Color Changing Asters

Fall is here and the asters are out in force. Their floral displays are some of the last we will see before the first fall frost takes its toll. Their beauty is something of legend and I could sit in a field and stare at them for hours. In doing so, an interesting pattern becomes apparent. Have you ever noticed that the disc flowers of the many aster species gradually turn from yellow to red? Whereas this certainly correlates with age, there must be some sort of evolutionary reason for this.

Indeed, there is. If you sat and watched as bees hurriedly dashed from plant to plant, you may notice that they seem to prefer flowers with yellow discs over those with red. The plot thickens. What about these different colored discs makes them more or less appealing to bees desperately in need of fuel? The answer is pollen.

A closer observation would reveal that yellow disks contain more pollen than those with red discs. Of course, this does relate to age. Flowers with red discs are older and have already had most of their pollen removed. In this way, the color change seems to be signaling that the older flowers are not worth visiting. Certainly the bees notice this. But why go through the trouble of keeping spent flowers? Why not speed up senescence and pour that extra energy into seed production?

Well, its all about cues. Bees being the epitome of search image foragers are more likely to visit plants with larger floral displays. By retaining these old, spent flowers, the asters are maintaining a larger sign post that ensures continued pollinator visitation and thus increases their chances of cross pollination. The bees simply learn over time to ignore the red disc flowers once they have landed. In this way, they maximize their benefit as well.

Further Reading: [1]

Fall Leaves of the Putty-Root Orchid

Whereas most plants here in the Northern Hemisphere have largely geared down for the long winter, there is one species that has only recently begun a new stage of growth. Though it may seem damaging to produce leaves when a hard frost is just around the corner, that is exactly what this plant is doing. What's even more bizarre is that the plant in question is an orchid.

The putty-rood orchid (Aplectrum hyemale) may seem strange to most. Though it flowers during the same time as most of our terrestrial orchids (May through June), its display can be hard to track down. In fact, lacking any knowledge of a specific location, it is more likely that you will stumble across one before you pick it out of the hustle and bustle on the forest floor.

Flowering occurs at a different time than leaf out. The solitary flower stalk gives way to a single leaf starting in late summer or early fall. Why the heck would this plant start its photosynthetic lifecycle when everything else is about ready to go dormant? The answer is competition. Summer is not a bright season for those growing on the forest floor. This is especially true for a plant that only produces a single leaf.

What the putty-root is doing with its oddly timed leaf production is taking advantage of a dormant canopy. With trees and herbaceous leaves out of the way, the putty-root is able to soak up as much sun as it can get. This is a similar strategy adopted by spring ephemerals around the globe. But what does the plant have to gain from having leaves in the fall? Why not wait until spring to leaf out?

As it turns out, it simply doesn't have to. The photosynthetic machinery within the leaves of the putty-root perform exceptionally well at low temperatures. Whereas most plants simply can't photosynthesize when it starts getting too cold, the putty-root is able to photosynthesize at temperatures as low as 2° C (35.6° F)! Not only does this enable the plant to get a jump start come spring, its also able to make food throughout most of fall and even early winter.

There does seem to be a limit to this. Once temperatures drop below 2° C, the machinery can't keep up and photosynthesis grinds to a halt. This is further complicated by the fact that the leaves are often buried under snow for months at a time. Certainly its mycorrhizal associations help feed the plant, even when it isn’t actively photosynthesizing. Regardless, this strategy is a great way of getting an extra kick while everything else is slowing down. Stories such as this bring to mind the story of the tortoise and the hare. Sometimes slow and steady really does win the race!

Photo Credit: Lance Merry (www.lancemerry.com)

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

American Witch Hazel

With October nearly over, temperatures are starting to dip. The asters and goldenrods have traded their floral displays for their wind-dispersed seeds that take advantage of the fall breeze. Alas, floral displays in the northern hemisphere are nearly over. There is one major show left for those living in eastern North America. From October through November (and even into December in some regions) one species of understory shrub puts forth a display reminiscent of a firework extravaganza if the fireworks only came in yellow.

I am, of course, talking about American witch hazel (Hamamelis virginiana). This wonderful shade-loving shrub goes largely unnoticed throughout the summer. Come fall, however, it makes up for its subtle appearance by offering up some of the last flowers of the season. Seemingly overnight their branches become adorned with unique little flowers whose petals shoot out like four little party streamers. They somehow manage to look both modest and showy all at once.

It may seem strange for any plant to be flowering so late. What possible advantage could this entail? Some experts believe that late flowering evolved as a way for American witch hazel to avoid competition with other flowering plants. Indeed, it certainly attracts its fair share of pollinators in desperate search of a late season meal. Flies and bees make up a majority of pollinator visits. It could also be possible that American witch hazel flowers so late to avoid hybridizing with its spring-flowering cousin, the Ozark witch hazel (Hamamelis vernalis). Regardless of its "intentions," this fall flowering strategy comes at a cost.

Despite garnishing a fair amount of pollinator attention, American witch hazel doesn't have enough time following pollination to produce fruit before winter hits. As such, fertilization of the ovaries is delayed until May the following year. The fruits, which are contained in woody capsules, spend the entire growing season maturing into viable propagules. Once mature, the seed capsules begin to dry until they become so taught that the capsule bursts. If you are lucky and attentive enough, you may be able to hear a small snap as the seeds are forcibly ejected from the capsule.

What's more, fruit set in this species is rather low. Analyses of over 40,000 witch hazel flowers showed that less than 1% produced viable seeds. Despite all of this, American witch hazel is nonetheless a successful species in eastern North American forests. It is proof that evolution need not be all or nothing. Any slight advantage is still an advantage. This hardy shrub is, at the end of the day, a survivor.

Further Reading:
http://www.amjbot.org/content/89/1/67.abstract

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 so make sure to visit and like Northern Naturalists.

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