POLLEN AND SPORES — LEOPOLD* AND SCOTT 305 



Although the grains of certain unrelated plants are similar enough to 

 be virtually indistinguishable, this situation is not common enough 

 to be a major problem. Pollen grains of the flowering plants are in 

 general radially or bilaterally symmetrical, although a few asymmetri- 

 cal forms are known. Many pollen grains are basically spheroidal, 

 but modification into various other geometric shapes is common, and 

 flattening as a result of compression is usual in fossil material. The 

 appearance of a single pollen grain may vary depending upon whether 

 it is seen in polar or in equatorial view. The appearance of many 

 pollen grains reflects the presence of pores and/or furrows (colpi), 

 which may function as exits for the pollen tube at germination of the 

 grain. Various combinations of these apertures occur ; three to many 

 furrows and/or pores are common in pollen of dicotyledonous plants, 

 and one-furrowed grains occur frequently in monocotyledonous and 

 gymnospermous plants. Pollen grains of other gymnospermous (co- 

 niferous) plants have elaborate bladders or wings. Some basic 

 structural features of typical pollen grains are illustrated in figure 1. 



Spores of mosses and ferns commonly bear a triradiate tetrad scar, 

 representing the lines of contact of the four spores produced as a 

 result of the two successive divisions of the spore mother cell. Most 

 pollen grains are also produced in tetrads, but with the exception of 

 a few extinct gymnosperms, do not retain the triradiate scar. Spores 

 with a single scar (monolete) and without a scar (alete) also occur. 

 Some examples of the basic shapes of modern spores are shown at the 

 top of figure 1. 



The tremendous variety in wall texture, shape, and configuration 

 provides a reliable basis for the categorization of many isolated spores 

 and pollen grains, either in terms of their natural affinities or into 

 morphological types. Both approaches are utilized by palynologists. 

 Natural affinities must be determined for the interpretation of pa- 

 leoecological and floristic information, though stratigraphic correla- 

 tions can be carried out by the matching of morphological types with 

 little regard for their relationships to the parent plants. 



DISPERSAL OF POLLEN AND SPORES 



Basic to interpretation of a fossil pollen assemblage is an under- 

 standing of the factors affecting the original representation of spores 

 and pollen at the locality. This representation is determined by a 

 complex of factors, including the proportion of wind- and insect- 

 pollinated plants in the contributing vegetation, the total pollen 

 production of individual plants and their relative abundance in the 

 contributing vegetation, and the meteorological and other conditions 

 affecting distance of transport. 



Spores of ferns and mosses are disseminated by wind or water, 

 but pollen is distributed either by the wind, water, insects, or oc- 



