Boyle. — Studies in the Physiology of Parasitism. VI. 339 
On potato-mush agar and malt extract agar growth proceeds somewhat 
slowly at first, forming a dense felted mass of mycelium in contrast to the 
active sparse growth on prune juice agar. When an area approximately 
equal to that of a penny is covered in the centre of the dish, growth 
appears to be inhibited. About a week later a number of sclerotia are 
formed along the outer margins of the mycelium, which then extends 
a short distance farther, when growth is again inhibited and another circle 
of sclerotia is formed. Thus after about three or four weeks the mycelium 
reaches the edge of the culture medium and a series of concentric rings of 
sclerotia are formed coinciding with the successive zones of growth. As 
many as 40-50 sclerotia may be formed in a single, 4 in., Petri dish. After 
a period of rest the sclerotia germinate and give rise to apothecia from 
which ascospores are produced. These, as well as the mycelium of the 
fungus, are capable of bringing about infection of susceptible plants (12). 
Leaves of Scarlet Runner Bean (. Phaseolus coccineus) and Broad Bean 
( Vida Faba ) were used as material for infection. The leaves were cut 
from healthy plants, washed in sterile tap-water so as to remove any 
extraneous material, and were placed in a dry atmosphere for a few minutes. 
They were then placed on damp blotting-paper in sterile Petri dishes. As 
was observed by de Bary (4), the fungus mycelium makes very poor growth 
in pure water, and consequently infections were made in drops of turnip 
juice. The turnip juice was prepared by steaming lb. of peeled and 
chopped turnips for one hour. The steamed pieces were placed in a muslin 
bag and the juice expressed. From this quantity of turnips 130 c.c. of-turnip 
juice were obtained. One part of sterile turnip juice added to nine parts 
of sterile distilled water was the nutrient medium used. Infections were 
also made in drops of potato-mush agar placed on the surface of bean 
leaves. A further method adopted was to place cut leaves in Petri dish 
cultures of the fungus so that the upper surface of the leaves came directly 
into contact with the growing ends of the hyphae. 
It is difficult to control the hyphal concentration, and hence the. stage 
of infection in an infection drop, when pieces of mycelium are used for 
inoculation. As a rule, blackened areas are apparent under liquid infection 
drops after about 30 hours When this stage was reached it was 
invariably found that the fungus had already penetrated into the host and 
was causing disorganization of the tissues. As the present investigation is 
concerned with the earlier stages of penetration the material was fixed at 
intervals from 17 hours after infection — usually in Flemming's stronger 
solution diluted with an equal volume of water. Carnoy’s fixing fluid and 
absolute alcohol containing 25 per cent, by volume of glacial acetic acid 
were also used and gave very satisfactory results. The material was 
embedded in paraffin and sections cut 5 m. thick. The sections were 
stained with gentian violet followed by Sudan III (13). The latter stains 
