308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



ences in number and buoyancy usually result in an exaggerated rep- 

 resentation of the wind-pollinated types in the pollen rain in relation 

 to the numerical importance of the parent plants in the source vege- 

 tation. Conversely, insect- and bird-pollinated species, which are 

 particularly important in the arboreal vegetation of tropical regions, 

 may not be well represented in the sedimentary record. 



The distances to which pollen grains may travel vary widely with 

 the nature of the grains, location of the source plants, and weather 

 conditions. For example, light grains tend to travel farther than 

 heavier ones, and pollen produced by plants forming the forest canopy 

 is more favorably situated for long-distance dispersal than pollen 

 originating in the undercover. Anthers typically open during dry, 

 sunny weather when thermal updrafts may be present to raise the 

 pollen to altitudes favoring extended transport. Mixing of pollen 

 grains in the air produces a more or less representative sample of 

 the regional vegetation. The fall of pollen from the air is hastened 

 by such factors as rain, increase in the relative humidity, and decrease 

 in wind velocity. By far the major amount of pollen is deposited 

 in the immediate area of the producing plants, and most pollen is 

 removed from the air within a distance of 50 to 100 kilometers (Faegri 

 and Iversen, 1950). Long-distance transport of single grains for 

 distances of as much as 1,000 kilometers is on record (Erdtman, 

 1954), but these rare occurrences do not appreciably affect the re- 

 liability of a mass sample of pollen. In general, however, the oc- 

 currence of fossil pollen is a less reliable indication that a particular 

 plant grew in the immediate vicinity than is the presence of leaves or 

 other detached parts. 



Within the temperate zone it has been shown that the density of 

 pollen in the air is greatest over the continents and falls off rapidly 

 as one travels out to sea. Erdtman (1954) cites an example in which 

 the pollen content of the air over the coastal plain of eastern Sweden 

 was several thousand times greater than the amount present in the 

 air 200 miles west of the European coast at the same latitude. The 

 density of pollen in the air in an inland mountainous area in Norway 

 has been investigated by Faegri (Faegri and Iversen, 1950), who 

 reports that the tree pollen fallout at collecting stations in the 

 montane forest belt was 13 times greater than the fallout received 

 by stations at and above timberline (fig. 3) . 



Factors affecting the relationship of the pollen rain to the source 

 vegetation have been listed by Kuyl et al. (1955). These authors 

 point out in part that pollen may be retransported after its original 

 deposition but before it is incorporated into sediments. This second- 

 ary transport may be by wind, or if the pollen falls into running 

 water, it may be carried long distances in the stream before final 



