22 



revert altogether to the type of another species. In an extreme case, it has even 

 been reported that from one and the same seedpod were raised under cultiva- 

 tion several forms which from wild collected specimens had been described as 

 separate species. Such regretable mistakes can be avoided only if the study 

 of plants under cultivation is given more serious consideration by the tax- 

 onomist. On the other hand, differences appear under cultivation which can 

 not be observed under natural conditions, and for which, though they are of 

 the greatest importance to the horticulturist, the taxonomist does not even 

 offer a term, for instance, variation in hardiness. This variation as shown by 

 the famous hardy race of the Lebanon Cedar at the Arnold Arboretum, and 

 as I observed, also by Cornus kousa, Cryptomeria japonica, and various other 

 woody plants, remains constant in the offspring, yet it can not be expressed 

 by a name. The inheritable growth habits of trees is another rock on which 

 botanical nomenclature splits. At the Forestry Experiment Station, Wagen- 

 inger, Holland, I was very much impressed by a beautiful set of races of Quercus 

 robur and Fagus sylvatica. One of these races always developed more than one 

 leader which, of course, made it worthless for forestation purposes. Another 

 always grew as straight as a candle, and a third, again, always grew crooked 

 and scrubby. These peculiarities of growth were invariably passed on to the 

 respective offspring as they had been through several generations. Such differ- 

 ences in hardiness and growth habit which can be relied upon to remain con- 

 stant, are of much greater importance to the horticulturist than many of the 

 characteristics which botanists consider sufficient for the separation of species, 

 yet there are no taxonomic terms by which they can be distinguished. 



All these rather well known facts I mention merely to give my point more 

 carrying power. What I wanted to present to you in particular is an observa- 

 tion which I made at the Morton Arboretum, Illinois, and which, I believe, 

 will prove of interest to you. To me it brought home more clearly than any- 

 thing else, how difficult it is to define the term "species." This observation 

 concerns a very widely cultivated, ornamental shurb, the Choke-berry, Aronia. 



There are two species of Aronia which interest the horticulturist for their 

 ornamental value: 



First, Aronia melanocarpa with black fruits which ripen in August and 

 drop early. Its glabrous leaves are usually lustrous above and do not develop 

 a conspicuous fall-coloring. This species usually grows in swamps, though oc- 

 casionally in dry uplands. 



Second, Aronia arbutifolia with bright red berries which ripen in Septem- 

 ber. Usually the berries of this species are still green when those of A. melano- 

 carpa are falling, and they remain on the bush until late in the winter. The dull 

 green leaves, which are densely grayish pubescent beneath, color brilliant 

 scarlet in the fall. This species usually grows in swamps and lowlands. 



So far so good. These two species are very distinct and can not be mis- 

 taken 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 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, 



