symbiotic relationship with bacteria living on their bodies. These bacteria interact with the tissues of the leaf and alter the production of cytokinins. In the leaf, cytokinins inhibit leaf senescence. When the plant switches into dormancy mode, cytokinin production is shut down. The bacteria, however, actually ramp up cytokinin production throughout the tissues surrounding the larva. The result of which is a small region or "island" of tissue with prolonged photosynthetic life. 

Because of this, the larvae are able to go on feeding well into the fall when food would otherwise become nonexistent. By harboring these bacteria, the moths are able to get more out of each seasons reproductive efforts instead of simply stopping once fall hits. This is the first ever evidence of insect bacterial endosymbionts have been shown to manipulate plant physiology, though it most certainly will not be the last. 

I would like to thank Charley Eiseman for the use of this photo as well as inspiring this post. Charley is the man behind one of my all time favorite blogs Bug Tracks (https://bugtracks.wordpress.com/) so make sure to visit and like Northern Naturalists.

Further Reading:
https://bugtracks.wordpress.com/2014/01/29/green-islands/

https://bugtracks.wordpress.com/2015/10/13/green-islands-part-2-and-another-mystery-moth/

http://rspb.royalsocietypublishing.org/content/early/2010/03/24/rspb.2010.0214.short

http://onlinelibrary.wiley.com/doi/10.1002/ece3.1580/epdf

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3908980/