Wild On The Prairie: Plants
Plant adaptations to heat, wind, and drought
July 9, 2000
With the recent rains, GREEN magically appeared. How in the world can this happen? Everything looked dead! The drought has been an ongoing ecological catastrophe -- it has been three long years since pastures greened up after a rainy spell. It seemed the horrors would never end. But now, plants have grown, bloomed and set seed -- all in the space of three weeks! My, my, my what a blessing what a gift how wondrous!
A country gardener fighting newly germinated Careless Weeds in her flowerbed called to ask, How long can seeds lie dormant? I knew that a 2000-year-old lotus seed found in a grave in China germinated. The gardener then asked a much tougher question to answer. How is it possible that they can live that long?" I had to tell her I would call her back.
Part of the reason that I love my job at the Sibley Nature Center rests in the fact I can spend time researching answers to questions like this. An hour spent flipping through books is productive work and is, in fact, part of my job description.
What is the physiological basis for seed dormancy? The sugars stored in the seed form a matrix similar to glass -- liquid glass, that is. In effect, suspended animation occurs plant cryogenics, as it were. Wow!!! How cool! Learning is thrilling! What a novel way to think of a seed a tiny crystal of life. The knowledge gives an interior light and glowing warmth to any seed we hold. The mind can imagine this liquid glass -- bright and hot too hot and bright to hold.
While researching seed dormancy, I stumbled across another tidbit of information that startled me. Many xerophytic plants transpire at more rapid rates than mesophytes. Desert plants have more water escaping from their leaves than other plants? No -- no way! The very idea seemed absurd.
At Sibley, we often talk about xeromorphism. Plants of the desert have many different mechanisms to prevent the loss of moisture: sunken stomata (pores), hairy leaves, waxy leaves, small leaves, succulent leaves, big tubers, highly dissected leaves, silver leaves, etc. Whole programs for school children have been built around this topic. Had we been wrong? Or was this author crazy?
After it rains, plants absorb increased soil moisture through their roots. Heat from the sun warms the plants moisture-laden leaf cells, causing their internal vapor pressure to rise above that of the outside air. The increased heat that is generated within the leaf cell is whisked away as pores in the leaf act like teakettle spouts. Once the internal pressure is released, the plants xeromorphic characteristics kick in to prevent more moisture from escaping. Growth stops when soil moisture is no longer available, and the plant returns to enduring the usual conditions of an arid climate.
Some bushy plants can survive droughts of a year or more. Some are aphyllous -- leafless. Allthorn (the worlds largest is on Stan Smith's Buffalo Basin Ranch southwest of town) has green branches that photosynthesize. Prickly Pear Cactus has tiny leaves that appear early in the growing season tiny tubercles where spines will later grow. Wolfberry only puts on leaves after a rain, as does Ocotillo, which some use as an ornamental specimen. Wolfberry can put on and lose leaves (and produce blooms and fruit) three times in a year, depending upon how the rainy periods are spaced. Mesquite survives by extending its roots deep and far 90 feet down and 75 feet horizontally.
Creosote Bush (Larrea tridentata) is one of the most drought resistant plants existing anywhere in the world. It can be found in great numbers in the gravelly soils of the southwestern U.S., almost to the exclusion of other plants. Its small, bright yellow flowers appear whenever it rains.
Creosote Bush has a pungently aromatic scent arising from the resin in its evergreen leaves that many find to be quite pleasant. Resinous plants are often found in the desert since their thick sap slows transpiration. Myrrh is another such plant that is native to the deserts of the Middle East. The scent of Larrea is the scent of rain in the desert. To me, it is intoxicating. I rub the crushed leaves of the plant on me as cologne, and Deborah makes a bath soap full of its resins and dried leaves just for me.
The xylem cells in Larreas roots also contain resin and have thick walls, enabling these cells to hold four times as much moisture as the xylem cells in the roots of forest plants. In addition, Larrea has a symbiotic relationship with fungi that collect water near its roots.
When drought occurs, stomata on the leaves of Larrea close for longer and longer periods during the day, until eventually they are open only in the early morning. Because stomata allow the escape of oxygen and water during photosynthesis, this process is slowed when the release valves remain closed for long periods of time.
This slowing of photosynthesis could cause problems, but Larrea has developed a mechanism to lessen the potentially harmful effect. Although its lower, interior leaves wilt and turn yellow, they continue to hang on to the plant for several weeks. Decay of the wilted leaves produces nitrogen, which the plant transfers to the remaining healthy leaves. If the drought worsens, the remaining leaves go dormant. Their resin protects them, both from drying out, and also from the effects of sudden changes in osmotic pressure when moisture does return.
Larreas reactions to rain are fascinating. On as little as 2/10ths of an inch of rain, Larrea can grow -- a 1/2 inch rain causes Larrea to grow as much as 2 inches. Even so, the plant contains growth inhibitors that prevent it from growing so much that it could not survive the next period of drought. Because moisture appears to be the factor that triggers production of the plants growth inhibitors, I wonder if too much moisture causes the growth inhibitors to kill the plant. Larrea dies if it is planted in garden soil and watered regularly.
Massive carpets of newly germinated Larrea have been seen in the wild. These occur after repeated summer monsoon downpours, when ground temperature is near 150 degrees Fahrenheit and the humidity at ground level is at 90%. I love the irony that the most drought-tolerant plant on earth can only germinate in the steamy conditions of a sauna!
I consider Larrea to be a totem, a revered symbol of what it means to adapt to the strict demands of life in the desert.
A number of desert plants are cleistogamous, or self-fertilizing. Puccoon has showy, sterile yellow blooms in March. The self-fertilizing blooms look like buds and appear in early summer. The cleistogamous flowers of some plants never even open.
Angels Trumpets and Moonpod are two charming plants of the 4-Oclock family found on the southern Llano Estacado. Both form small, low-growing mounds with long tubular white flowers that open thirty minutes before sundown. Both are pollinated by Hummingbird Moths (also known as Sphinx Moths, whose caterpillars are Hornworms). Both species have both cleistogamous and chasmogamous flowers. Chasmogamous means cross-fertilizing with the flowers of other plants.
Both species will produce seed when the moth visits their opened flowers. When no rain falls, the cleistogamous flowers appear short, stubby tubes that never open into the trumpet shape.
This sort of floral behavior is rare in the 4-Oclock family. Cleistogamous 4-Oclocks only occur in Chihuahuan Desert species such as ours. Why does it not occur in 4-Oclocks elsewhere in the world? I love such mysteries. Unknowable and unanswerable mysteries are holy, wondrous and humbling.