DoRAN: ON THE GERMINATION OF FUNGOUS SPORES 327 
of temperature. Johnson (26) and Melhus and Durrell (7) 
preferred to use the growth or length of the germ tubes as an 
indicator of the temperature reaction. To test the relative 
merits of these indicators, both were used by the writer in the 
case of germinating conidia of Venturia inaequalis. When the 
optimum temperature for germination was based upon length 
of germ tube rather than upon number of spores germinating 
the optimum was less sharply defined. The writer believes that 
to consider length of germ tube rather than number of spores 
germinating involves growth rather than germination and so 
should be studied separately, for there is no reason to believe 
that the optimum temperature for the growth of a fungus is the 
same as the optimum temperatures for spore germination. 
Melhus (22) found the optimum temperature for the germination 
of the conidia of Phytophthora infestans to be 12°-13° C. Jones, 
Giddings and Lutman (30) found that the optimum temperature 
for the growth of this fungus is between 16° and 19° C. The 
cardinal temperatures for the germination of the conidia of 
Botrytis cinerea were found to be minimum 7° C. to 17° C., an 
maximum 26° C. For the germination of the spores of Rhizopus 
nigricans, the minimum temperature was found to be 10° C. 
and the optimum 19° C. to 20° C. All later studies of the germ- 
ination of Botrytis cinerea were made at 15° C. and of the germ- 
ination of Rhizopus nigricans at 19° C. 
Time relation—The length of time required for the process 
of spore germination is in itself an important factor. If before 
the elapse of the necessary time period the environmental 
conditions depart from the optimum, the germination process is 
impeded or stopped and the spore may even be killed. In all 
laboratory studies of spore germination, if conditions in nature 
are to be duplicated, no more time need be allowed for germina- 
tion than that in which nearly optimal conditions are likely to 
continue in nature. Probably most fungous spores begin and 
complete germination between darkness and daybreak. One of 
the principal natural protections of plants from pathogenic 
fungi is the failure of environmental conditions to remain be- 
tween minimum and maximum limits long enough for spore 
germination. Infection does not occur if during the protrusion 
and growth of the germ tube the necessary precipitated moisture 
evaporates or the air falls below a certain degree of humidity 
or the temperature passes beyond minimum or maximum limits. 
