C. H. Ostenfeld. 
114 
clean films from a surface, several preliminary casts should be taken 
and discarded, all the loose material will be picked off with the 
earlier films, and the surface left quite clean. 
It seems probable that a renewed examination of fossil plants 
by some of the methods just described will result in great additions 
to our knowledge of their structure and affinities. The elaboration 
of these and similar methods is giving a new impetus to the study 
of what has been hitherto regarded as very unpromising material 
from a botanical standpoint. Most of the methods described in 
these notes were devised or improved by Professor Nathorst of 
Stockholm, who has done so much for the study of Jurassic plants 
and to whom the thanks of the author is due. 
THE INTERNATIONAL PHYTOGEOGRAPHICAL 
EXCURSION IN THE BRITISH ISLES. 
VI— Some Remarks on the Floristic Results of 
the Excursion. 
By C. H. Ostenfeld. 
Owing to the excellent manner in which Mr. Tansley had 
arranged the conditions for preparing herbarium plants during our 
excursion, it was possible, in spite of the short time, to make 
ample collections at all points of our extensive trip. To me it was 
of special interest to get as many British plants as possible, as I 
was anxious to compare them with more northern and with Danish 
plants. I devoted special attention to the forms of the more 
critical species, and have brought home a large number of such 
plants. My leading idea in so doing is that a study of the geogra¬ 
phical distribution of the “ elementary ” species or forms may be 
of value with regard to the interpretation of the immigration of the 
postglacial flora and the paths which this may have followed. As 
illustrations of my point of view the two races of Helianthemum 
nummularium, viz., var. hirsutum and var. tomentosum may be 
cited. In Denmark we have the former almost alone, while in the 
British Isles only the latter is found ; this means that one variety 
has travelled in a western direction, the other in a northern, 
since both occur in Central Europe, Very much the same is the 
case with the two varieties of Lamlum galeobdolou, of which var. 
