F.C. Stern A Study of the Genus Paeonia

DISCUSSION OF THE GEOGRAPHICAL DISTRIBUTION

On reviewing the geographical distribution of the members of the genus, the cytology of the species provides some -interesting indications of the relationships of the different species to one another. It has not been possible to obtain the chromosome numbers of all the species, but sufficient are now known to make their study suggestive.

Maps a-z

In North America two species, both herbaceous and both diploids, are unlike other herbaceous species and have more in common with shrubby species of the Delavayi group from the mountains of western China, which are also diploids. Both the American and these Chinese shrubby species have in common fleshy discs, comparatively small petals and cylindrical seed. The other shrubby species from the mountains of western China, the Suffruticosa group, are also all diploids. The districts of central and eastern Asia are rich in paeonies ; besides the shrubby species which are only found in the high mountains of south-eastern Asia, the herbaceous species are represented by the Lactiflora, Mairei, Obovata, and Anomala groups. All the species which have been examined in these groups are diploids, with the exception of P.obovata and its variety Willmottiae.

An interesting point is that all the species which have more than one flower to a stem, P.lactiflora, P.emodi, P.Veitchii and their varieties, are diploids.

The Obovata group illustrates a phenomenon which occurs in five other groups, namely that one of the species is a diploid and the remainder of the species in the group are tetraploids ; in each case the species are closely related morphologically and are found in the same geographical area.

In this group P.japonica, a diploid, is possibly confined to Japan, but the tetraploid P.obovata and its variety extend across eastern Siberia, Manchuria, N. China, Korea and perhaps Japan. The tetraploids are larger and stronger growing plants than the diploid.

The same phenomenon occurs in the Wittmanniana group, the yellow-flowered paeonies of the Caucasus region. In this group P.Mlokosewitschi, which is confined to one small district, is a diploid, while the tetraploids, P.Wittmanniana and its two varieties, are more widely distributed.

In southern Europe and the islands of the Mediterranean there are four further groups of paeonies in which the same phenomenon occurs. The Officinalis group, which extends from Crete along the Mediterranean coast to the centre of Spain, contains one diploid, P.Clusii, confined to Crete and neighbouring island, yet the tetraploid, P.officinalis, has a wide distribution. The chromosome numbers of P.humilis and its variety are not known.

In the Russi group, the species of which are indigenous to the islands of the western Mediterranean, the diploid P.Cambessedesii is confined to the Balearic Islands, while P.Russi and its varieties, which are tetraploids, extend over Corsica, Sardinia and Sicily.

The newly described species from the Island of Rhodes, P.rhodia, is a diploid and has much in common with the species in the Arietina group, P.arietina, a tetraploid, and P.arietina var. orientalis (chromosomes not known). P.rhodia has only so far been reported from the Island of Rhodes, but P.arietina has a wide distribution from Bosnia eastwards across Asia Minor to Armenia, while P.arietina var. orientalis is mainly found in Syria and also on the islands of Cyprus and Crete.

In the remainder of Europe and in Asia Minor there are a number of species which are solitary, not closely connected with other species, although the tetraploid P.mascula from central Europe, and the diploid P.daurica from the Crimea and surrounding districts, have many characters [end page 33] in common and have here been put in the same grouP.These species, however, are not quite so closely connected as the members of the other groups mentioned above.

P.banatica, from the Banat, south of Hungary, might be the connecting link between P.mascula and P.arietina, a tetraploid which extends from Bosnia to Syria. Although P.mascula and P.arietina have no characters in common with each other, P.banatica shares some characters with both these species.

There are two paeonies in south-eastern Europe, the Balkan species P.peregrina, a tetraploid, and P.tenuifolia, a diploid, which are solitary and not closely connected with any other species. The same thing occurs in south-western Europe and North Africa, the diploid P.Broteri, confined to southern Spain and Portugal, is unlike any other species and is in no way connected with the tetraploid, P.coriacea, from southern Spain and North Africa.

The cytological aspect of the species of the genus may be shown as follows :—

1. The species indigenous to America are diploids.

2. The Asiatic species (other than those in the Caucasus and Asia Minor) include nine diploids with five diploid varieties or subspecies, and one tetraploid with one tetraploid variety or subspecies.

3. The species in Europe with North Africa, Asia Minor and the Caucasus include seven diploids and ten tetraploids with four tetraploid varieties or subspecies.

The accompanying tables show how the different species, grouped together by their morphological characters have, where there are tetraploids and diploids in the group, one diploid species and one or more tetraploid species in each grouP.In these groups the diploid and allied tetraploid species do not always occupy the same territory but are in the same geographical area. Where there are both tetraploid and diploid species the diploids have the more restricted range. P.Clusii, P.Cambessedesii, P.rhodia and P.japonica are confined to islands, P.daurica to the Crimea and nearby districts and P.Mlokosewitschi to one small district in the central Caucasus. The tetraploid species, on the other hand, have a wider range. This same phenomenon is discussed by Anderson and Sax (1936) in reviewing the Tradescantia species allied to T. virginiana in North America ; they show that the diploid species have in each case a comparatively narrow range and the tetraploids have a very wide distribution ; they calculate that the average range of the diploids is about 83,000 square miles while that of the tetraploids is about 376,000 square miles.

In Europe the diploids P.Broteri and P.tenuifolia have no tetraploid species allied to them and the tetraploids P.coriacea and P.peregrina have no allied diploid species. In the U.S.A. and in Asia all the groups except one (the Obovata group already mentioned) are composed entirely of diploid species. The distribution of these diploid species is comparatively restricted except in the case of P.anomala and P.anomala var. intermedia which have a wide distribution from Lapland to Chinese Turkestan.

It is difficult to draw any definite conclusion from these facts, but it may be permissible to speculate on a possible explanation of them.

Manton (1934), in discussing " the problem of Biscutella laevigata " and the distribution of the species in central Europe, says that the diploid and tetraploid forms do not occupy the same territory and in few places do the localities even abut; the tetraploid forms have spread into an area which was covered largely by the Alpine Ice Sheet while the diploid forms are practically confined to unglaciated regions ; it is suggested that the diploid forms are interglacial if not [end page 34] preglacial relics, survivors of the preglacial period ; the tetraploid forms occur in nearly the whole area occupied by the Alpine Ice Sheet during the glacial period, therefore these tetraploids must be post-glacial immigrants to many of their present districts and may still be spreading. The diploid forms give little evidence of expansion of territory and show a highly disrupted " Areal " centring round river systems known to be favourable climatically and to be rich in " relicts " of various kinds.

This explanation may be applicable to the distribution of paeony species to explain the reasons why the tetraploid species in Europe are so much more numerous than the diploid species and why in Asia and America the diploid species so greatly outnumber the tetraploids.

The suggestion is that in Europe in the glacial period the diploid species, which are presupposed to be pre-glacial relics, were pushed down on to the islands and into other warm outlying districts south of the glacial area. The tetraploid species which are presupposed to have arisen from the diploid ones proved themselves better adapted to the post-glacial conditions and have spread more rapidly than the diploids owing to their more vigorous habits. The European tetraploid species in cultivation are stronger growing plants and increase faster than the European diploid species. The tetraploid species have been responsible for the post-glacial migrations northwards ; it was not possible for them to spread south except to the mountains of Morocco and Algeria, because of the Mediterranean and the desert conditions of the rest of North Africa. This hypothesis would explain how groups of paeonies which are closely allied morphologically are found in the same geographical areas in Europe with one diploid species confined to a limited area and one or more tetraploid species with a much larger distribution; for instance, P.Cambessedesii is restricted to the Balearic Islands and the other species and varieties of the Russi group cover the larger area of the islands of Corsica, Sardinia and Sicily. Similarly, the diploid P.Clusii is restricted to the island of Crete and neighbouring island, while the other species of the Officinalis group cover the maritime districts of Albania and Trieste, through northern Italy, southern France and central Spain. Also P.rhodia is restricted to the Island of Rhodes, while the other species of the Arietina group cover the whole area from Bosnia across Asia Minor to Syria. Again, the diploid P.Mlokosewitschi is restricted to one small district in the central Caucasus and the other species and varieties of the Wittmanniana group cover the whole of the Caucasus range, even stretching down to the mountains of northern Persia. The same phenomenon occurs in eastern Asia with the Obovata group ; the diploid species P.japonica is only found in Japan, but P.obovata and its variety are found over a wider area on the mainland of eastern Asia.

The remainder of the paeony species in Asia and western North America are diploids, some covering large areas and some small. It is supposed that these species have not been displaced during the glacial period but survived in areas not covered by the glacial sheet. The diploid species from the mainland in Asia are strong growing plants in cultivation and increase rapidly.

In Europe there are two diploid species—P.Broteri and P.tenuifolia—which have no allied tetraploid species ; there are also two tetraploid species which have no allied diploid species. From these two diploids it would appear that no tetraploid forms have arisen while the tetraploid species may have arisen from diploid forms which have died out. This hypothesis is put forward as a speculative suggestion to explain the main features of the geographical distribution of the genus.[end page 035]