LONG-DISTANCE DISPERSAL 



plants. Moreover, none of these lichens inhabited rocks : all were epiphytes 

 which probably arrived on driftwood. Evidently lichens are poorly 

 equipped for wind transport in comparison with Myxomycetes, which were 

 represented by twenty-eight species (7 per cent of the world's list). Only 

 two rusts were recorded (0-02 per cent of those known), but these obligate 

 parasites must needs wait for their flowering-plant hosts. 



QUANTITATIVE STUDIES 



Ample qualitative evidence exists of mass transport of microbes by 

 wind over large distances, but there are few quantitative data. The work 

 of Zogg (1949) on one of the maize rusts in the Upper Rhine Valley is of 

 exceptional interest, both for the distance studied and for the complex 

 topography of the area. 



In that part of Switzerland, Pticcinia sorghi overwinters in its aecidial 

 stage on the wild Oxalis stricta^ which grows in the level area where the 

 Rhine flows into Lake Constance. In early summer, near-by maize 

 becomes infected and a uredospore focus is established from which the 

 fungus is spread by wind up the narrow Rhine Valley. At various dates 

 in the 1945 and 1947 seasons, Zogg measured the incidence of infection 

 for 66 km. (up as far as Chur). Three striking facts about the gradients 

 observed are: (i) the general decrease in the number of uredosori per 

 plant with increasing distance from the source; (2) the flattening of the 

 gradient as a result of secondary spread later in the season; and (3) the 

 great irregularity of the gradient, because the incidence of the rust de- 

 creased locally wherever the valley widens, and increased again where it 

 narrows : a feature which Zogg attributes, no doubt rightly, to the nozzle 

 eflfect of the valley increasing spore concentration, though other ecocH- 

 matic factors may play a part. In the terminology adopted in the present 

 book, both area dose (A.D.) and efficiency of deposition (E) would be 

 increased where the valley narrows. 



The diffusion theory developed in Chapter XIII referred to dispersal 

 over a level plain, and these modifications, imposed on a spore-cloud by 

 dispersal up an alpine valley, are therefore particularly interesting. In 

 comparison with the distance travelled, the focus of infection near Lake 

 Constance w^ould appear as a point source, though some flattening of 

 the gradient would be expected near the source. As the winds blow charac- 

 teristically up or down the valley, it seems appropriate to consider d,^ 

 (deposition downwind of source). Plotting Zogg's data on a log.-log. 

 scale, we find that the linear regression (calculated as log. y = 5-356 

 — I -8 1 8 log. x) is compatible for the slope of d.^ as predicted by our theory 

 (Chapter XIII). We take this to indicate that in the valley diffusion by 

 atmospheric turbulence proceeds much as elsewhere, but that deposition is 

 subject to pronounced fluctuations associated with the width of the valley. 



Observations over similar distances above the sea come from Hessel- 

 man (1919), who traced the transport of tree pollens from the Scandinavian 



185 



