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Grasses – an evolutionary history
As a child of the prairie, I can’t help loving grass. Perhaps it’s the simplicity
of a grassy landscape that I find appealing, but seemingly simple things are always
more complex than they first appear. Grasses provide about a quarter of the caloric
intake for Americans – and even more in developing parts of the world – so are immensely
important. Yet they are relative newcomers to the flora of the world.
On my tour of the western states this summer I saw lots of grasslands. In fact, I
often adjusted my route to see many of the national grassland preserves scattered
throughout the west. My drive through the Nebraska Sandhills, a place I have visited
a number of times, equals anyplace I visited on the trip in terms of sheer beauty.
Only a person who loves grasses could make such a claim.
Grasses, with about 12,000 species and 780 genera, are the fourth largest plant family
and one of the most diverse. One diminutive species is native to Antarctica while
giant timber bamboos of Asia grow to 120 feet tall. But mostly grasses grow in open
prairie lands and often require a specialist to correctly identify. It is estimated
that grasses cover about 20 percent of the surface area of the earth. To humankind
they are indispensable. Rice and wheat are critical sources of human food, barley
gives us beer, corn gives us chips for dipping and alcohol to burn in our cars, bluestems
fatten our cattle, bermudagrass provides for lawns and sports fields, and on and on.
Grasses are evolutionary newcomers to the world stage, only becoming an important
addition to the world flora with the demise of the dinosaurs about 66 million years
ago. With its windborne pollen, grasses are distinctive and easy to find in ancient
soil deposits if you know where to look. A few late Cretaceous age grass pollen samples
have been identified, indicating that grasses evolved from their ancient ancestors
as a forest-edge plant that grew in moist areas. There was a steady increase in the
presence of grass pollen in the record until about 15 million years ago when grasses
The cause for this explosion was both environmental and physiological. The earth’s
ever-changing orbit had wobbled a bit far from the sun and the poles piled high with
ice, leaving much more surface area exposed on the continents. But, not only was the
world colder, but it was drier. Trees were variable enough to tolerate the cold, but
they had problems with the drier climate. Most forests need at least 30 inches of
annual precipitation to survive. Grasslands usually expand in areas with less rainfall.
With this large expanse of open ground, grasses expanded their range. But, sometime
about this time a new type of photosynthesis appeared in grasses – C4 photosynthesis.
Estimates vary about when this actually happen. The clearest direct evidence indicates
it was sometime around 12.5 million years ago, but the biological clock technique
using estimates of mutation rates, pushes it back to about 25 million. The oldest
documented grassland is 30 million years old and found in China. To complicate things,
C4 photosynthesis is thought to have originated independently over 20 different times
in different grass species.
Most plants (about 97 percent of all plant species), including about half of the described
species of grasses have old fashioned C3 photosynthesis. The first product of conventional
photosynthesis is a three-carbon compound. In this type of photosynthesis, about 20
percent of the chemical energy generated is lost because oxygen is present and demanding
its due in a process called photorespiration. C4 plants, because of an anatomical
adaptation to the vascular sheath and the addition of a unique enzyme system, keep
oxygen out of the system, producing a four-carbon compound called malate that is the
precursor to the production of a six-carbon sugar. Not only is the efficiency of photosynthesis
improved, but these C4 plants use water more efficiently because they can close their
stomata during the hottest part of the day and still continue to fix carbon dioxide
Most of what we think of as warm season grasses – bermudagrass, corn, sugar cane,
most prairie grasses, crabgrass, Johnson grass, etc. – take advantage of C4 photosynthesis.
Cool season grasses – wheat, rice, oats, fescues, cheatgrass, etc. – are C3 plants.
As a rule, C4 plants are intolerant of shade and do better in hotter, drier environments
than their C3 counterparts. The world’s great prairie provinces became especially
prominent in the landscape as C4 plants began to flex their muscles during the past
6 to 12 million years. The Nebraska Sandhills had their last major deposit of sand
about 10,000 years ago with the retreat of the last glaciers from the Rocky Mountains.
These deep sands, sometimes forming mounds 300 feet tall, are stabilized by the deep-rooted
C4 grasses, remind me of an open ocean sea of grass.
Gerald Klingaman is a retired Arkansas Extension Horticulturist and retired Operations
Director for the Botanical Garden of the Ozarks. After more than two decades of penning the popular Plant of the Week column, he’s taking a new direction, offering views on nature as he pokes about the
state and nation. Views and opinions reflect those of the author and are not those
of the University of Arkansas System Division of Agriculture. If you have questions
or comments for Dr. Klingaman about these articles contact him at email@example.com.