368 G. ERDTMAN 



earthquakes, etc., and non-intermittent changes caused by erosion and 

 deposition, etc., also favor certain apocrats. They may invade wide stretches 

 of land and remain for a long time as the dominating element if the area in 

 question — hke the Alvar steppe on the Isle of Oland, Sweden — offers some 

 kind of perpetual instabiUty (in Oland the shallow soil is broken up each 

 year by frost action). Usually, however, they are confined to narrow belts, 

 as, for example, the Sea Buckthorn {Hippophae rhamnoides) in the tension zone 

 between sea and forest along the slowly emerging shores of the Gulf of 

 Bothnia. 



But conditions have changed. In this respect Hippophae provides an 

 instructive example. As the continental ice withdrew from Sweden, this 

 species followed the receding ice-border not only along the coasts but 

 practically all over the country. Fossil Hippophae pollen grains were identified 

 by Halden (1917), but it was von Post (1918) who emphasized the importance 

 of these pollen grains as indicators of open, treeless areas. This, it seems, is 

 the first important contribution to the theme "Palynology and Pleistocene 

 Ecology". 



Surface samples. The present often provides clues for unlocking the 

 secrets of the past. However, attempts to draw historical conclusions from the 

 actual distribution of biota have not always been successful. It is impossible, 

 for instance, to draw any conclusions from the maps of the present-day 

 distribution of Artemisia spp., Centaurea cyanus, chenopodiaceous plants, 

 Ephedra sp., Helianthemum cf. oeJandicum, etc., regarding the former occur- 

 rence of these species in the wake of the receding ice in northwestern Europe. 



On the other hand, it is easy to study, practically all over the globe, the 

 way in which the vegetation is reflected by the "pollen rain" of the present day. 

 The amount and composition of the pollen rain can be ascertained by trapping 

 pollen grains in different ways, including the collection of those that sink in 

 lakes, etc. The composition of pollen rains, the effect of long-distance trans- 

 port, etc., can also be studied by subjecting surface samples from Uving bogs 

 (hchen thaUi, moss tufts, and similar material) to pollen analysis. 



In connection with the theory of pollen analysis, the effect of long-distance 

 transport of pollen grains was first studied by Hesselman (1919). A comparison 

 between the forests and the distribution of the different tree species in a certain 

 area on one hand, and the pollen spectra in surface samples from the same 

 area on the other was made a few years later (Erdtman, 1921). In 1943 a 

 study of different types of pollen spectra was pubhshed, including "habita- 

 tion spectra" particularly rich in pollen grains of "apocrats" (Erdtman, 

 1943). 



Some apocrats produce pollen grains in great profusion, e.g. Ambrosia, 

 Artemisia, Urtica, and certain chenopodiaceous species. In northern Sweden 

 many surface samples contain at least a trace of Artemisia pollen although 

 botanists roaming about in this part of the country may search for days 



