POLLEN DISTRIBUTION AT DIFFERENT ALTITUDES mean diameters of birch pollen on the night flight, A to (Table III). The size-range recorded by Rempe varied from z3µ to 27-5 µ, so it is evident that even at night the sorting effect was not great—a difference in altitude of r,000 metres was associated with a drop of only 4.5µ (or 1/9th 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 Polyporus squamosus rose nearly to°C. in to hours, and he regarded parasitism by maggots as a heat-generating adaptation favouring dispersal. Buller (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.