FROMME: THE CULTURE OF CEREAL RUSTS 513 
remained at nine days throughout February and March. The 
sequence of stages during a nine days incubation period may be 
divided into two periods. First, a period of vegetative develop- 
ment during which no evidence of infection*is seen. Second, a 
fruiting period during which the stages in the formation of the 
pustules are apparent. The first period occupies five days after 
inoculation and the first visible evidences of pustule formation 
become apparent on the sixth day. The leaves on this day have 
a faint mottled appearance which is due to the presence of small 
areas that are lighter in color than the surrounding leaf tissue. 
These areas are visible only by transmitted light. On the seventh 
day the light areas become more conspicuous and their boundaries 
more sharply defined. The areas become slightly swollen on the 
eighth day and a light orange color is apparent. During the next 
twenty-four hours the development of the pustules is rapid. They 
continue to swell until the epidermis of the leaf is ruptured by a 
longitudinal slit and the orange mass of spores is extruded. The 
mass of spores hangs together for a time but breaks apart on drying 
and falls from the leaf as separate spores or in small groups when 
the leaf is disturbed. 
OBSERVATIONS ON CONDITIONS AFFECTING SPORE GERMINATION, 
INFECTION, AND RATE OF DEVELOPMENT 
Effect of moisture 
It has been demonstrated a number of times during this culture 
work that a humid atmosphere provided by covering with a bell 
jar is necessary to secure infection. Abundant moisture may be 
supplied at the time of inoculation and still the plants will not 
become infected, in the greenhouse, unless they are covered soon 
afterwards. The drops of water apparently dry up before germi- 
nation and infection result. The per cent of saturation in the 
greenhouse in which my experiments were made as obtained from 
the hygrometer records, averages about 75~80 per cent with a 
temperature range between 55 per cent. and 85 per cent. Al- 
though the conidia of Erysiphe graminis infect the cereals spon- 
taneously under these conditions, the uredospores of P. coronifera 
will not do so. 
To test the possibility of providing conditions of humidity 
