MULDBJERG DWELLING PLACE—TROELS-SMITH 587 
micron being 1/1,000 mm. The fact that pollen from the same species 
is all alike and is more or less different from that of related species 
makes it possible to determine from which plant a given pollen grain 
originated. 
Although it is possible to establish the presence of pollen grains 
in a peat sample just by stirring it in water and placing a drop of the 
liquid under a microscope, it is necessary to apply a careful chemical 
treatment for two reasons. First, the pollen must be concentrated 
sufficiently so that only a few slides need be counted. This is done by 
removing, by various treatments, the extraneous plant remains and 
the possible clay and lime in the samples; the pollen grains are so 
resistant that they can stand treatment with both hydrofluoric acid 
and concentrated sulfuric acid without being destroyed. Next the 
samples are treated chemically to make the pollen grains swell so 
that certain details become more marked. Often it is profitable to add 
some dyestuff which will stain the pollen but not the other components 
of the peat thus facilitating the counting of the grains. 
When the chemical treatment is concluded, microscope slides are 
prepared. A drop is placed on the slide and a thin cover glass is 
fixed on the top of it with molten wax. In this way the slide is 
sealed and can be kept for decades. 
The pollen grains are now ready to be counted. From each 
sample are counted on an average 2,000 to 3,000 pollen grains, which 
usually requires two days by a skilled pollen analyst. The common 
pollen grains, i.e., those of the forest trees and a number of herbs, are 
so characteristic that a determination can be made immediately, but 
now and then pollen grains are encountered which have not been ob- 
served before, and then the difficulties arise. Determination of the 
species of the pollen requires, of course, a thorough knowledge of the 
pollen of contemporary plants. The study of modern pollen, there- 
fore, must be carried on together with the study of fossil pollen. This 
necessitates the preparation of slides of pollen of contemporary 
plants, and the classification of the pollen according to shape, struc- 
ture, and surface sculpture. However, it is often necessary to retain 
the unknown pollen types, so-called X-pollen, not immediately identi- 
fiable, with a description, a drawing, or a photograph for final de- 
termination at a later time. 
By counting the number of pollen grains from the different plant 
species, knowledge is gained of the various trees and herbs which 
were growing in the surroundings of the bog when the peat sample 
was deposited, and the relative abundance of each species. 
SEED ANALYSES 
The peat samples which are taken for investigation of seeds and 
fruits also have to undergo a chemical treatment. They are placed 
536608—60——_41 
