American Rose Annual 55: 78-85 (1970)
Species Roses in The United States and Their Relation to Modern Roses1
By Dr. Walter H. Lewis2
Missouri Botanical Garden
speech presented to the American Rose Society National Convention, Houston,
Texas, April 19, 1969.
2Dr. Lewis is Director of the Herbarium at the Missouri Botanical Garden and Professor of Botany at Washington University (St. Louis).
There is very little direct relation between our native and our modern roses. Why should this be and what, if anything, might we do about it?
Certainly the first reason for the lack of involvement of native American species in horticultural roses is historic and perhaps even cultural. European horticulturists had their own stocks and, by the end of the 18th century, those of Rosa chinensis and R. gigantea from China. At that time, only four native American species had been described and all were only vaguely known to the taxonomic world. None could compete with the much hybridized and selected stocks from Eurasia. Add to this the appeal of importing from the sophisticated East rather than from the wilds of Indian-infested colonial America and we can understand why these Old World roses were routinely used in plant breeding by the Victorian era. Surely the Queen of Flowers, I could imagine hearing our predecessors say, deserves more than just a minor input from the backwaters of the New World!
Species roses ancestral to the cultivated roses; the number of species in each subdivision is indicated next to its rectangle, the size of which is proportional to the number of species each contains (from Dr. Ann P. Wylie, Master's Memorial Lecture, 1954).
So our involvement has been minor indeed in the evolution of modern roses: no American stock is found in the gallicas, damasks, musks, cabbages, mosses, and dog roses, all of which originated from European species, and the noisettes, bourbons and teas which originated from Oriental-European crosses as did our whole range of hybrid classes: the hybrid chinas, hybrid perpetuals, and hybrid teas. Even our dwarf and hybrid polyanthas and hybrid musks are derived from these old stocks after direct crosses with native Oriental or European species (Fig. 1).
What of our native species? Are they too difficult to find, too difficult to understand taxonomically, too unattractive, or too sterile to come into some use? What really is the situation. Certainly our species are difficult to understand and classify, but not insurmountably so (see Fig. 2 as an example of the great variation in stem covering and thorns for an American species R. stellata) and surely no worse than their Eurasian counterparts. Some find our species reasonably attractive to the eye and by and large they are readily found and brought into cultivation. More of these points later, but first let us look at the sterility question. Generally, our native roses are quite fertile when selfed or crossed with other individuals of the same species and they may be fairly fertile if different species are crossed. This is really quite similar to our experiences with species elsewhere. What must be taken into consideration, however, is the chromosome number of the species and attempt to cross individuals having only the same number for best, surest, and fastest results in plant breeding.
We have many species with two sets of chromosomes, the diploid group (one set with 7 chromosomes and usually expressed as 2n = 14, the diploid number), but only one has been used in breeding. Our prairie rose, R. setigera, a species common in the midwest (Fig. 3), was used to develop a group of climbing roses, the earliest of which was known as the 'Queen of the Prairies' when crossed with a R. gallica hybrid. Unfortunately R. setigera is the only native American rose plagued with sterility problems even though the chromosomes function normally. Other diploid species which could serve as sources of new genes include R. nitida, R. palustris, and R. foliolosa with their natural distributions illustrated by Fig. 4 and also R. blanda, R. woodsii, and R. pisocarpa, each similarly illustrated by Fig. 5. Of all only R. blanda has been used in a limited way in rose work, either as a hardy, often thorniess understock or in crosses with other diploid species. For example, the Japanese R. multiflora x R. blanda produces interesting seedlings, yet these can only be considered novelty byways in no way competing with or as yet adding to the main events in the science of rose breeding.
|Various floral branches for individuals of the R. stellata complex, magnified 4X (from Ann. Missouri Bot. Gard. 52: 103, 1965).|
Turning to the second group of roses with 4 sets of chromosomes, the tetraploids, we have fewer species but still a wide distribution across the continent (Fig. 6). Each is complicated taxonomically and is what we call a plastic species, but each has attractive forms that might be directly adaptable to the garden, for example, the double-flowered mutant of R. arkansana. But again this approach of introducing such novelties into the trade does little to incorporate certain fine characteristics of these roses into the main stream.
Still a third group having 6 sets of chromosomes is known in North America, the hexaploids. There are but two species (their distributions in Fig. 7). Both have been used in breeding programs with some success. 'Cantab' for instance is from a cross of the west coast R. nutkana, a vigorous species with large pink petals and large red hips, with the H.T. 'Red Letter Day.' And there are other such hybrids, each of particular interest to the sophisticated rose grower or to the grower with special needs.
What we have on the continent, then, are three major groups of native roses plus a minor octoploid variant (with 8 chromosome sets) of the hexaploids. Looking at their total ranges (Fig. 8) we find that the hexaploids and octoploid are largely boreal in distribution, the many diploids rather widely ranging though not so northern in distribution, and the tetraploids more nearly restricted to the United States.
If we assume that the plants are available for study, that they are desirable as roses, and that they have no complex sterility barriers, then we should also assume that there is need for a new surge of wide-cross breeding programs concentrating on species and modern roses with the same chromosome number. Since the majority of our modern H.T's, H.P.'s, and H. Polyanthas are tetraploids it only seems logical to choose -native tetraploids rather than diploids or hexaploids. Chances of sterility are far less and long periods of selection to tetraploidize our plants (as was true for the original H.T.) is unnecessary. So let us turn to our 4 native tetraploid species. Do they have desirable qualities worthy of incorporation into modern roses?
Fig. 3 - Pressed specimen of R. setigera in the Missouri Bot. Gdn. Herbarium.
Fig. 4 - Distributions of R. nitida, R. palustris, and R. foliolosa.
Fig. 5 - Distributions of R. blanda, R. woodsii, and R. pisocarpa.
Fig. 6 - Distributions of R. virginiana, R. carolina, R. arkansana, and R. californica.
|Figure 7 - R. virginiana from the author's collection at the Blandy Experimental Farm, Univ. of Virginia, Boyce (photo by Alan Casper, Acting Director of the Farm).|
Fig. 8 - Distributions of R. acicularis and R. nutkana.
Fig. 9 - Distributions of species according to their chromosome number as diploids, tetraploids, hexaploids, or rarely as an octoploid.
My attention would focus on the typical form of R. virginiana which is a hardy species, having a compact, small shrubby habit, glossy somewhat disease resistant foliage, and deep rose petals. The fragrance is exquisite. The species is common from New England to Newfoundland and less frequently as far south as Delaware; it transplants readily and is highly fertile.
Why not start breeding with such an attractive American rosesee what would result after crossing it with hybrid teas, hybrid polyanthas and others? It is clear that those in the past who have attempted to use new species in developing garden varieties have rarely had immediate encouragement, but then our knowledge today of cytogenetics and of plant breeding techniques is surely greater than even a generation ago. Much time will be saved, for example, by choosing a tetraploid like R. virginiana than in seeking out a diploid or hexaploid or willy nilly any plant perhaps of unknown chromosome number and attempting to cross these with our modern roses.
The time is now to start a sensibly planned, long term program which actually sets out to explore the possibilities of our American species in modern rose breeding and to at long last incorporate some germplasm from this continent into the Queen of Flowers. If someone will accept the challenge of this project, a half a century from now we shall be able to observe with joy what North American roses have contributed to horticulture. Perhaps a whole new class of roses will emerge, but regardless of this outcome, who could reject our R. virginiana (Fig. 7).