94 J. A. NANNFELDT 



larger. At least five of the six southern races show a distinct affinity to each 

 other and contrast markedly to the four northern which show a similar 

 affinity inter se. Dr. Knaben's conclusion is that each of the ten taxonomically 

 recognized races survived the Last Glaciation in a refuge of its own and that 

 the lesser differences between the local populations have arisen on the spots 

 where they now grow. This theory is very suggestive but sounds perhaps too 

 good to be true in every detail. It is impossible to calculate even roughly how 

 long these two stages of differentiation have taken, but it is now known 

 that in very small populations differences in chromosome structure may arise 

 in a surprisingly short time. Nevertheless, her establishing of the two 

 stages of differentiation is a most important discovery. The only possible 

 explanation to this remarkable fact is that it is connected with the migratory 

 history of the poppies. The additional fact that the northern races group 

 themselves into one group and the southern races (or at least five of the six) 

 into another, is most tempting to try to explain by historical reasons as well. 

 Also the two other species of Scandinavian mountain poppies behave in a 

 similar way, although they are not bicentric but northern. Our knowledge of 

 the three species outside Scandinavia is not as detailed but their behavior 

 there seems to be modified, i.a. by the circumstance that the populations are 

 often much larger and not so isolated from each other. 



Another group showing differentiation within the Scandes is the poly- 

 morphous Poa arctica-comphx, studied by me some 20 years ago (Nannfeldt, 

 1940) from a taxonomic point of view and then submitted to a cytological 

 study by Nygren (1950). This complex is totally absent from the Alps and the 

 other southern mountains of Europe, and in the Scandes it is typically bicentric 

 (Fig. 5). In the southern part there occur three very distinct and uniform 

 taxa, viz. subsp. depauperata, subsp. elongata and subsp. stricta, whereas in 

 the northern part the polymorphy is more continuous, only a minority of the 

 specimens being referable to distinct lower taxa. The explanation of this 

 difference is given by Nygren's cytological studies. One of the southern taxa, 

 subsp. stricta, is viviparous and has a very small area. It has the lowest 

 chromosome number (2/? = 39) known in the whole complex and forms its 

 embryo-sacs sexually. Due to its vivipary no seeds are ever formed and it 

 propagates exclusively by bulbils. Morphology, chromosome number and 

 lack of aposporous embryo-sacs suggest a very isolated and probably very 

 old type. The two other taxa are non-viviparous and propagate thus exclusively 

 by seeds. Their areas are small but much larger than that of subsp. stricta, and 

 one of them, subsp. depauperata, has been found on Iceland by Love (1947), 

 which affords still another example of a close connection between the Scandes 

 and Iceland. Nygren has found that in both the embryo-sac mother-cells always 

 degenerate very early and are substituted by aposporous embryo-sacs in 

 which the egg-cell divides so early that chance fertilization becomes impossible. 

 All embryos are thus formed asexually. 



