eGarden © 2004 Ed Mues

Galls: Most Unusual, but Noticeable, Homes for Mystery Guests

I remember when I was a youngster walking through Chisolm Park in College Point, Queens, near where I grew up. Mom and dad and brother, Jim, and sister, Judy, and I were all making our way either along the salty water's edge, or across the meadow, or, exploring the boundary of the woods. Many times I came across these thin stemmed, upright plants with a nearly spherical swelling in the middle of the stem. Maybe only one or two stems in a hundred had these. It was part of the stem. I could see the cells of the stem spread, stretch and grow outward, totally out of sync with the stem an inch or so above and below this structure. I always wondered. It was mysterious.

Many years later I owned a plant store. One day I purchased from a traveling salesman an oddish slab of wood, some 5 or 6 inches in diameter, maybe two inches thick with bark similar to pine. The other side, the cut side, was flat. The salesman told me it was a piece of "burl" from a redwood tree. If I placed it in a dish of water, fresh cut side down, it would sprout, and be fragrant of pine. It did. Small green feathery redwood trees sprouted from the gnarled rounded, brown surface. How curious.

I would imagine that anyone who spends a moderate amount of time outdoors has at one time or another encountered unusual looking swellings or bumps on tree trunks, stems, or leaves. Some are the exception rather than the rule. Others are so common that as many maple and oak leaves, as not, possess them. They are highly noticeable; they are seen as outgrowths of the plant part: unnatural, maybe even deformities, but looking a part of the whole.

They are called galls. Pliny, the elder, a Roman writer and naturalist, wrote about the spherical growths called gallnuts in the first century. He believed prophesies of the coming year could be deduced from their contents. In the late 17th century Marcello Malphighi discovered that some galls are produced after the vegetative tissue is punctured by an insect that injects a substance. This provoked the interest of zoologists because it turns out most galls are caused by animals (insects). Botanists took an equal interest because the excessive development of abnormal plant tissue was perceived as a problem akin to tumors or cancers.

So, what are galls? Galls are a plants response to an irritation from an insect's feeding or laying an egg, or from an infection caused by a bacterium or fungus or virus. We do know that the rapid abnormal growth results from an imbalance of the plant's growth regulators. We don't know what exactly stimulates this response. It may be a chemical secretion or a toxin that spurs on the heightened cell division.

Thousands of different galls are caused by insects. The gall causing response remains one of the most perplexing mysteries in entomology and in botany. Something else we know is that in most cases the plant's instinct appears in the great majority of instances to isolate the outside invader and grow it a home with food and shelter provided.

Consider the legumes. The nodules that make atmospheric nitrogen available for plant use (nitrogen fixing) are root galls caused by beneficial bacteria. It was once believed that potatoes were fungi caused by root galls. Superstitions abounded. So, too, some remarkable uses. The Aleppo gall, or gallnut, of the eastern Mediterranean was an article of trade since the 5th century B.C. It provided a widely used medicine as a tonic, poison, antidote, and strong astringent.

Galls have long had economic importance both positively and negatively. Dyes for tattoos and fabrics go back to African Somalia and Turkey. The Greeks used the Aleppo gall for dying wool, hair and skins and even today they are used in large quantities to dye leather and seal skins. The richest of all sources of tannin, many of these galls contain from 30 to 70 percent. Inks made from them are so durable and permanent that some countries have enacted laws requiring certain records only be recorded with inks made from gallnuts.

Many serious plant diseases result from infection from bacteria and fungi that cause gall formation. They can form on leaves, twigs, branches, stems, roots and can in some cases disrupt water and nutrient circulation. Some may girdle a plant part such a black knot on prunus species. Others may stunt small plant parts. The ones I found as a youngster were goldenrod ball galls caused by a fly. The redwood slab was cut from a burl, a term used for a gall seen as a swelling of varying size and shape on tree trunks and branches.

Galls and other stem swellings, also called tumors, may be introduced in response to bacteria, fungi, nematodes, mites, insects (flies, wasps, aphids, etc.) mistletoe, viruses, and cultural and environmental insults such as lightning strikes, lawn mower and weed trimmer injury, ice and snow damage, deicing salt injury. Many other causes remain unexplained.

The "black knot" mentioned earlier acts and grows as if parasitic. The "pine-pine gall rust", looking more like a burl, can persist for years, enlarging year after year until the supporting branches die from girdling. The abundance of shapes these sometimes complicated structures take is staggering.

Some look like miniature pineapples, others like flowers, grubs, globes, Hershey kisses, horns, wheat seed heads, warts, peas, fuzzy caterpillars, kidneys, pleats, cones, masses of wool with brown kernels, small potatoes, clusters of BBs and on and on.

Some are shiny, some not. Red, lavender, brown, yellow, green, blue, gold. Some look like small pouches. Some are corky, woody, spongy, spindly. There are more than fifty associated with willows. More than 150 are on Compositae (Daisy family). More than 300 on oaks. All different from one another. And so on, throughout the plant world.

Very often galls caused by insects are occupied by a larva that has hatched from an egg. It is protected from predators ( in most cases) and has a plentiful supply of food until it matures and eats a hole through the outer covering into the outside world to emerge as an adult.

Sometimes galls are inhabited by creatures that are not the offspring of the instigator, but parasites of the same, or sometimes even just guests (termed "inquilines") visiting and sharing the large supply of food without injuring the original inhabitant.

Some galls have very complex structures, entirely plant tissue; in some unknown way, the invaders or secretions from them control and direct the forms and shapes of these temporary homes as they grow. The life cycles of some of the inhabitants is equally complicated. The "grape phylloxera", responsible for the destruction of many thousands of acres of the most valuable vineyards in Europe and America, is caused by an aphid that spends the winter as an egg on the cane of the same plant. There are also gall nodules on the roots. The eggs hatch and move to new leaves developing in spring and new galls are formed. From the nodules on the roots, winged adults emerge from the soil and mate and lay eggs on the canes.

In another case, some older galls become covered with algae which supports large populations of mites that survive on decaying organic matter.

Recent thinking has it that the plant under attack produces extra tissue for the larva or foreign invader to eat in order to protect more of its own vital tissue on the remainder of the plant. A plant defense, to be sure. It is beyond the scope of this brief article to explore the world of galls in greater depth. But, I'll take my leave reporting a provocative speculation on plant-insect relationships.

Until 1996, the prevailing theory held that about 125 million years ago hungry insects spurred plants to invent (evolve) attractive flowers which bribed insects into carrying pollen by offering them meals of nectar. Both color and fragrance attracted them. This is when the fossil record indicates the flowering plants first appeared and paired up with insects for their mutual benefit.

On August 6, 1996 paleobotanists uncovered evidence that plants and insects had a relationship as far back as 302 million years ago. It was not one for mutual benefit, so far as we can tell. It was a drama of attack and defense. In the fossilized remains of long extinct tree ferns, fleshy protuberances on the fern fronds were found, and close examination of these extremely well preserved samples showed them to closely resemble modern galls with the remains of larva and their fecal matter.

The fern is known. The insect is not, but we can probably guess it was a fly or wasp from that era that sought food and protection for its offspring, perhaps the one thing the plant could offer. What the plant may have gotten in return may have been a very altruistic gift to the plant world of a distant future.

From The Garden of Ed. Submitted for publication in The Towne Crier on Decermber 1, 2004

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