Bulldozers and Bacteria: The Ecology of Sweet Fern
Peter Del Tredici
Comptonia peregrina, a common roadside plant in eastern North America, provides a case study both of how nature copes with disturbance to the land and of just how convoluted the study of this process can be.
Sweet fern, Comptonia peregrina, is a shrubby soils. The first is its use of nitrogen gas from member of the Myricaceae, or bayberry family. the atmosphere to produce mtrates-a feat it Its common name is derived from the pleasing accomplishes by forming root nodules in symbi- fragrance that its tiny, resin-filled, glandular otic association with nitrogen-fixing bacteria. hairs give off when crushed or rubbed, and from The second is an ability to propagate itself veg- its coarsely lobed, somewhat fern-like leaves. etatively by means of long, thick roots that run Comptoma, a distinctly unprepossessing plant, an inch or so beneath the soil surface. These has a natural range that covers a large portion of shallow roots form numerous buds in the fall eastern North America. Forming a rough tri- that grow into shoots the following spring. angle, the eastern flank of this range extends Under the right conditions, Comptonia behaves from Prince Edward Island and Nova Scotia as a shrubby groundcover, spreading over large south into the mountains of north Georgia; the areas by means of these root suckers. western edge reaches from the southern Appala- chians north through Tennessee and Minnesota Historical Considerations all the way to central Manitoba; and the north- Sweet fern’s distinctive form and pungent odor ern edge runs from the Canadian plains through made a strong impression on the early European central Ontario and Quebec to the Atlantic settlers of North America. Nowhere is this (Elias 1971 Sweet fern typically grows to three more apparent than in a passage from a book or four feet in height and, over time, forms written in 1654 by one Edward Johnson, Won- extensive colonies-up to twenty feet across- derworking Providence of Sion’s Saviour in from suckers produced by its roots. New England. Johnson was presenting a second- As to habitat, sweet fern shows a strong pref- hand account of the arduous journey made in erence for dry, sandy soils with full exposure to 1636 by the first English settlers of Concord, the sun. These sites, which include dry, piney Massachusetts, led by Captain Simon Willard. woods, exposed mountain slopes, abandoned Starting from Boston, they traveled by boat as pastures, pine barrens, highway bankings, gravel far as Watertown and then made their way over- pits, weathered mine tailings, and cut-over land, more or less followmg the meandering forested land, have typically experienced some Charles River. Johnson describes (and undoubt- form of disturbance in either the recent or dis- edly embellishes) a scene in which the wearied tant past (Schramm 1966; Schwintzer 1989). pilgrims confront "a scorching plame, yet not so Two attributes equip Comptoma for the plaine, but that the ragged bushes scratch their pioneering role of a colonizer of disturbed legs fouly, even to wearing their stockings to
The charactemstics that mspmed the common name sweet fern-tmy, resm-fllled hams and fern-like leaves- can be seen m this plate from Franz Schmidt’s Osterreichs Allgememe Baumzucht (Vienna, 1792). The plant we know as Comptoma peregnna is labelled under a hybrid of the two names given it by Carolus Lmnaeus m his Species Plantarum. It was Charles L’Hemtier who demonstrated that the plant did not belong m either of the genera suggested by Linnaeus. 4
their bare skin m two or three hours." Those had gotten a violent toothache. To cure it she without "bootes or buskings ... have had the boiled the Myrica aspleniifolia, and tied it, as bloud trickle downe at every step." And injury hot as she could bear it, on the whole cheek. was compounded when "the sun casts such a She said that remedy had often cured the tooth- reflecting heate from the sweet ferne, whose ache before." The medicinal use of sweet fern scent is very strong, that some herewith have must have been widespread, given that later beene very nere fainting, although very able authors and travelers make frequent reference bodies to undergoe much travel."1/ to its use not only by various tribes of Native John Josselyn’s reference to sweet fern in his Americans, but also by European settlers classic work New-Englands Rarities Discov- (Erichsen-Brown 1979). ered, written in 1672, is considerably more William Bartram mentions sweet fern only benign: "Sweet Fern, the Roots run one within once in his Travels, but more sigmficantly, he another like a Net, being very long and spread- offered it for sale in his famous Catalogue of ing abroad under the upper crust of the Earth, American Trees, Shrubs, and Herbaceous sweet in taste, but withal astringent, much Plants, published in 1783 (Fry 1996). In this hunted after by our Swine: The Scotch-men that broadside, Bartram listed sweet fern under a are in New-England have told me that it grows hybrid of the two Lmnaean names, Liquidam- in Scotland." Josselyn was an astute observer, as bar Aspleni Folia, noting that it grew on "Light his description of the spreading roots of the dry sandy Ridges." Two years later, Humphrey plant clearly indicates. His Scottish informants, Marshall produced the first detailed description however, were dead wrong; sweet fern is native of the sweet fern in his book, Arbustrum only to eastern North America. Americanum, also using Bartram’s hybrid It was Carolus Linnaeus who assigned the name, Liquidambar asplenifolia. Marshall’s first modern scientific name to sweet fern, publication, which is considered the first book which he did in Species Plantarum, published in by an American about American trees and 1753. Unfortunately, he confused the situation shrubs, brings to a close the early history of by accidentally giving the plant two names, Liq- Comptonia. Later botanical authors continued uidambar peregnna on page 999 and Myrica tinkering with the name, but added little origi- asplenifolia on page 1024. Subsequent authors nal information to the basic understanding of were left to choose which name to use. The the plant itself. currently accepted name of sweet fern’s Sweet Fern genus, Comptonia, was established in 1789 by Desperately Seeking the French botanist Charles L’Heritier, who My own involvement with sweet fern began in demonstrated that the plant did not belong in 1971 when I started working for the late Dr. either of the genera suggested by Lmnaeus. John Torrey at the Harvard Forest in Petersham, L’Heritier’s name commemorates Henry Massachusetts, just after he had shifted the Compton (1632-1713~, Bishop of London, a focus of his research from root physiology to lover of trees and an early supporter of botanical nitrogen fixation. He selected Comptonia as his research and exploration. experimental subject and hired me to grow it in Linnaeus’ student Peter Kalm, who may the laboratory. At that time, the symbiosis of well have collected the specimens on which legumes with the nitrogen-fixing Rhizobium Linnaeus’ original description was based, pro- bacteria was well understood, but almost noth- vided a particularly interesting reference to ing was known about nitrogen fixation by the sweet fern in his book, Travels into North so-called nonlegumes that form a symbiotic America, written in 1770. In this work, a report association with a totally different type of bac- of his travels between 1747 and 1750, Kalm terium in the genus Frankia. When Dr. Torrey’s noted the medicinal use of sweet fern by indig- project started, no one, despite seventy years of enous people: "Among the Iroquois, or Five trying, had succeeded in isolating the causative Nations, on the Mohawk River, I saw a young bacterium from a nonleguminous root nodule Indian woman, who by frequent drinking of tea or in culturing it independent of its host. This 5
Sweet fern is seen with quaking aspen growing along Route 2 in Concord, Massachusetts failure was the block that held up progress in them between the thumb and the forefinger to researching the subject. extract the seeds. With an overabundance of enthusiasm and a Once we had managed to collect enough seeds dearth of experience, I was hired to bring sweet to work with, the next hurdle was to get them fern into the greenhouse-domesticate it, if to germinate. We tried all the standard tech- you will-so that we could study the mtrogen- niques for stimulating seed germination in fixation process in a controlled environment. To woody plants and all of them failed. Subsequent cultivate Comptonia under laboratory condi- research with excised embryos grown in a tions, we couldn’t just dig up plants from the sterile culture demonstrated that the failure field because the roots were always contami- resulted from the presence of chemical inhibi- nated with fungi and bacteria other than the one tors located m the innermost seed coat. These we wanted to study. No, Dr. Torrey insisted, we inhibitors are not unique to Comptonia. In most had to grow the plant from seed in sterile sand. temperate plants, however, chilling effectively In central Massachusetts, sweet fern’s seeds, counteracts the inhibitors-not the case with technically considered to be frmts, ripen around sweet fern seeds. It was only when Dr. Torrey the fourth of July. They are light brown in color, suggested treating the seeds with gibberellic four-to-five millimeters long, and, as they acid, a naturally occurring plant growth regula- mature, they become enveloped in a burrlike tor, that we were able to get any of them to structure that is covered with long, green bracts. sprout. Eventually we learned that soaking The burrs are soft to the touch and give off a scarified seeds m a dilute solution of gibberellic delicious, almost spicy scent when one rubs acid for twenty-four hours would produce up to 6
80 percent germination (Del Tredici and Torrey The lack of any obvious dispersal mechanism 1976). While these results were satisfying in left buried seeds as the only likely explanation that they allowed the research program to move for the seedlings in the Connecticut clearcut. forward, they were also frustrating because we The question was, how did they get there? In could not relate the gibberellic acid treatment to nature, most Comptonia seeds come to rest the way the seeds behaved in nature. within a half meter of the parent that produced The problem stumped me for some time. In them and are soon buried in the leaf litter that four years of studying Comptonia I had exam- collects beneath the plant. As I see it, deep ined thousands of plants all across New England chemical inhibition prevents germination for but had never found a wild seedling. Invariably, several years, by which time the seeds are well every small plant I found was attached to a root covered. The litter contributes to delayed germi- that emanated from an established plant. For nation either indirectly, by excluding light, or whatever reason, I never found Comptonia seed- directly, by giving off specific chemicals that lings under an existing clump of sweet fern. In suppress germination. In either case, a buried frustration, I stopped thinking about the prob- seed will not sprout unless brought to the sur- lem of seed germination in nature until one day face after its own internal dormant state has in the spring of 1976, on a walk in the woods in been neutralized. In the Connecticut woods northwest Connecticut, I came upon a site where I found my sweet fern seedlings, this res- where hundred-year-old white pines (Pinus urrection was facilitated, albeit inadvertently, strobus) had been clearcut and then bulldozed by the state forester who upon completion of the autumn before. Among all the weeds and the logging operation had the whole area bull- whatnot that were emerging, I was amazed to dozed to encourage the "natural" regeneration see seedlings of sweet fern growing, their coty- of white pine seedlings. ledons still attached. There were no adult plants Clearly bulldozing was just what the sweet to be found, just seedlings. In all, I counted fern seeds needed. They had been deposited in 194 of them in an area of less than an acre (Del the soil before the pines grew up, while the land Tredici 1977). was in pasture, and then germinated after the According to my reasoning, these seedlings logging operation brought them to the surface. must have arisen either from dormant seeds On the basis of ring counts of the cut pine trees, buried in the soil (the so-called seedbank) or I estimated that the canopy of pines had closed from seeds brought in by some dispersal agent. about seventy years before I came on the scene, Given the relatively large size of the sweet fern the point when sweet fern would have disap- seed and its lack of any specialized dispersal peared from the site because of insufficient structures, transport by rain or wind could be sunlight. Seventy years, then, is a minimum ruled out; and its inconspicuous appearance and estimate of the time the seeds could survive in lack of fleshy coverings make dispersal by ani- the soil. I have no idea what the maximum is. mals extremely limited. Indeed, the only animal It is clear, however, that soil disturbance is an ever reported to eat the sweet fern seeds is the absolute reqmrement for the germination of yellow-shafted flicker (Colaptes auratus), a Comptonia seeds. Henry David Thoreau made ground-feeding member of the woodpecker fam- essentially the same observation m his journal ily. One F. E. Beal examined 684 flicker stom- on October 22, 1860: "I notice that the first achs in 1911 and found an undisclosed number shrubs and trees to spring up in the sand on rail- of Comptonia seeds in one of them. However, in road cuts in the woods are sweet-fern, birches, order to explain by animal dispersal the 194 mllows, and aspens, and pines, white and pitch; seedlings that appeared just one year after but all but the last two chiefly disappear in the clearcutting, one would need to postulate a size- thick wood that follows." All of the above able flock of flickers roaming the countryside, species, save Comptoma, have wind-dispersed eating sweet fern and defecating exclusively on seeds that exhibit no capacity for long-term sur- this one acre in the woods. vival in the soil. Clearly sweet fern’s buried seed 7
fixation in Comptonia. Using gibberellic acid to stimulate germination, we were able to produce abundant nodule growth on vigorous seedlings that were grown with their roots dangling in a nutrient mist (aeroponics). This system, unlike water culture (hydro- ponics), allowed the plant roots to develop the hairs through which the bacteria penetrated the root itself (Zobel et al. 1974). By repeatedly subcultur- ing the nodules from one mist box to the next, we eventually 1/ were able to produce "clean" nodules that were relatively free of other microbial con- taminants (Callaham and Torrey 1977; Bowes et al. 1977). These nodules were then surface-sterilized, macerated together with special digestive enzymes, and incubated on an elaborately formulated nutri- ent agar. After three weeks of culture, Dale Callaham, who did the isolation work, observed several small colonies of bacteria with filamentous growth. While the unusual morphology of this organism clearly resembled that of an actinobacterium, it was unlike that had been Sweet fern in frmt at the height of summer. any previously described. It was not until we strategy, which evolved in response to natural had obtained a second generation of functional disturbance such as fire and erosion, had nodules by re-innoculating fresh Comptonia adapted well to the human-induced changes of seedlings with a culture of the isolated bacte- the twentieth century. Sweet fern, as a pioneer rium that we knew we had the real thmg. species, can play an important role in revitaliz- This conclusion was corroborated when we ing land that has been traumatically stripped of isolated the filamentous bacteria from the sec- its plant cover. ond-generation nodules and found them to be identical to those of the first generation. It was Nitrogen Fixation only by following this elaborate procedure- Eventually, after seven years of work, Dr. referred to as fulfilling Koch’s postulates-that Torrey’s research team succeeded in isolating we could prove that we had the causative organ- the bacterium that is responsible for nitrogen ism in hand. These successful results, published 8
in 1978, marked the conclusion of nearly seventy years of frus- trated attempts to isolate a Frankia bacterium from its host plant. This breakthrough opened wide the floodgates of research on actinorhizal plants, whose important role in colonizing bare, nutrient-poor ground was just starting to be appreciated. Most of the nitrogen fixed by these plants enters the nutrient cycle slowly through the de- composition of fallen leaves, twigs, branches, and fine roots, but over time the contribution of actinorhizal plants to the total ecosystem nitrogen bud- get can be substantial. Research on red alder (Alnus rubra) in A micrograph of the Frankia bacteria showing its long, branchmg filaments the Pacific Northwest, for under Nomarski phase mterference optics at a magmfication of ISOOx example, has shown that pure stands of the tree can add up to 280 pounds of There are several reasons for this, not least the nitrogen per acre per year to the forest plant’s reputation for being difficult to propa- (Schwintzer and Tjepkema 1990). It is important gate. Germination from seed, as shown above, is to keep in mind, however, that nitrogen-fixing virtually impossible, and digging the plant up plants can typically hold their own against com- from the wild is seldom successful, given the petition only when soil conditions are poor. On ropy nature of its root system. It wasn’t until the fertile ground they seem to lose some of their early 1970s that a research team at the Univer- competitive advantage to other trees and sity of Massachusetts, Amherst, developed tech- shrubs. In a very real sense, nitrogen-fixing niques that allowed for the plant’s commercial plants sow the seeds of their own replacement production (Hyde et al. 1972). by elevating the nitrogen content of the soil. The authors of that study were seeking to identify plants that would rapidly cover high- and Cultivation Propagation way bankings, and sweet fern was one of the Sweet fern’s ability to propagate itself from root plants that interested them. They designed an suckers is another important component of its experiment to determine both the best time of colonization strategy. Once the plant gets a foot- year to take root cuttings as well as their opti- hold in a location to its liking, it comes to domi- mal size. Two different-sized cuttings were col- nate the area by sending up numerous root lected twice a month for a period of one year: suckers. The ever-observant Henry Thoreau three inches long by one-quarter-inch diameter made note of this on March 18, 1860: "The and three inches long by one-eighth-inch diam- sweet fern grows in large, dense, more or less eter. Forty-five days after the cuttings had been rounded or oval patches in dry land. You will see stuck in individual pots, they were checked to three or four such patches in a single old field. It see whether they had produced leafy shoots. " is now quite perfect in my old bean-field." No significant difference was found in the William Bartram’s 1783 offering of sweet fern number of shoots produced by the two different notwithstanding, the plant has never made cutting sizes over the course of the year, but the much of an impression in the nursery industry. time of cutting was highly influential. At least 9
80 percent of the root cuttings taken between shrub cover on fresh roadcuts and bankmgs. The February 24 and May 1 produced shoots, while authors found that root pieces of sweet fern those taken between May 15 and August 1 pro- could be stuck directly into a bare bank in early duced few or no shoots. Cuttings taken between spring. According to recommended procedure, August 15 and December 10 produced good-to- root cuttings of Comptonia, which can be any- poor percentages of shoots, depending on the where from one-sixteenth to one-quarter of an date the cuttings were made. (No cuttings were inch in diameter and four to six inches long, taken between December 10 and February 24 should be planted an inch deep and six inches because the ground was frozen.) Based on these apart and mulched with two to three inches of results, the authors recommended that root cut- wood chips. If this "direct stick" procedure is tings be taken before the parent plant started to followed, sweet fern will produce a closed, leaf out, around May in the Boston area. Root weed-resistant canopy within three to six years. cuttings made after the stock plant’s leaves A Problem emerged produced shoots in very low percent- Pathological ages. Their observations clearly suggest the The final chapter in the Comptonia story pits existence of an inhibitory hormone produced by one plant against another in a battle to the the leaves that suppressed the development of death. It concerns a disease that I became aware the root buds into shoots. of only after publishing an article advocating sweet fern for landscape use. To my surprise, Uses: A Landscape Community Approach several plant pathologists wrote to chide me for Frank Egler, working with researchers at the my recommendation. Sweet fern, it turns out, Connecticut College Arboretum m New Lon- is the alternate host of a fungus, Cronartinm don, was among the first to recognize the poten- comptoniae, that causes sweet fern blister rust tial role that sweet fern, as well as other on hard pines with needles in bundles of two suckering shrubs, could play in the formation of or three. In the Northeast, jack pine (Pinus low-mamtenance, naturalistic plantings along banksiana) and pitch pine (P. rigida) can be highway bankings and power company rights- infected, as well as other introduced hard pines. of-way (Kenfield 1966; Niering and Goodwin In the South, shortleaf pine (P. echinata) and 1974). In the course of their studies of old-field loblolly pine (P. taeda) can be seriously infected. succession m the Northeast, the authors devel- During the course of its life cycle the blister oped techniques-specifically the use of herbi- rust has two hosts, the susceptible pine species cides to selectively kill trees-to "arrest" the and either sweet fern or its swamp-dwelling successional process at the shrub stage of devel- relative, sweet gale (Myrica gale). The fungus opment. Their goal was to manage existing lives one stage of its life on the leaves of the vegetation to form a distinctively beautiful, sweet fern and the second inside the stem of the low-growing landscape that would not interfere pine tree. Although Comptonia is only slightly with power lines or highway sightlines. In New affected by the fungus, the susceptible pine can England, these low-maintenance associations be seriously damaged or even killed. commonly include, along with sweet fern, the Control of the disease is difficult, given sweet following woody plants: pitch pine (Pinus fern’s wide natural range, but the forestry litera- rigida), red cedar (Juniperus virginiana), gray ture makes a few simple recommendations, birch (Betula populifolia), meadowsweet (Spirea including taking care not to plant infected pine sp.), bayberry (Myrica pensylvamca), sumacs trees and clearing out sweet fern colonies (Rhus sp.), low and highbush blueberries within a quarter mile of any commercial hard (Vaccinium angustifolium and corybosum), and pine plantation. In a report on the susceptibility quaking aspen (Populus tremuloides). of loblolly pine to sweet fern blister rust, J. D. The University of Massachusetts group took Artman and T. N. Reeder (1977) observed that the Connecticut College concept further by sweet fern "may become a major ground cover working out specialized techmques for actually when dry sites are intensively prepared for planting-as opposed to simply managing-the planting." What the authors mean by intensive 10
A few last leaves clmg to the stems of Comptonia peregrina even through the snows of winter. site preparation is, of course, bulldozing before with its pleasing fragrance as with its supposed planting trees. This observation, buried deep medicinal attributes. within a technical report, confirmed once again A second trait of sweet fern, one that catches the intimate relationship between Comptonia the eye rather than the nose, is its tendency to and catastrophic disturbance. hold onto its leaves late into the growing sea- son. Even in the middle of winter one can find a Conclusion few leaves clinging to the stems of the plant. No discussion of Comptoma would be complete Thoreau described this feature in his journal without saying something about its effect entry for January 14, 1860, along with his on the human senses. As the first settlers of response to it: "Those little groves of sweet-fern Concord learned all too well, the scent of still thickly leafed, whose tops now rise above Comptoma on a warm summer’s day can be the snow, are an interesting warm brown-red overwhelming-a thick, resinous pungency that now, like the reddest oak leaves. Even this is an borders on the unpleasant. More spicy than agreeable sight to the walker over snowy fields sweet, the warm scent conjures up the fullness and hillsides. It had a wild and jagged leaf, alter- of summer, which no doubt explains why nately serrated. A warm reddish color revealed Comptonia foliage is often dried for use in by the snow." And finally, in a passage that sachets and potpourris. I suspect, too, that the moves from mundane detail into emotional use of Comptonia as tea by Native Americans description, Thoreau writes of the sweet fern and Europeans may have had as much to do stem, densely covered with fine hairs: "As 11I
nature generally, on the advent of frost, puts (Myncaceae) in culture. Amencan Journal of 64: 476-482. on a russet and tawny dress, so is not man Botany clad more in harmony with nature in the fall in Hyde, L C., J. Troll, and J M. Zak. 1972. Growing a suit or the different hues of Vermont tawny sweet fern in low-fertility soils Amencan gray? I would fain see him glitter like a sweet- Nurseryman 136 (6): 12, 30-36. fern twig between me and the sun" (October 16, 1859). Johnson, E. 1654. Wonderworkmg Promdence of Sion’s Savior in New England. London: Nath. Brooke References Josselyn, J. (1672) 1972. New-Englands Ramties Artman, J. D., and T. N Reeder, Jr 1977 Sweetfern Discovered. Boston: Massachusetts Historical blister rust found in young loblolly pine Society plantations m Maryland and Delaware. Journal 75: 136-138. of Forestry Kalm, P. ( 1770~ 1987. Travels mto North America, ed. A. B. Benson. NY: Dover. Beal, F. E 1911. Food of woodpeckers of the Umted States USDA Bulletin 37. Biological Survey Kenfield, W. G. 1966. The Wild Gardener m the Wild Landscape NY: Hafner. Bowes, B., D. Callaham, and J. G. Torrey 1977. Time- lapse photographic observations of mor- Lmnaeus, C. /1753~ 1957. Species Plantarum. Facsimile phogenesis in root nodules of Comptonia ed., 2 vols. London: Ray Society. peregnna, the sweet fern. Botanical Gazette 137:262-268. Marshall, H. 1785. Arbustrum Americanum, the Amencan Grove Philadelphia: Hafner. Callaham, D , and J. G. Torrey 1977. Prenodule formation and nodule in primary development Nrenng, W. A., and R. H. Goodwin 1974 Creation of roots of Comptonia (Myncaceae). Canadian relatively stable shrublands with herbicides: Journal of Botany 55: 2306-2318. arresting "succession" on nghts-of-way and pastureland. Ecology 55: 784-795 Callaham, D., P. Del Tredici, and G. Torrey. 1978 Isolation and cultivation in vitro of the Schramm, J. R. 1966. Plant colonization studies on root nodulation in actmomycete causing black wastes from anthracite in Science 199: 899-902 mining Comptonia Pennsylvania. Transactions of the Amencan Philosophical Society 56/1/: 1-194. Del Tredici, P. 1977. The buried seeds of Comptonia the sweet fern. Bulletin the peregnna, of Schwintzer, C. 1989. All field-collected actinorhizae Torrey Botamcal Club 104: 270-275. exammed on Comptoma peregnna and Mynca pensylvamca m Mame are spore negative Del Tredici, P., and f G. Torrey. 1976. On the Canadian /ournal of Botany 67: 1460-1464. germination of seeds of Comptonia peregnna, the sweet fern. Botamcal Gazette 137. 262- and J. D. 1990 The of Frankia 268 Tjepkema. Biology and Actmorhizal Plants San Diego: Academic Press. Elias, T. S. 1971. The genera of Myncaceae in the southeastern Umted States. the Journal of Thoreau, H. D. 1962. The fournal D Thoreau, Arnold Arboretum 52: 305-318. of Henry 1837-1861, ed. B. Torrey and F. H. Allen. N.Y.: Dover Publ., repnnt of the 1906 edition. Enchsen-Brown, C. 1979. Medicmal and other uses of North Amencan Toronto: General plants Zobel, R. W., P. Del Tredici, and J. G Torrey. 1976. Publ. Co. Method for growmg plants aeroponically. Plant Physiology 57: 344-346 Fry, J. T 1996 Bartram’s garden catalogue of North Amencan plants. Journal of Garden History 1 G~ 1 1-66
Goforth, P. L , and J. G. Torrey. 1977. The development Peter Del Tredici is Director of Living Collections at the of isolated roots of Comptonia peregnna Arnold Arboretum.