SEDIMENTATION IN STILL AIR 



mean diameters of birch pollen on the night flight, A lo (Table III). 

 The size-range recorded by Rempe varied from 23 ^ to 27-5 |U., so it is 

 evident that even at night the sorting effect was not great — a difference in 

 altitude of 1,000 metres was associated with a drop of only 4-5 fi (or i/gth 

 of the mean diameter), with an estimated terminal velocity differing 

 between 1-6 and 2-3 cm. per sec. 



Quite insignificant convection currents may be enough to counteract 

 the terminal velocity of fall of small spores. Falck (1904) believed that the 

 fruit-bodies of the larger fungi generate sufficient heat to induce convection 

 currents which could carry their spores upwards. The temperature of an 

 insulated mass of Polyporiis squamosus rose nearly io°C. in 10 hours, and 

 he regarded parasitism by maggots as a heat-generating adaptation 

 favouring dispersal. BuUer (1909) justly criticized this view, but field 

 experiments are needed to determine whether the pilei of agarics modify 

 air-flow by their own heat, by absorbing solar radiation, or by their 

 shape generating stationary eddies in an air stream. The colours of agarics 

 are usually considered to be functionless, but the presence of dark colours 

 among species inhabiting burnt ground suggests that this character may 

 have been selected during evolution. It is not impossible that both 

 colours and shapes of agaric fruit-bodies are partly adaptive. 



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