The Magazine of the Arnold Arboretum VOLUME 71 • NUMBER 1 The Magazine of the Arnold Arboretum VOLUME 71 • NUMBER 1 • 2013 CONTENTS Arnoldia (ISSN 0004–2633; USPS 866–100) 2 Mutants in our Midst is published quarterly by the Arnold Arboretum William E. Friedman of Harvard University. Periodicals postage paid at Boston, Massachusetts. 15 Rediscovering Rhododendron Dell, Part 2 Subscriptions are $20.00 per calendar year Kyle Port domestic, $25.00 foreign, payable in advance. Remittances may be made in U.S. dollars, by check drawn on a U.S. bank; by international 26 The World of Mosses money order; or by Visa, Mastercard, or American Stephanie Stuber Express. Send orders, remittances, requests to purchase back issues, change-of-address notices, 36 Chamaecyparis obtusa ‘Chabo-hiba’ and all other subscription-related communica- 877-37: A Venerable Survivor tions to Circulation Manager, Arnoldia, Arnold Arboretum, 125 Arborway, Boston, MA 02130- Peter Del Tredici 3500. Telephone 617.524.1718; fax 617.524.1418; e-mail [email protected] Front cover: Mountain laurel (Kalmia latifolia) nor- Arnold Arboretum members receive a subscrip- mally bears cup-shaped flowers like these, but Arnold tion to Arnoldia as a membership benefit. To Arboretum Director William (Ned) Friedman writes become a member or receive more information, about a fascinating mutant form at the Arboretum please call Wendy Krauss at 617.384.5766 or (starting on page two). Photo by Nancy Rose. email [email protected] Inside front cover: The common name for Polytrichum Postmaster: Send address changes to commune is haircap moss, which refers to the abun- Arnoldia Circulation Manager dant hairs on the calyptra of its showy sporophytes. The Arnold Arboretum Photo by Stephanie Stuber. 125 Arborway Boston, MA 02130–3500 Inside back cover: Part of the Larz Anderson Bonsai Nancy Rose, Editor Collection, Chamaecyparis obtusa ‘Chabo-hiba’ acces- Andy Winther, Designer sion 877-37 arrived in the United States from Japan 100 years ago. Photo by Dave Henderson. Editorial Committee Phyllis Andersen Back cover: Expanding flower buds form a striking Peter Del Tredici pattern on Rhododendron ‘Cynthia’ accession 813-72-B. Michael S. Dosmann Photo by Kyle Port. William (Ned) Friedman Kanchi N. Gandhi Copyright © 2013. The President and Fellows of Harvard College ANT Mutants in our Midst Y R B EOFF William E. Friedman G hat is horticulture? At its core, it is a human celebration, whether con- Wscious or unconscious, of the very fact of evolution. It is thousands of years of detect- ing and rejoicing in the rare: the selection of the novel form that somehow pleases the human aesthetic or serves to feed the world. Although often overlooked, many of the wonderful horti- cultural varieties that grow in botanical gardens (as well as in backyard gardens) are premier examples of the amazing and ongoing process of evolution: random mutations that lead, on the rarest of occasions, to novel and desirable biological characteristics—as opposed to novel and neutral or undesirable characteristics. Charles Darwin was an avid consumer of hor- ticultural literature and information, and was a frequent correspondent with the most eminent horticulturists of the nineteenth century. Over the course of his life, he wrote 55 notes and arti- cles in the Gardeners’ Chronicle and Agricul- tural Gazette, one of the most widely circulated Charles Darwin wrote about many horticultural topics horticultural periodicals of his time. He cov- including variegated pelargoniums, which were very ered everything from how pea and bean flowers popular in the Victorian era. ‘Mrs. Pollack’, seen here, are pollinated (Darwin 1857, 1858, 1866) to the was introduced in 1858. origin of variant forms of roses in cultivation (Darwin 1868). He wrote of his observations “Florists have attended in some instances of and interest in the origin of double-flowered to the leaves of their plant, and have thus produced the most elegant and symmetrical forms (Darwin 1843) and variegated leaves (Dar- patterns of white, red, and green, which, as in win 1844). No horticultural phenomenon was the case of the pelargonium, are sometimes beyond his interest. Indeed, Darwin looked to strictly inherited.” the world of horticulture and plant domestica- ——Charles Darwin, The Variation of Animals tion in order to gain critical insights into the and Plants Under Domestication, 1868 generation of variation and the process of natu- ral selection that underlie evolutionary change. In essence, Darwin was intensely interested in living collection contains wild-collected trees, mutants in our midst. shrubs, and vines, as well as a spectacular set of horticultural varieties whose very presence is EVOLUTION AT THE ARBORETUM the result of human discovery and propagation The Arnold Arboretum of Harvard University of desirable variants. Many of these horticul- hosts a remarkable collection of more than tural varieties are the result of the never-ending 15,000 accessioned woody temperate plants process of spontaneous mutations that occur distributed in over 2,000 different species. This in all organisms and serve to create novel Variety, Form, or Cultivar? HOW TO NAME a variant plant is the topic of some taxonomic debate and often results in multiple versions of the plant’s name. As taxonomic understanding and interpretation changes through the years it often results in changed nomenclature, reflected in the International Code of Nomenclature and the International Code of Nomenclature for Cultivated Plants. A quick reference search finds the white-flowered redbud mentioned in this article listed as Cercis canadensis var. alba, Cercis canaden- sis f.[forma] alba, or Cercis canadensis ‘Alba’(a cultivar name). The same range of synonyms are found for the mutant Kalmia latifolia (var. polypetala, f. polypetala, or ‘Polypetala’) featured later in the article. To add to the confusion, in common usage the words “variety” and “form” are often broadly applied (“I like pink varieties of roses”) or used when referring to a cultivar. For this article, I have used the scientific names as they appear in the Arboretum’s collections database. RBORETUM A ORRIS , M ER Y E W. M W. AUL P Eastern redbud (Cercis canadensis) blooms throughout its canopy, producing a spectacular spring display. traits—the very stuff of evolution. These vari- the parent plant. The source of the biological ant plants, referred to as “sports,” arise in a sin- novelty is random mutation, and subsequent gle generation and have undergone a dramatic vegetative propagation (e.g., grafting, rooting of change in phenotype (the biological properties cuttings, tissue culture) allows the new form to of the organism) from the parent plant and spe- be cloned for further dissemination. cies. Typically, sports are discovered as a single Since arriving at the Arnold Arboretum in branching system on a tree or shrub that dif- January 2011, I have fallen in love with these fers significantly in its morphology, coloration, wonderful horticultural results of random or other biological properties from the rest of genetic mutations and the creation of novelty 4 Arnoldia 71/1 • August 2013 in plants. And in turn, I have come to see the that of the rest of the plant. Such buds can Arboretum (and all botanical gardens) as among be propagated by grafting, &c., and sometimes the best places to actually observe evolution, by seed. These “sports” are extremely rare and importantly, how evolution works. A walk under nature…” in any woodland would indeed expose the ram- Charles Darwin, On the Origin of Species, bler to mutant forms of plants, but most of 1859 these would be so subtle as to evade the senses “Many cases have been recorded of a whole of all but the most acute observer. On the other plant, or single branch, or bud, suddenly pro- hand, a walk through the Arboretum essentially ducing flowers different from the proper type concentrates the opportunity to witness the in colour, form, size, doubleness, or other char- results of evolution—many of our horticul- acter. Half the flower, or a smaller segment, tural gems are representatives of the even rarer sometimes changes colour.” forms of mutations that are dramatic and easily Charles Darwin, The Variation of Animals observable. In this article, I will examine two and Plants under Domestication, 1868 cases of mutants in our midst at the Arboretum. The eastern redbud, Cercis canadensis (pea Each is the result of what is likely to be a single family, Fabaceae), is a widely distributed small genetic mutation that caused a major change in tree species native to the eastern and midwest- the color or morphology of the plant that bears ern United States from Connecticut south to the aberrant copy of the gene. Florida and over to Oklahoma and parts of Texas. Every spring, it can be counted on for THE REDBUD AND THE ORIGIN OF its clusters of pink and magenta flowers that NOVEL FLOWER COLOR appear throughout the leafless canopy just prior “A long list could easily be given of “sporting to the production of new leafy shoots. The plants;” by this term gardeners mean a single Arnold Arboretum has more than twenty acces- bud or offset, which suddenly assumes a new sioned specimens of Cercis canadensis. One of and sometimes very different character from these trees (accession 10-68-B), however, has had something remarkable X A S occur—it has undergone a ILES spontaneous (and random) M mutation that changes the color of the flowers from the normal (“wild- type”) pink and magenta to mostly white. For several decades after its establishment in the Arboretum collections, this specimen produced the characteristic clusters of pink and magenta flow- ers on all of its two-year and older woody branches. However, beginning about ten years ago (see below for details on how this was About a decade ago, a mutation that eliminated most of the synthesis of red pigmenta- determined), one of the tion in flowers occurred on a branch of an Arboretum redbud (Cercis canadensis, acces- branches on this tree began sion 10-68-B ), producing pink-tinged white flowers on that branch.