We
saw a couple of really interesting plant adaptations in the Black Hills. The
first is the white buttercup, which filled some of the shallow ponds of Little
Spearfish Creek.
The
underwater leaves were finely dissected, which increases their surface area for
absorbing dissolved carbon dioxide. Terrestrial leaves could also absorb more
carbon dioxide this way but would lose too much water, which is not a problem
for an aquatic plant. The flowers, however, float on top of the water where
insects can find and pollinate them.
Another
set of adaptations is represented by the quaking aspen, Populus tremuloides. When you see a whole clump of aspens, perhaps
covering several acres, it may be a single genetic individual that has spread
by underground stems. All of the aboveground stems are the same height. The
resulting genetic uniformity has its risks: diseases can spread rapidly not
only because of the genetic monoculture but because of whichever underground
connections may remain intact. But the connections help to stabilize the
massive clone: the stems (ramets) growing in favorable locations can help to
support the ramets growing in unfavorable locations, very much like the phalanx
of an army. Many plants have this adaptation, but aspens are famous because
they have such massive clonal growth.
Other
aspen adaptations sound more like just-so stories. The first is the ability of
their leaves (as well as those of cottonwoods, which are in the same genus) to
quake in the slightest breath of wind. They do this because of their flat
petioles. The standard explanation is that this allows the leaves to absorb
more carbon dioxide. I am confident that this is true, but I can only ask why,
if quaking is such a wonderful adaptation, that more plants do not employ it?
The
second is a nearly unique bark adaptation. The bark looks greenish and is
capable of carrying out photosynthesis. Underneath a tight white layer of
tissue, there is an intensely green layer, at least in the Black Hills
populations. Apparently the photosynthesis in this layer is less inhibited by
cold temperatures than in leaves and might benefit the tree in early spring or
late autumn. I could not readily find references about what aspens might do
with the sugar their bark produces; perhaps they load it directly into the
adjacent phloem and send it down to the roots and underground stems. Apparently
this layer was actively producing sugar, for an ant quickly found the shallow
wound that I made in the bark.
I
do not doubt the value of the adaptation, but once again I ask, if it is such a
great adaptation, why don’t other trees have it? Of course, many young stems do
have it. We cut down a mulberry tree in our yard, which was inconveniently
disturbing the foundation of our house, and hundreds of thick green stems
immediately sprouted back. Even older mulberry stems have a green layer—but not,
to my knowledge, the trunks, as in aspen.
I
would think that green trunks, like quaking leaves, would be easy to evolve,
and should be more common, if our facile adaptive just-so stories were entirely
correct. It appears to me that a few plants have evolved great adaptations
within evolutionary constraints.
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