E. V. Wulff — 116 — Historical Plant Geography 



(Stakman el al., 1923) made some interesting observations from air- 

 planes as to tiie number of spores in the upper layers of air. The 

 spores were caught on ordinary microscope slides smeared lightly with 

 vaseline on one side. The slides were exposed for a definite period of 

 time, after which they were returned to the sHde box to be analyzed 

 later in the laboratory. The observations were made in Texas in 192 1 

 in different months of the year, and it was found that up to an alti- 

 tude of 11,000 feet above the surface of the earth the atmosphere con- 

 tains numerous spores and pollen grains, as many as 10 being caught 

 on a slide exposed for 5 minutes, while still higher (observations were 

 made at altitudes as high as 16,500 feet) no more that one or two were 

 caught on a slide exposed for 10 minutes. At low altitudes, say 1,000 

 feet above the surface of the earth, as many as 450 were caught on 

 sHdes exposed for 20 or 30 minutes. 



For flowering plants such slight atmospheric currents cannot be of as 

 much significance, since their seeds, even the very smallest, are too 

 heavy to remain suspended in the air. Stronger air currents are needed 

 for their dispersal, it being understood, of course, that the distance 

 seeds may be transported by wind from the place of their origin de- 

 pends on the velocity of the wind, on the size and weight of the seeds, 

 on whether or not they possess special adaptations for flight, and on 

 the effectiveness of these adaptations. 



The transport by wind not only of seeds but even of small animals 

 and other objects for considerable distances has been observed on 

 numerous occasions. We shall cite a few instances, taken, for the most 

 part, from a review of the literature on this subject made by Beguinot 

 (191 2). VisiANi described the transport by wind into Mesopotamia of 

 considerable quantities of various parts of Lecanora esculenta; Kold- 

 WEG found leaves off the coast of Greenland 15 km. from the shore; 

 SvERDRUP found leaves of cereals on the polar ice at a great distance 

 from any land. Vogler (1901) reports several cases of the transport 

 of leaves into the upper altitudes of the Swiss Alps, considerably 

 higher than the forest line. Thus, beech leaves were found at an alti- 

 tude of 2,490 m., 10 km. from the nearest habitat of beeches, and 

 leaves of Alnus viridis, Sorbus aria, willows, etc. at an altitude of 

 2,585 m., 15-20 km. from the nearest trees. In the Alps on the Franco- 

 Swiss boundary (on the summit of Les Cornettes de Bise) Beauverd 

 observed on the snow at an altitude of 2,439 ^- beech leaves and maple 

 samaras, the nearest habitat of such trees being 4 km. away and about 

 1,000 m. lower. Mirbel reports the wind-transport into Spain of seeds 

 of plants growing on the northern shore of Africa. Wallace tells of 

 large quantities of seeds of some unknown species of Salix being 

 carried into Shanghai by the wind. Warming found on the eastern 

 shore of Jutland faded flowers and fruits of species of Calluna and 

 Erica, blown by the wind across the Cattegat from Sweden. 



Vogler (1901) devoted a special paper to the problem of the 

 significance of wind in the distribution of alpine vegetation. From his 

 statistical study of the flora of Switzerland he found that among true 

 alpine species 59.5 per cent possess adaptations for the wind-dispersal 

 of their seeds or fruits, while among non-alpine species this percentage 

 is only 37.9. Another interesting conclusion made on the basis of the 



