Monday, April 13, 2015

Dichotomous key (VII.c) - The world of internodes cont'd (Color)

Color - To start, everyone should read through this beautiful portrait of colors through the seasons. This time of year can come with color changes nearly as drastic (though not as abundant) as in the fall. The bark of aspens begins to take a strong green hue, red-osier dogwood bark deepens in red, and the boxelder twigs have taken on a stronger purple. The primary reason for the color change is photosynthesis. This early into spring, it's still a bit risky to produce leaves for fear of a hard frost. This is particularly true for species with compound leaves, which have a lot more energy invested in a single leaf than trees with simple leaves (and indeed ash is one of the last to leaf out).

Color change - While color can be important in distinguishing one species from another, it can also be helpful in determining the age of a section of branch. The two photos immediately above show this color change over time in sugar maple. At the bottom of each section you can see the terminal bud scale scars that wrap completely around the branch (they kind of look like Jane Fonda's bunched up socks). In total, the whole branch was 6 years old, the brownest on the left was the youngest and progressively getting older to the right. The photo above it is the branch before I cut it. The color changes as the bark thickens and cork is added. As the bark thickens on a branch or stem, photosynthesis is inhibit and that section of the plant may no longer exhibit the dramatic shift in color in the spring.

In some species this color change indicates response to ecological conditions. The photo at the very top is a shrub willow, which I unfortunately do not know which species. Each of the sections is from just a single year's growth - the stem was about four and a half feet! The stem greens as photosynthesis in the bark ramps up (the green is due to the production of chlorophyll). The response of the plant that results in the gradation to red is from the production of anthocyanins in the bark. These accessory pigments, which give red maple and many other leaves their red or purple fall colors, act as protection from the sun (see below for more details). 

Photosynthetic, greenish bark of quaking aspen with a snow flea! (first warm day of the year, 65deg)

Green: I always remind my students that when you see green on a plant it means photosynthesis is happening there. More precisely it indicates the presence of chloroplasts, which are the site of photosynthesis. Regardless the important thing to remember is that where you see green, that part of the plant is harnessing sunlight to make sugars. So whether it's a ripening green banana photosynthesizing energy to help itself grow rather than siphon all its energy budget from the leaf production or a vibrant green twig of boxelder photosynthesizing in the early spring before leaf out, the color is always due to the presence of chlorophyll. Above is the yellowish green bark of a mature quaking aspen. It maintains thin bark to continue photosynthesis along the stem for many years by having the bark turn to powder. If you rub the bark you're hands will feel soapy and you can see the white of the bark on your hands. This can actually be used as sunscreen!

White pine twig. When wet shows more dramatically the yellowish hue of its bark
Plants with dramatically green bark include:
  • boxelder
  • aspens, cottonwoods, willows
  • striped maple (at maturity, twigs are purplish)
  • white pine

Red: There are many other colors besides green. Some plants on red all the way around and up and down the stem, as in red-osier dogwood, while others are facultatively red, as in the reddish pigment in raspberry branches. On raspberries, you'll often see a stark contrast from the red color on the top (exposed to the sun) surface versus the green color on the bottom, shaded part of the stem. I have a hunch that in dogwoods the red is also a warning to would-be predators of the toxicity of the plant. Since, as shrubs, their stems are always within munching range of most herbivores mouths, it would behoove them to have strong defenses and strong warnings.

Plants with vibrant red bark:
  • Red-osier dogwood
  • Silky dogwood (above)
  • Raspberry

Alt-leaf dogwood showing controst between the lighter bottom of the stem and the darker, exposed portion. 
Purple in trees is caused by the same process as reds, though in purples there is a higher concentration of chlorophyll. The following plants have a purplish tint.
  • boxelder (kind of gets a lot of dramatic color)
  • alternate-leaf dogwood (pictured above and below)
  • striped maple

Yellow: The dead twigs of alternate-leaf dogwood (Cornus alternifolia) turn yellow. I haven't been able to find any corroborating evidence of this, but I would assume that the branches of alternate-leaf dogwood, which are a deep purple when alive, are rich in carotenoids, accessory pigments that allow a plant to capture a wider range of wavelengths of light. When the twig dies for whatever reason, the cholorophyll dies too and is it breaks down reveals the beautiful hidden palette of oranges and yellows. 

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