Bradford (Callery) Pear

A Bradford Pear tree on a ridge above the Shenandoah River probably escaped from a nearby development.

Common Name: Bradford pear, Callery pear, Braford Callery pear – The common name Bradford is eponymous, given to the tree to recognize the horticulturist who was the head of the USDA Plant Introduction Station in Glen Dale, Maryland where the cultivar of the Callery pear was first bred. Pear is from pirum, the Latin word for the fruit.

Scientific Name: Pyrus calleryana – The generic name is a variant of the Latin word for pear tree. The species name honors Joseph Callery (Giuseppe Calleri in his native Italian), a Catholic missionary to China who collected specimens of Asian native plants during his tenure there. He is recognized for having introduced the Callery pear to Europe in the 19^th century.

Potpourri: Just as kudzu gained notoriety as the vine that ate the South, the Bradford pear is rapidly becoming the shrub that swallowed suburbia. As the Callery pear, it was originally imported from China in the early 20^th century as an integral part of a United States Department of Agriculture (USDA) program to save the commercial pear industry from the devastation of a bacterial blight. The plan to use robust root stock resistant to blight from Callery pears grafted with the commercial, French pear was sound, as a similar method had been used in the late 19^th century to resolve the “Great French Wine Blight” using American vine roots resistant to the American insect pest (a type of aphid) that caused it. [1] The French pear trees grafted to Chinese root stock flourished. Had it ended there, the monoculture stands of white petaled trees and shrubs that line many roads and dominate disturbed areas would never have occurred. The history of Bradford pear, like that of kudzu, is a cautionary tale of human intervention in ecosystems without a full understanding of the complexities of nature and evolution.

Before invasives demonstrated their ability to devastate native flora and fauna in the aughts of the 21^st century, moving species randomly around the globe, sometimes purposely, was not only tolerated, but encouraged. Tomatoes, corn, and potatoes originated in the New World to become staples of European cuisine just as wheat, cotton and rice were imported and widely planted in the Americas. The genus and species scientific classification system of Carolinus Linnaeus still in use after over three hundred years was undertaken to organize the thousands of newly discovered plants submitted and named by field naturalists augmented by a list of descriptive nouns and adjectives, mostly in Latin. [2] The intentional importation of alien plants was mostly benign, with the exception of plants like dandelion, plantain, and garlic mustard that spread, crowding out the native species due to their superior resiliency. Bradford pears occupy a middle ground, having been introduced with good purpose, then intentionally hybridized to satisfy consumer demand for landscape trees.

The story begins in the decades following the Civil War as Conestoga wagons forged ever westward to colonize the verdant valleys on the windward side of the Sierra Nevada. As herding gave way to agriculture in the late 19^th century, the search for plants that would flourish there became the mission of the purposely established USDA Foreign Seed and Plant Introduction Office. Due to similarities in climate, east Asia was considered the best potential source for candidate botanicals. A stout-hearted Dutch immigrant gardener cum naturalist named Frank Meyer with no fear of travel to remote areas to forage in relative isolation was recruited to undertake the mission. Like Darwin’s mission to collect specimens around the Pacific rim on HMS Beagle from 1841 to 1846, Meyer collected a wide variety of cereal grains, leguminous vegetables and fruits between 1905 and 1915 that eventually led to many of the food crops that have been cultivated in North America for over a century. Meyer’s initial forays did not seek out pears, as they were already well established in California, Washington and Oregon. [3]

The genus Pyrus probably originated in the Tian Shen Mountains in the Xinjiang Province of western China. Pear trees hybridized as they spread throughout Eurasia as a natural progression by animals, especially birds, eating the much smaller fruit of wild pear trees and defecating its seeds. Cultivation of larger, sweeter pears preferred by Europeans predates historical records but probably started in Mesopotamia. The resultant European pear (P. communis) was brought to the colonies of the Americas by both the British and French from the east and the Spanish from the south giving rise to orchards as early as the 18^th century. Pear groves proliferated, particularly in the Pacific Northwest to make pears the third most consumed fruit in the United States, trailing only apples and peaches. As with most commercially grown fruit trees, grafting is used to grow an appealing pear variant onto rootstock selected for its hardiness Since pear trees are therefore essentially identical clones―a genetic monoculture―they are subject to epidemics as a microbe that infests one will spread to them all. This is precisely what happened to the pear industry in the early 1900s. [4][5]

Meyer returned to the United States in 1916 to observe the effects of the fire blight caused by the bacterium Erwinia amylovora on pear orchards. Professor Frank Reimer of Oregon State University had initiated a program in 1912 to find a North American pear species that was resistant to the blight to no avail and enlisted the aid of Meyer to find an Asian pear species. When he returned to China in 1918, Meyer focused on Pyrus calleryana, noting in a communiqué to the USDA that the “form from the Yangtze Valley seems to be better suited for Oregon than the one from South China.” After trekking though China for some months to seek out the tree, he concluded in another letter that “Pyrus calleryana is simply a marvel. One finds it growing under all sorts of conditions; one time on dry, sterile mountain slopes; then again with its roots in standing water at the edge of a pond.” In 1918, he proceeded to collect and ship 100 pounds of seeds back to the USDA for testing and began his journey home. He never made it, falling overboard to his death from a ship on the Yangtse River. His body was found thirty miles downriver on June 9, 1918. His colleague Reimer wrote “Mr. Meyer was one man in many thousands. He possessed a great brain and also a great heart.” [6] A suitable epitaph would be Frank Pearseed to stand beside John Chapman of Appleseed fame in the pantheon of American agronomy.

The showy spring florescence is one of the appeals of Bradford pear as a landscaping tree.

The seeds sent by Meyer were provided to USDA Plant Introduction Stations in Corvallis, Oregon and Glenn Dale, Maryland to assess the viability of P. calleryana as both root stock and as a new pear variant. The root stock proved to be resilient to the ravages of fire blight and was subsequently used to reestablish of pear orchards, saving the pear industry from devastation. Over three decades later, one of the Callery pear trees planted in Maryland caught the attention of a USDA employee named John Creech. Noting the glossiness of the leaves, the aesthetic, geometric balance of its spreading branches, and the lack of sharply spurred twigs that were typical of pear trees, he concluded that it would make an exceptional landscaping tree. By grafting branches from the original tree onto rootstock of P. calleryana, he cloned a cultivar variant that he named the Bradford pear in honor of a horticulturist employed by the Glenn Dale facility. The landscaping tree was commercially released in 1962 and quickly became popular due to the attributes that drew Creech. According to a respected and seminal plant guide, “The Bradford pear, a selection of P. calleryana, has recently become popular as an ornamental because of its profuse spring flowers and red fall color.” [7]

Bradford pear thicket along a road in Maryland

The Bradford pear became one of the primary trees lining the streets of American suburban sprawl built outward, a mecca from the noise and congestion of cities in the second half of 20^th century. To satisfy the insatiable demand for variety in the cookie cutter sameness of burgeoning developments, twenty four variants of the Callery pear were introduced with catchy names ranging from Whitehouse to Autumn Blaze to augment the original Bradford cultivar. Like the ancestral Callery pears of China lauded by Meyer for their stamina and ubiquity, the Bradford pear and its variants were indomitable, thriving in poor soil that could be wet or dry, acidic or alkaline, resistant to disease, and reliably radiating branches of bouquets in spring and brilliant red fall foliage reminiscent of New England’s maples in autumn. Millions were planted across the country from California to Connecticut. [8] By 2015, Bradford pears had become the third most popular tree in New York City with a population of 58,000. The transition from desirable landscaping tree to pernicious pest occurred slowly, as the phalanxes of flowering white trees lining major roads could no longer be dismissed as part of a normal spring renaissance. [9]

What happened was hybridization. This was unexpected but could have been anticipated. Since Bradford pears and their ilk were clones in having been propagated by grafting small branches onto robust root stock (mostly P. calleryana), they were not able to cross pollinate and produce seeded fruits due to genetic incompatibility. However, the different horticultural cultivars were produced from pear seeds that were originally gathered by Meyer from all over China resulted in hybrids with different genotypes. This is the essence of the genetic diversity that Darwin first observed among the different specie of finches in the Galapagos Islands. As long as different hybrids are within individual bumble bee collection zones, chances are that eventually the pollen from a Bradford pear will find itself in the ovule of a receptive clone. The resultant fruit with its now mutant and fertile seeds, carried away for consumption by birds, especially European starlings and American robins, spread wherever and whenever the birds went, literally. Eventually, as a matter of evolutionary dynamics, a variant emerged that was super survivable. The Bradford/Callery pear has been listed as a “plant invader” by the US Fish and Wildlife Service in the mid Atlantic states since 2008. [10]

There is a certain amount of irony in introducing Callery pears to save the commercial pear industry from fire blight and then hybridizing it to create the hardy and aesthetic Bradford pear that has become a pernicious invasive. While other trees, like “tree of heaven” ailanthus and “empress tree” royal paulownia, have been introduced and become invasive, Bradford pears are unique in having been created by USDA plant breeders as a perceived public service. Contributing to the irony is that the fire blight bacterium that was the reason for the introduction of P. calleryana in the first place has reemerged as a major problem in the commercial pear industry due to its own evolutionary mutations. Walt Kelly’s Pogo cartoon for the first Earth Day provides the adage of the age: “We have met the enemy and he is us.” Barry Commoner, the father of ecology, proposed the law that “everything is connected to everything else,” a testimony to the complexities of the natural world. The only option at this point is to stop planting Bradford pears and their peers intentionally as landscape trees and to remove them whenever they spread into new habitats. There is evidence that the word is out. A sign posted at the trailhead at a Virginia State Park read “Wanted, Dead, not Alive, Callery Pear,” asking the public to “be on the lookout for this invasive intruder” and alert park staff so that it can be removed. The enemy strikes back.

References:

Lukacs, P. Inventing Wine, A New History of one of the World’s Most Ancient Pleasures W. W. Norton and Company, New York, 2012, pp 169-174
Wilson, C. and Loomis, W. Botany, 4th edition, Holt, Rinehart, and Winston, New York, 1967, pp 365-367.
Culley, T. “The Rise and Fall of the Ornamental Callery Pear,” Arnoldia, Volume 74 Issue 3, 18 February 2017. https://arboretum.harvard.edu/stories/the-rise-and-fall-of-the-ornamental-callery-pear-tree/
U.S. Department of Agriculture. “Pyrus Crop Germplasm Committee: Report and genetic vulnerability statement, September 2004” September 2004, Germ Resources Information Network (GRIN), pages 5-7
Little, E. Field Guide to North American Trees, Alfred A. Knopf, New York, 1993, p 509
Meyer, F. N. 1918. South China Explorations: Typescript, July 25, 1916–September 21, 1918. The National Agricultural Library. Available online at: https://archive.org/details/CAT10662165MeyerSouthChinaExplorations
Brown, R. and Brown, M. Woody Plants of Maryland, University of Maryland, Port City Press, Baltimore, Maryland, 1999, p. 132.
Higgins, A “Scientists thought they had created the perfect tree. But it became a nightmare” Seattle Times. 17 September 2018. https://www.seattletimes.com/nation-world/scientists-thought-they-had-created-the-perfect-tree-but-it-became-a-nightmare/
McConnaughey, J. “Invasive Callery pear trees become a real menace” Washington Post, 17 May 2022

Hemlock for a Happy New Year

Hemlocks are among the many pines and fir evergreens that are symbolic of the holiday season. This hemlock is a new generation growing to replace those lost to an invasive species and a devastating hurricane at Limberlost in Shenandoah National Park.

Common Name: Eastern Hemlock, Canada hemlock, Hemlock spruce – Hemlock is the name for the hop plant in both the Germanic (homele) and Finno-Ugric (humala) language groups. The hop plant is the source of “hops” used for centuries across much of northern Europe to impart a bitter flavor to liquors made from malted grain. The small flowers of the hop plant are similar to the flowers of the poison hemlock (Conium maculatum) which shares the same etymology and from which the hemlock tree gets its name (by indirect association). In other words, the poison hemlock looks like and was named for the hop plant and the hemlock tree shares a number of attributes with poison hemlock. The Carolina hemlock is very similar and difficult to distinguish from its collocated cousin.

Scientific Name: Tsuga canadensis – The generic name is from the Japanese word for the larch tree which, like the hemlock, is a member of the pine family. Most of the other trees in the genus Tsuga are indigenous to east Asia, primarily Japan. The species name is reference to the first classification of the tree in the Linnaean taxonomic system based on a specimen first sighted and identified in Canada. The Carolina hemlock is Tsuga caroliniana first distinguished in the Appalachian uplands further south.

Potpourri: Hemlocks are members of the ubiquitous Pinaceae or pine family which consists of conifer or cone-bearing trees that grow throughout the temperate regions of both the Northern and Southern Hemispheres and in mountainous tropical regions. The Pine family includes pines (Pinus), spruce (Picea), firs (Abies), hemlocks (Tsuga), larches (Larix), and Douglas-firs (Pseudotsuga or false hemlock). [1] Since they are large trees that grow in dense clusters, they are among the most important trees of the timber industry, providing 75 percent of all lumber, and 90 percent of paper pulp. There are over 200 species worldwide of which about 60 are indigenous to North America. Pine family trees are self-pollinating, or monoecious, contributing to their evolutionary success at the expense of genetic diversity. The “naked seeds” that literally define the Gymnosperms (gymno is Greek―gymnasiums were places for naked exercise) are at the base of the female pinecone scales fertilized by male cone pollen wind-blown from the same tree. The pollen that is deposited on the megasporangium of the female cone in the spring ceases growth through the winter, consummating fertilization the following year. [2] In good time, you get a pine.

Hemlocks can most easily be distinguished by their needles, a term referring to the narrow, pointed leaves that, except for the larch, do not fall off over winter giving rise to the more general term evergreen. Hemlocks needles are short, arrayed in two neat rows, one of nature’s better options for higher mountains and boreal forests. However, needles do have a lifespan. Pine trees lose about one fourth of their needles every year resulting in trails coated with a soft cushion of decaying needles that suppresses almost all other plant growth, one of the best treads for foot travel. The “evergreen” needle as a leaf form is an evolutionary result of several factors involving both latitude and geology. The primary determinant is the length of the growing season, which can vary from as short as 65 days in New England to an average of 250 days in the southeast. All things being equal, a plant will trend toward greater leaf area exposed to as much sunlight as possible. Photosynthesis in the chloroplast cells of the leaves converts sun photon energy to the hydrocarbon molecules of biology. Broadleaf trees grow where they can, and evergreen needle trees grow where they can’t.

When the non-growth colder season approaches, broadleaf trees are better off wintering over with bare branches, having adequate time to replenish their foliage the following spring. In northern latitudes, there is simply not enough time to restock the canopy with sun gatherers, so they persist year-round as narrow needle-like leaves. Temperature is a second factor due primarily to physics; when the freezing point is reached, the uptake of water is squelched and growth is curtailed. Since average temperature drops about 3 degrees F every 1,000 feet, mountainous terrain has the same effect as latitude on the growing season so evergreens also prevail in higher elevations. Needle trees are also favored in northern latitudes and uplands because they are winterized with wax-coated needles and resin-infused wood and roots. The conical shape of many conifer trees with their one dimensional needles are also better at survival in heavy snowpack. It should be noted that the pine barrens of New Jersey and the wide expanses of scrub pines across the south are neither mountainous nor northern. Some species of pine thrive in dry sandy soils where periodic wildfires have historically been the norm. Their cones are serotinous, which means that they evolved to burst open after a fire to spread the seeds of restoration, eventually becoming the dominant species. [3]

That hemlock trees have the same name as the poisonous hemlock plant cannot be a matter of chance etymology. They have some things in common, but not the notorious toxins of the latter. The “drinking of the hemlock” was the standard method of execution in Ancient Greece. One of history’s most enduring dramas is the trial of Socrates by the popular court or dikasterion comprised of 500 Athenian citizens in 399 BCE. He was prosecuted for undermining religious faith in the “gods that the state recognizes” by introducing new “demonical beings” and for “corrupting the youth” and found guilty by a slight majority. The hemlock execution of Socrates is considered by many historians to mark the end of the Golden Age of Greece. [4] Poison hemlock was thus well known throughout Europe by the Middle Ages both for its toxicity, and, in small doses, for treatment of a variety of ailments. There is evidence of its use for the treatment of cancer, as a narcotic or analgesic, and even as an anti-aphrodisiac (perhaps by killing the object of desire). [5] Because of this, many Europeans were familiar with its shape when growing and its smell when ground into powder. However, since there were no hemlock trees in Europe, it took the discovery and exploration of the Americas to associate the poison hemlock plant with its namesake tree.

The hemlocks of North America were almost certainly first sighted along riparian riverbanks by French explorers who penetrated the mainland by sailing up the St. Lawrence from the North Atlantic in the 16^th century. Their knowledge of the smell and branching pattern of the poison hemlock led to applying the familiar name to the unfamiliar evergreen tree due to its similar characteristics. This is corroborated by the British Cyclopedia of 1836 in noting that the hemlock tree was “so called from its branches in tenuity and position resembling the foliage of the common hemlock.” Conium, the genus of the poison hemlock, was purposely chosen because the plant looked like a miniature cone-bearing tree. In the New World, where there were so many new and strange plants, any means of distinguishing one species from another by using a mnemonic brought some order to the chaos. To differentiate the evergreen version of hemlock from its doppelgänger, the compound name “hemlock spruce” was applied. [6] Spruce trees of the genus Picea prevail in boreal forests across North America and Eurasia. Spruce is an anglicized version of “from Prussia” due to the prevalence of native spruce trees along the Baltic Sea near present day Lithuania. Prussia was the ancestral home of the medieval Teutonic Knights that grew in prestige and power, uniting the disparate Germanic states to form a unified Germany in the 19^th century. The hemlock spruce is called Pruche du Canada in Quebec, further evidence of Prussian origin. It was later moved from the spruce to the pine family.

Eastern hemlock or hemlock spruce is the most shade tolerant of all tree species and can survive with as little as 5 percent full sunlight. Since the conversion of solar energy to produce hydrocarbon energy is the foundation of life, its lack can only be compensated for by slow growth. Like Treebeard, the ent of Tolkien’s mythical Fangorn Forest, hemlock growth is slow but inexorable. A one-inch diameter (usually reported as dbh―diameter at breast height―to account for irregularities) hemlock can be over 100 years old. Since hemlocks can grow to over six feet dbh with a height of over 150 feet, it follows that longevity is another characteristic trait. The record age for a hemlock is 988 years, older than Noah’s 969-year-old grandfather Methuselah, the epitome of lifetime endurance. Once established, a hemlock canopy blocks sunlight from penetrating to the understory, snuffing out most arboreal competition. The subsequent microclimate of dense shade with a deep duff layer retains moisture and sustains uniformly reduced ambient temperatures. Not surprisingly, the relatively exacting moisture and temperature requirements for hemlock germination are met by the conditions that they create. [7] But there is more to forest soil management than trees. There are also fungi.

Hemlock polypore growing on dead hemlock.

Pine family trees like hemlock are connected through their root systems with fungi that surround them, an arrangement know as ectomycorrhizal, “outside fungus root” in Greek. About 90 percent of all plants form mutualistic partnerships with fungi to gain access to essential soil nutrients like phosphorus and nitrogen, with the plant providing up to ten percent of its hydrocarbon sugar output to root fungi in return. For most plants, the mycorrhizal relationship is an option that results in more robust growth. For trees of the Pine family like hemlock, the mycorrhizal relationship is universal. Many different species of fungi are involved with the roots of any given tree. While there have been no studies for hemlocks, the closely related Douglas firs (Pseudotsuga menziesii) are estimated to have over 2.000 different species of associated fungi. [8] The kingdom Fungi is not uniformly benign, however, as all living things must find their niche in the tangled web of life as a matter of survival. The subsurface soils kept moist by the hulking hemlocks are an ideal habitat for mold, another broad category of fungi. Seven species of fungi attack the seeds of hemlock resting on the moist soil awaiting the magic of germination. One mold species, Aureobasidum pullulans, was found growing on almost three fourths of all hemlock seeds, impeding their full function. Hemlocks, when they eventually keel over, provide yet another form of fungi, the saprophytes that feed on the dead. Were it not for the fungi that consume the cellulose and lignin from which tree trunks are made, the world would be covered with tree trunks and none of their carbon would be returned to the atmosphere. Because hemlocks are so pervasive, one species of fungus aptly named Ganoderma tsugae or hemlock polypore, subsists exclusively on its deadwood. Also called varnish shelf, it is one of the most recognizable of all fungi and is closely related to one of the most important fungi in Asian medicine (see full article for further details).

Hemlock growing adjacent to fallen old growth hemlock trunk in foreground.

The hemlock is listed on the International Union for Conservation of Nature Red List as near threatened. [9] This surprising state of affairs is not the result of clear cutting and overharvesting, although human impact has surely had deleterious effects. The high point of hemlock harvest was at the turn of the last century when the wood was used primarily for home construction roofs and flooring. As the population surged in the decades that followed and newspapers of the golden age of Hearst and Pulitzer proliferated, hemlocks became one of the primary sources for paper pulp. The effects are exemplified by Michigan’s growing stock decreasing by over 70 percent between 1935 and 1955, a result of the slow growth of hemlock relative to its removal. However, the real culprit that threatens hemlocks is a sap sucking insect closely related to aphids, the bane of gardeners and food for ladybugs. The woolly adelgid was probably introduced from Japan in the early 1950s somewhere in New England and has now spread to 19 states and two Canadian provinces.[10] The larvae of the adelgid suck the body fluids from hemlock needles at their base, covering themselves with a fluffy white layer (hence woolly) to protect against predation (see full article for further details). A death by a literal thousand cuts ensues that can take decades but is in most cases inevitable. The hemlocks of Limberlost were the only old growth tract in Shenandoah National Park. They had been so weakened by woolly adelgids that they toppled during hurricane Fran in 1996. The hemlocks are just starting to recover almost thirty years later (note fallen hemlock trunk in foreground in photo).

Unlike its poisonous namesake, hemlock is not only edible but salubrious. It has been attested that the entire Pine family “comprises one of the most vital groups of edibles in the world.” [11] This would mostly apply to northern latitudes where the paucity of winter food could result in starvation absent the resort to eating pine tree inner bark, a thin layer called the cambium. The nutritious cambium is responsible for the formation of the water transport xylem on the inside and the hydrocarbon food transport phloem on the outside; in other words, it makes the tree trunk. For soft wood pine trees stripping off the outer bark layer to gain access to the cambium can be readily accomplished with primitive scraping tools. The native peoples of North America collected cambium which was cut into strips eaten either raw, cooked, or dried and ground into flour to make bread, a practice adopted by early colonists. The Adirondack Mountains of New York derive from the Mohawk word haterỏntaks, which means “they eat trees.” The healthful benefits of hemlocks and other pines are further enhanced by high concentrations of anti-inflammatory tannins and anti-oxidant ascorbic acid/vitamin C in all parts of the tree. The various Indian tribes had diverse uses, extending from pine tea tea to treat colds to thick pinesap paste applied to wounds as poultice.[12] One early colonist wrote in his diary in the mid 19^th century that “I never caught a cold yet. I recommend, from experience, a hemlock-bed, and hemlock-tea, with a dash of whiskey in it merely to assist the flavor, as the best preventive.” [13]

References:

1. Little, E. The Audubon Field Guide to North American Trees, Eastern Region, Alfred A. Knopf, 1980, pp 276-301.

2. Wilson, C. and Loomis, W. Botany, Holt, Rinehart and Winston, New York,1967, pp 549-570

3. Kricher, J. and Morrison, G. A Field Guide to Eastern Forests of North America, Peterson Field Guide Series, Houghton Mifflin Company, Boston. 1988, pp 9-10.

4. Durant, W. The Life of Greece, Simon and Schuster, New York, 1966, pp 452-456.

5. Foster, S. and Duke, J. Medicinal Plants and Herbs of Eastern and Central North America. Peterson Field Guide Series. Houghton Mifflin Company, Boston, 2000, pp 68-69.

6. Earle, C. Tsuga, The Gymnosperm Database, 2018, at https://www.conifers.org/pi/Tsuga.php

7. Godman, T. and Lancaster, K. “Pinaceae, Pine Family” U.S. Forest Service Report at https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_1/tsuga/canadensis.htm

8. Kendrick, B. The Fifth Kingdom, Focus Publishing, Newburyport, Massachusetts, 2000. Pp 257-278.

9. https://www.iucnredlist.org/species/42431/2979676

10. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.131718/Tsuga_canadensis

11. Angier, B. and Foster, K. Edible Wild Plants, Stackpole Books, Mechanicsburg, Pennsylvania, 2008, pp 168-169.

12.Ethnobotany Data Base at http://naeb.brit.org/uses/search/?string=tsuga+canadensis

13. Harris, M. Botanica, North America, Harper Collins, New York, 2003, pp 44-46.