DISPERSAL IN CUP FUNGI 297
The dung-inhabiting — the coprophilous — Discomycetes
seem all to have the same kind of dispersal, as also have
most of the other fungi of the dung flora. The spores
discharged into the air are carried by wind on to the
surrounding grass. This is eaten by herbivores. The spores
pass uninjured through the animal and germinate in the
dung. Here the distance of passive aerial transport is small.
Indeed, some of the coprophilous Ascomycetes have such
powerful spore-guns that the distance of discharge is suffi-
cient to carry the spores to the surrounding herbage without
the assistance of the wind. This is particularly true of
Ascobolus immersus, in which spores are shot to a distance
of 60 cm. This relatively great distance of discharge is
associated with the large size of the spore projectile. The
distance (" d ") to which a minute spherical body discharged
with a given initial velocity is shot is expressed by d = Kr2
where "K" is a constant and "r" the radius of the pro-
jectile. The principal factor limiting the flight of the
projectile is air resistance. The large range of the spore-gun
in Ascobolus immersus is not the result of any greater vio-
lence of discharge, but simply of the large size of the
projectile, in part due to the spores being very big and in
part to the fact that they are bound together by mucilage
into a single mass.
In some other coprophilous Discomycetes, the number of
asci in each apothecium is even more reduced than in species
of Ascobolus. Thus in Thelebolus nanus there is usually
only one ascus formed at a time. This contains 256 spores
forming a single mass which is shot to a distance of about
7 cm.
We have seen that phototropism of the asci in Ascobolus
has been known for a long time. However, in more recent
years our knowledge of this phenomenon in Discomycetes
has been considerably extended by the work of the late
Professor A. H. E. Buller. Buller is one of the great figures
of modern Mycology. By always studying the fungus in
the living state, he has presented a detailed picture of fruit-
bodies as mechanisms concerned with the production and
liberation of spores. His seven volumes of "Researches on
Fungi" (1909-1950), beautifully illustrated, represent one
of the great contributions to mycological literature. Part