338 PLANT SOCIOLOGY 



Thus the proportions of the dominant tree species for an appreciable 

 distance around the moor can be shown with certainty.^ The quantity 

 of pollen produced by different trees, its capacity for distribution by the 

 wind, and its degree of preservation in the peat are factors that are 

 sure to influence results of pollen analysis. Some of these factors are 

 decidedly difficult of evaluation. Pines and hazels produce large 

 amounts of pollen, whereas Ulmus and Tilia produce relatively little; 

 moreover, some pollens, such as those of Fraxinus, Prunus, and Juni- 

 perus, are not well preserved. But after considering all the possible 

 sources of error, pollen analyses yield a rather convincing picture of tree 

 life for a considerable distance around the locality of the peat bogs 

 studied. 



Pollen Statistics. Pollen Diagrams. — The current methods of 

 pollen analysis go back to Lagerheim (1902) and have been described 

 by Von Post, Erdtman, Rudolph and Firbas, Stark, Keller, Furrer, 

 Bertsch, Liidi, and others. Erdtman (1927, 1930) has given a very 

 complete bibliography of the literature, and Rudolph (1931) has 

 compiled a good summary of the results which apply to the forests of 

 middle Europe. 



The sampling of the peat in the bog is accomplished by a peat 

 borer. Several types have been used, but one of the best is the Hiller 

 peat auger, developed in Sweden. It has a closed chamber that may 

 be opened at the exact depth at which a sample is desired, and with it 

 clean samples may be obtained to a depth of 8 to 12 m. The samples 

 are placed at once in numbered glass tubes, about 7.5 cm. long and 1.2 

 cm. in diameter, corked at both ends. For the pollen count a small 

 sample (1 cc.) is taken and boiled in 10 per cent caustic potash for a few 

 minutes and a portion placed on a glass slide, the surplus water evap- 

 orated, and a few drops of glycerin added. For counting the pollen a 

 magnification of 250 to 500 diameters is necessary, and a microscope 

 equipped with a mechanical stage greatly facilitates the labor. Trust- 

 worthy percentages are obtained if about 150 pollen grains are counted. 

 The results are expressed graphically in the pollen diagrams (Erdtman, 

 1931). 



Pollen Analysis and the Development of Vegetation. — Pollen analysis 

 is obviously incapable of giving more than a crude outhne of succession 

 of forest trees and their relative abundance during the formation of the 

 peat bed. One can hardly, therefore, discuss the development of 

 vegetation, especially as the pollen is not only autochthonous (depos- 

 ited in the place of its origin) but also allochthonous (brought together) 



1 KuDRjASCHEW (Ber. Wiss. Meeresinst. 12: 192.5) has proved the presence of 

 pollen of Abies, Picea, Alnus, and Tilia on the arctic island of Nowaja Senilja. 



