304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



single catkin (e. g., birch ; Erdtman, 1954) . Only minute proportions 

 of this quantity of pollen grains complete their role in the repro- 

 ductive cycle of the plant, the excess being the primary source of the 

 pollen rain incorporated into the sedimentary record. Most plants 

 adapted to pollination by insects produce fewer pollen grains per 

 flower, although some insect-pollinated plants produce enough pollen 

 to be represented regularly in the pollen rain. 



Spores, produced by so-called lower plants ranging from the fungi 

 through the ferns, lycopsids (club mosses), and sphenopsids (horse- 

 tails), may represent different aspects of the life cycle in different 

 groups but have in common their function as a means of dispersal. 

 Some species among the ferns, lycopsids, and sphenopsids are hetero- 

 sporous, producing two kinds of spores differing in function, struc- 

 ture, and usually in size. The female or megaspores are typically 

 large, ranging from about 150 to several hundred microns in max- 

 imum dimension ; the male or microspores are usually smaller, from 

 about 25 to 100 microns in their maximum dimension, and are 

 produced in far greater numbers than megaspores. However, 

 sex, not size, is the fundamental difference between megaspores and 

 microspores. 



Megaspores are usually less abundant and less widely disseminated 

 than microspores. Although they have been described from younger 

 beds (Dijkstra, 1951), megaspores are most important as micro- 

 fossils in the Paleozoic. They were produced in numbers by arbores- 

 cent lycopsid and sphenopsid plants that were important components 

 in the vegetation forming the Carboniferous coals. 



The persistence of pollen and spores in numbers in sedimentary 

 rocks of diverse geological ages is due to the remarkable resistance 

 of their walls to most degradative processes. The walls of pollen 

 grains and spores are composed of a waxlike compound, a chemically 

 undefined polymer of stable, long-chain molecules. This compound, 

 one of the most enduring organic substances found in nature, is 

 resistant to acidic or basic solutions. It is, however, susceptible to 

 oxidation resulting from prolonged exposure to air; consequently, 

 pollen and spores are best preserved when deposited in relatively 

 anaerobic environments. 



The wall of a modern pollen grain is complex structurally, usually 

 consisting of an outer, 2-layered exine and an inner inline. Post- 

 mortem changes result in the degradation of both the contents of the 

 pollen grain and its intine, so that only the exine remains in fossil 

 material. Modern pollen grains can be treated chemically to leave 

 only the exine for comparison with fossil pollen. 



A great diversity of shapes and morphological features is found 

 among the pollen and spores produced by the many kinds of plants. 



