MORTON ARBORETUM BULLETIN POPULAR INFORMATION

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1 Vol. II No. 8 MORTON ARBORETUM JOY MORTON, Founder BULLETIN OF POPULAR INFORMATION By H. TEUSCHER, Botanist LISLE, ILLINOIS Sept. 12, 1927 Subscription $1 a year, loc a copy. Copyrighted by Morton Arboretum WHAT IS A SPECIES? This is a question which is embarrassing to every botanist. Most probably he will answer with a sigh: "I wished I knew." I hardly.need to emphasize that I do not mean this as a reproach to anybody, and I hope that it will not be too shocking to the amateur botanist who seldom is aware of the difficulties encountered by the scientist in his attempt to classify living beings into genera, species, varieties, etc. Gardeners and plant-lovers, whom in the main I have in mind when speaking of amateur botanists, are prone to nurse the belief that a plant-species is a very definite thing, and they curse the botanist for "wilfully" and "violently" changing around the names of plants which they had just managed to remember. Of course the changing o:fi names is regrettable. Nobody will deny that. And names,should not be changed if it can be avoided. But the point is that so very often the change can not be avoided. Our knowledge of most plants is still incomplete, and to a large extent will probably always remain incomplete Jike all human knowledge. A change in the name of a plant generally signifies a step forward in knowledge, it means that some additional data concerning that particular plant have been assembled. A new name is first applied to a plant which has been found to vary in certain prominent characters from other closely related plants. According to the prominence of the distinguishing characters we give the new plant the rank of a species or only of a variety. But variations may be occasioned by numerous external or inter.nal factors, and only intensive study of a plant in the field as well as under cultivation can enable us, often, to decide definitely "how good" a supposed species or v.ariety is. As long as we do not know this, maybe for many generations, the name serves at least to indicate that a variation has been observed which so far seems to be stable and 29

2 important. Further observation of the plant, especially in cultivation, and the due consideration of the physiological factors which may have caused the variation in structure, may force us to alter the place given to the plant in our system of classification, which, of course, will necessitate a change of the name. This is the kind of change of name which can not be avoided and the reason why plant names have to remain flexible and can not be standardized. What gives me occasion for this general outline of the difficulties encou:ptered by the botanist in the distinguishing and naming of species is an observation made in the collections of the Morton Arboretum which, I believe, is of general interest, since it concerns a very widely cultivated ornamental shrub. Aronia, the Choke-berry (not to be confused with the Chokecherry, which is a Prunus). There are two species of Aronia which 'interest us for their ornamental value: Aronia melanocarpa with black fruits which ripen in August and drop early. The glabrous leaves are usually lustrous above and do not develop a conspicuous fall coloring. The ~pecies usually grows in swamps, though occasionally also in dry uplands. Aronia arbutifollia with bright red berries which ripen in September. Usually the berries of this species are still green when those of A. melanocarpa are falling, but they remain on the bush until late.into the winter. The dull green leaves which are densely grayish pubescent beneath color brilliant scarlet in fall. The species usually grows in swamps and lowlands. So far so good. These two species are very distinct and can not be mistaken for one another. But there is a third species, if it is a species, which causes a great deal of confusion. Aronia atropurpurea. This is in all its characters well intermediate between the two others. Its fruits are neither red nor black, but purplish black. Its leaves and branches are quite densely pubescent when young, but get more or less glabrous when mature. It has all the appearance of a natural hybrid, but: In the first place, it breeds true from seeds. A hybrid is supposed to "split up," as the breeder says, and produce in its progeny a certain percentage of plants which resemble the grandparents, or at least one of them. The fact that it apparently does not "split up" is no proof, though, that it is not of hybrid origin. For that matter it could be constant through the dominance of certain characters in a certain combination. It could also be that it does split after all, but that the splitting has not been observed, because it has never been raised in quantity under scientific control. However that may be, there is another "but" which seems to be of a more serious nature. In its native haunts A. atropurpurea hardly ever is found near either of its supposed parents. The writer saw it himself in 30

3 ARONIA MELANOCARP A ELLIOTT VAR. GRANDIFOLIA SCHNEIDER The leaves of this variety are not only larger but also more lustrous above than those of the type. The whole shrub is of more vigorous growth and is, on account of its very good summer-foliage, of great ornamental value. The berries which are easily twice as large as those of the species, have a much milder taste, much less astringent, which suggests the possibility of developing a strain with sweet, edible fruits. It would undoubtedly add further to the garden value of this shrub if this could be accomplished.

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5 the White Mountains, New Hampshire, growing on exposed rocky ledges at about 3,000 feet elevation. Neither of the other two species occurred anywhere in the vicinity for several miles. Well, that settles it, most people will say, it is a true species. But that is not all yet. There exists a hybrid between the ~wo species which first was observed in Europe, where it originated in cultivation. It usually travels there under the name Aronia floribunda, and two varieties of it were distinguished: one which resembles more closely A. arbutifolia was naqied 'Var. typica. It is of more vigorous growth than either of the parents and has the densely pubescent leaves and branches of A. arbutifolia, but the fruits are purplish black. The leaves color very beautifully in fall. The other variety which is well intermediate between the parents was named var. glabrescens, because its leaves become glabrous when mature like those of A. melanocarpa. This latter variety is, as far as I can see, indistinguishable from the native American A. atropurpurea. If the two are planted together, it is quite impossible to say which is which. European botanists did not hesitate to identify A. atropurpurea with the hybrid A. floribunda. When speaking of A. floribunda they will simply state: This hybrid has also been found wild, growing naturally within the range of distribution of the two species. Of course, they usually have not observed it under natural conditions. American botanists, on the other hand, refuse to this day to accept hybrid origin for A. atropurpurea. They either do not mention the hybrid at all, or will say something like: There may exist hybrid forms between these two species which can not be distinguished from A. atropurpurea, but A. atropurpurea is a species. Now, where is the truth? It seems, as if the hybrid has never been recognized in American nurseries, but it occurs and perhaps just as frequently as in European nurseries. The Morton Arboretum has proof of that. Some six years ago we bought from an Eastern nursery 100 plants of Aronia melanocarpa. That is, we bought the plants under that name. What we got was an excellent line of the two forms of the hybrid. The plants were supposed to have been raised from seeds of A. melanocarpa. That could not be checked any more, but whatever their origin, the fact remains that here are the two very distinct varieties of the hybrid, agreeing perfectly in every character with the descriptions given in European literature. The var. glabrescens is almost entirely identical with A. atropurpurea, of which we have some wild collected plants: The two do not only look alike in general appearance and in leaf and branch characters, but they ripen their fruits at the same time-that is, just between. the two other species-their fruits are of the same size, shape, and color; they color their leaves in the same way in fall and drop them at the same time. 31

6 Whatever conclusions can be drawn from the above-outlined observations, which were not based on experiments, it seems to be beyond doubt that it is possible to reproduce experimentally the supposed species A. atropurpurea by cross-fertilizing the two other species. Now, if that were accomplished, would that be a proof that A. atropurpurea is of hybrid origin? Not necessarily, I believe. Why could not by some joke of nature an intermediate form originate as a mutant from one species which resembles a hybrid but is none? Should it be possible now to prove that, perhaps through microscopical investigation, what is the poor taxonomist to do? Shall he keep two plants separate under two different names which in outward appearance are exactly alike? Or shall he combine under one name two plants which in origin are fundamentally different? This illustrates, I believe, more plainly than anything else how far we still are from knowing what is a species. SOME PRACTICAL HINTS CONCERNING THE ARONIAS For those of my readers who want to realize some practical good out of all this more or less technical scramble, I have the following advice: Nurserymen: Concentrate on the cultivation of the two good species. Aronia melanocarpa with black berries and glabrous, lustrous leaves; and Aronia arbutifolia with bright red berries which ripen late, and densely pubescent leaves which color scarlet in fall. Throw out any intermediate forms which may turn up, because they are not worth cultivating. Of A. melanocarpa the var. grandifolia with larger leaves and very large berries, and of A. arbutifolia the var. macrocarpa with larger berries can be recommended most highly, as superior to the species. To the garden friend I wish to say : Plant A. melanocarpa mainly for its very good summer foliage, and remember for any color-scheme you may have in mind, that its fruits are deep black and ripen late in August. It shows no conspicuous fall coloring of the leaves. Plant A. arbutifolia in particular for its bright red berries which ripen usually not before the middle of September. and after those of A. melanocarpa have dropped. The fruits of A. arbutifolia remain on the shrub until late into the winter. Plant it, further, for the brilliant scarlet fall-coloring of the leaves. White and dark-lilac Michaelmas Daisies, Chrysanthemum uliginosum (Pyrethrum uliginosum, the Giant Daisy) or hardy Chrysanthemums in yellow and bronze are very excellent neighbors for this beautiful shrub. Both species of Aronia have to be planted in the full sun for best development. They do not by any means need to be set in swampy or moist ground. They do well in any ordinary garden.soil, but it is advisable to guard them against drought. They can stand considerable drought, if it has to be, but will get straggly then and will not be at their best. 32