The Power Of Plants

1. How Does An Oak Tree Seedling Split Open A Granite Boulder?

2. How Can A Soft Puffball Fungus Push Through Asphalt?

3. How Does A Clump Of Bamboo Buckle Its Concrete Retainer?

4. Will Planting A Clump Of Buddha's Belly Bamboo In A Concrete Sewer Pipe Make It Develop Fat Stems?

5. Why Would You Want Your Bamboo To Develop Fat Stems?

6. Which Tree Is Primarily Responsible For The Crumbling Of Ancient Maya Buildings?

7. What Is The Mechanism By Which Plants Are Able To Exert Terrific Forces On Inanimate Objects?

8. Are Lichens A Substantial Factor In The Decomposition Of Rock Surfaces To Form Soil?

9. Can Plants Exert Pressure On Politicians In Order To Preserve Their Dwindling Natural Habitats?

The process by which plant tissue expands in tight places involves water uptake through osmosis and imbibition, and an overall increase in size by cell division. Although it may take years (or even centuries), woody stems and roots can increase dramatically in girth by the activity of a cambial layer located just beneath the bark. In the case of an oak or pine tree splitting open a granite boulder, the amazing process starts with a seed that becomes lodged in a tiny crevice. Like a living, expanding wedge driven deeper and deeper into the crack, the boulder is literally forced apart by tremendous pressures. Although oaks have a very hard and dense wood with a specific gravity of 0.85-0.90, even a relatively soft-bodied, non-woody plant, such as a puffball fungus, can produce enough force to push through a thick layer of asphalt.

One of the main factors that initiates the rock-splitting scenario is imbibition--the remarkable process by which water molecules move into a porous, colloidal material and cause it to swell. Water (H-O-H) is a polar molecule with a negatively-charged oxygen atom (O) flanked by two positively-charged hydrogen atoms (H). Minute colloidal particles and long-chain polymer molecules within the walls and interiors of plant cells, such as cellulose, starch and lignin, develop electrical charges when they are wet. This is particularly true when they contain exposed hydroxyl (OH) groups with negatively-charged oxygen atoms and positively-charged hydrogen atoms. Like tiny magnets, the water molecules permeate these polymers, adhering to the charged surfaces as well as cohering to the positive and negative ends of adjacent water molecules. This influx of water molecules and chemical bonding (called hydrogen bonding) causes the cell wall and its contents to swell several times its original size.

The ability of polar water molecules to form hydrogen bonds explains water's remarkable "magnetic" properties which are absolutely vital for life on earth. In addition to the swelling and bursting of seed coats prior to germination, cohesive water molecules form continuous chains from the roots to the leaves of trees hundreds of feet above the ground. Without this strong force of attraction between adjacent water molecules, water could never rise in tall stems, and plant life on earth would be limited to low-growing species like mosses and liverworts. Because of its polarity, water is a very stable substance and tends to remain a liquid rather than changing into steam as in other liquids. Frozen water is lighter and floats, thus insulating the deeper water of lakes and ponds from freezing solid. Without this important attribute, aquatic animals living in lakes and ponds in cold climates could never survive the severe winter months. And because of its polarity, water is a universal solvent essential for the myriad of biochemical reactions necessary for life.

Have you ever packed lentils into a container and then allowed them to imbibe water. This is not recommended if you want your container to retain its original shape. Even a metal can may become bloated and distorted as the lentils expand with terrific pressures.

Every time you cook rice or beans in boiling water you are observing imbibition. The boiling rice grains and legumes absorb water and swell to several times their original size. Have you ever added more rice than the recipe called for, only to find that your favorite soup has turned into mush--even forcing the lid off of your pot? There are documented records of sailing ships loaded with rice that were literally split apart when water got into their cargo holds and was imbibed by the rice. Grain silos have also ruptured when water got inside and was imbibed by the cereals.

In case you are planning to demonstrate the power of imbibition to a classroom of students, lentils work very well when packed into a perforated plastic jar that is placed under water.

Most polished and "minute" rice have the outer bran layer removed and don't imbibe as much water. Although the value seems too high, the famous biologist George Gaylord Simpson (Life: An Introduction To Biology, 1965) states that imbibition in starch may develop pressures up to 15 tons per square inch. The actual pressure causing the rupturing of seed coats, as the seed content imbibes water and swells, is probably lower than Simpson's figure--but is still phenomenal. According to Kingsley Stern (Plant Biology, 1991), the imbibition pressure required to break a walnut seed coat is 600 pounds per square inch, and swollen cocklebur seeds develop forces 1,000 times the normal atmospheric pressure. This makes you wonder what kind of pressures are needed to split the seed coat of a macadamia nut. It is interesting to note that many desert wildflower seeds will not germinate until they have soaked long enough to rupture their outer coat, thus insuring that the developing seedling will have sufficient soil moisture during its ephemeral growing season. The delayed timing may also involve the leaching of certain inhibitor chemicals out of the seeds. Seeds of smoke trees (Psorothamnus spinosus) require scarification before they will imbibe enough water for germination. This is why smoke trees commonly grow along desert riverbeds where water from heavy rain storms and flash floods has carried the seeds over miles of abrasive gravel and rocks.

Imbibition is also involved in the bulk fiber of a healthy diet. The thickening and swelling of pectin and soluble fiber extracts such as Metamucil® and Hydrocil® involves imbibition. These plant products contain a mucilaginous, hemicellulose gum derived from the husks of psyllium seeds (Plantago psyllium and P. ovata). Psyllium powder readily absorbs water and forms a smooth bulky mass that moves through the intestinal tract. There are many other natural plant gums, complex carbohydrates with numerous hydroxyl (OH) sites for water molecules to bond with. Insoluble fiber comes largely from the indigestible cellulose cell walls of grains, fruits and vegetables. Although not soluble in water, the cellulose mass can imbibe water and swell. Both types of dietary fiber are key factors in being regular and maintaining a healthy large intestine with fewer visits to your gastroenterologist.

Several European & Mediterranean species of Plantago are naturalized in southern California, including P. major, P. lanceolata and P. ovata. There are also a few native species, including P. erecta and P. elongata (P. bigelovii). They typically have basal leaves and with flowers in slender, elongate, upright spikes.

Imbibition is employed in medicine to dilate the ear canals so they will drain properly. A slender porous cylinder called an "ear wick" is inserted into the blocked ear canal where it gradually imbibes water and swells. This same mechanism also involves one of the most unusual uses for brown algae. A slender cylinder of Laminaria japonica called "dilateria" is used to dilate the cervix in routine gynecological examinations. The cylinder of brown algae is inserted into the cervix where it imbibes water and swells. Laminaria has been preferred by many Japanese physicians for more than a century; they have found its gradual dilatation far less traumatic than the rapid dilatation induced by rigid dilators.

A slender cylinder of brown algae called "dilateria."

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One more amazing fact about imbibition concerns a plausible explanation for the excavation of large limestone blocks used in the construction of Egyptian and Mayan pyramids. The huge blocks were quarried by hammering rounded wooden stakes into holes made in the face of the stone, and then soaking the stakes with water. As the stakes swelled, the force created by imbibition was sufficient to split the rock.

In the natural environment, plants are constantly invading and colonizing new habitats--a phenomenon known as succession. Since lichens are among the first plants to colonize bare rock, they play an important role in primary succession. After lichens have etched and crumbled the rock surface for centuries, mineral soil and organic matter begins to accumulate. Then other plants such as mosses and grasses begin to grow, followed by herbs, hardy shrubs, and finally trees. Although lichens produce weak organic (phenolic) acids, it is doubtful that these acids have much effect on the etching of rocks, unless they are calcareous. For most rock surfaces, the etching process is probably mechanical. Crustose rock lichens are able to grow on bare rock, sinking their spreading thallus into every minute nook and cranny. Microscopic rock fragments intermeshed with lichen thallus become loosened by expansion and contraction, as the thallus is alternately moistened and dried. The rock-breaking power of plants is greatly enhanced when seeds fall into cracks and then germinate. This is particularly true in the case of woody shrubs and conifers with powerful expanding root systems. Throughout the subalpine Sierra Nevada there are forests of pine, fir, and hemlock growing in relatively shallow soils and duff overlaying solid granite. This massive bedrock of granite was scoured and polished by glaciers as recently as 12,000 years ago.

Close-up view of several crustose lichens slowly etching the surface of Santiago Peak Volcanic rock in the Coast Ranges of southern California. The lichens include lemon-yellow Acarospora schleicheri, brown A. bullata, and gray Dimelaena radiata.

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As opportunists, plants attempt to colonize every available habitat they can find. People are constantly battling this natural botanical invasion, especially when the "habitat" happens to be their home or property. Imagine the appearance of your house and yard if you didn't do any pruning for several years. Your house would probably be obscured by shrubs, trees and vines--unless you live in a low maintenance suburb landscaped with crushed rock. Landscapers carefully avoid planting certain invasive species, such as spreading bamboos and tropical fig trees, too close to sidewalks and dwellings. With huge surface roots and spreading underground rhizomes, these plants can readily destroy concrete driveways and rock walls.

Throughout tropical regions of the world there are at least 1,000 species of fig trees (Ficus) that form an important component of the rain forest canopy, providing food and habitats for a variety of animal species. The sweet fruits are relished by arboreal animals, and because of the laxative effect of the fruits, the seeds are often deposited high in the branches of neighboring trees. "Strangler figs" readily germinate on other trees and soon develop serpentine aerial roots that completely engulf the host tree like a writhing mass of boa constrictors. If there was ever a tree well-suited for destroying human structures it is the strangler fig. Seeds dispersed by fruit-eating birds and bats germinate in crevices high on the walls of buildings. The powerful aerial roots force their way between stone blocks, gradually reducing massive vertical walls to rubble. According to the famous neotropical botanist Paul C. Standley (Flora of Yucatan, 1930), strangler figs are perhaps the principal plants responsible for the destruction of ancient Maya buildings in Mexico and Central America. In fact, many ancient cities still remain partially or completely covered by huge strangler figs and other vegetation in these tropical forests. Even some of the British and French military forts built on strategic Caribbean islands during the 18th and 19th centuries have been leveled by strangler figs, including the common island species F. citrifolia and F. aurea.

A strangler fig Ficus cotinifolia growing on a limestone block wall in the ancient Maya city of Tulum in Yucatan, Mexico.

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When I began writing this article, I was primarily concerned with the remarkable forces exerted by plants on our environment. These are mostly physical forces produced by growing plants and the amazing pressures generated by imbibition. But then a former student, Senior Disparti, reminded me of another destructive plant power that has a pronounced effect on our environment. During the process of photosynthesis, hydrogen atoms are removed from water leaving oxygen gas. This reaction results in the vital atmospheric oxygen that surrounds our planet, and which all aerobic life forms depend upon. This life-giving oxygen is also a powerful agent resulting in the gradual chemical deterioration or oxidation of all natural and human-made structures on earth.

Athough this article focused heavily on the destructive power of plants, it was never meant to imply that plants are bad in any way. Most of these destructive forces are actually quite beneficial to the natural environment as plants produce more soil and colonize new regions, thus providing the habitats for countless animal species. We are absolutely indebted to plants for our existence by providing us with oxygen, food, beverages, textiles, paper, lumber and medicines. They also provide us with beautiful forests, grasslands and meadows, colorful flowers with sweet aromas, restful sounds as leaves rustle in gentle breezes, and shade on a hot summer day.