372 G. ERDTMAN 



cyanus, and Ephedra in Late Glacial deposits; of Artemisia, Chenopodiaceae 

 and Cruciferae side by side with Hippophae in Oland, Sweden, and in the 

 interior of southern Lapland just at or after the disappearance of the last 

 remnants of dead ice). Compare also the finding in the New Siberian Islands 

 of pollen grains of Artemisia and chenopodiaceous plants in layers which, it 

 seems, were laid down during a particularly apocratic regime. In our days they 

 no longer live in these places (cf. Gorodkov, 1954). 



What is needed are more detailed investigations carried out with refined 

 techniques and sound judgement, without exaggeration of the duration and 

 importance of open ground conditions and so forth. 



Pollen morphology, cytotaxonomy, and advanced microscopical techniques. 

 In Scandinavia the number of tree species is quite suitable with regard to 

 pollen statistics. In the U.S.A. the situation is often more complicated owing 

 to the great number of tree species. In Iceland conditions are not ideal either 

 because of the low number of species. Yet it is possible (cf., for example, 

 Praglowski, 1962) to distinguish pollen grains of Be tula nana from those of the 

 tree birches (B. pubescens and B. tortuosa), and — if the grains are typical and 

 well preserved — often also between the grains of the two latter species. 

 Hybridism — hitherto not adequately studied from a palynological point of 

 view — will no doubt make the matter more complicated. Attention should be 

 drawn to the importance of suitable embedding media, adequate optical 

 facilities, etc. (Berglund, Erdtman and Praglowski, 1959). Electron micro- 

 graphs exhibiting the fine rehef of the exine surface (PI. V, 1) are often very 

 instructive and helpful also to those working with an ordinary fight micro- 

 scope. They demonstrate sharply and precisely some of the features which 

 can be seen only dimly by means of the latter. Thus, the white dots and streaks, 

 e.g. in PI. I, 1, 2, 5, 7 and in PI. II, 1, 2, 4, 9 and 10, represent small spinules 

 and ridges of more or less the same shape as those shown in PI. V, 1 and in 

 some places in the UV micrograph PI. V, 2. 



Plates I-IV have no direct bearing on Icelandic Pleistocene problems. 

 They have been inserted, however, as examples of what might be done in the 

 way of making better and safer specific determinations based on pollen 

 grains. Thus, tetraploid pollen grains of Alnus glutinosa (PI. I) are con- 

 siderably larger than those of diploid specimens and usually also slightly 

 different morphologically (cf. the ringfike "arcus" the center of 9). "Tetraploid" 

 grains have, so far, only been noticed once in Swedish Post-glacial deposits 

 (unpubl.). They are of much the same shape as fossil grains described from 

 the Early Tertiary volcanic districts in Scotland (Simpson, 1961). Detached 

 "arcus" are also found in Alnus sieboldiana (Simpson, loc. cit.). The apertures 

 in A. glutinosa are, as a rule, more narrow than those in A. incana (PI. II). 

 In Sweden the Quercus robur-Q. petraea problem (cf. Pis. Ill and IV) has 

 not yet been tackled with pofien statistics. Investigations are, however, being 

 carried out in order to determine whether some of the features shown in the 



