338 Boyle . — Studies in the Physiology of Parasitism. VI. 
rise to a production of material which diffuses through the epidermis and 
exerts a chemotropic influence on the fungal hyphae. In the first investiga- 
tion of the present series on the physiology of parasitism carried out in this 
laboratory, Brown ( 2 ), working on a powerful extract which he obtained 
from germ tubes of Botrytis cinerea , showed that the extract contained no 
substance capable of dissolving cuticle. He found that such an extract 
had no effect on such delicate structures as rose petals as long as the cuticle 
remained unbroken. This important observation was followed by a careful 
microscopical study by Blackman and Welsford ( 1 ) of the early stages of 
penetration of the bean leaf by Botrytis cinerea. They found penetration 
of the cuticle effected solely by the mechanical pressure exerted by the 
germ tube. Later, Dey (6), working on the early stages of infection of 
French Bean pods by Colletotrichum Lindemuthiannm , found that the 
mechanism by which the 4 infection hypha 5 penetrated the host was similar 
to that employed by Botrytis cinerea. In both these microscopical 
investigations conidia were used for infection experiments. In view of the 
fact that Sclerotinia Liber tiana does not, as far as is known, infect its host 
by means of conidia, and also on account of the general acceptance of the 
view expressed by de Bary that the hyphae of this fungus possess the 
property of softening and dissolving cuticle, it seemed of importance to 
examine microscopically the stages of ‘ mass infection ’ by hyphae such as 
occurs here. 
Methods. 
Cultures of the fungus were made on potato-mush agar, prune juice 
agar, and malt extract agar. The potato-mush agar was prepared according 
to the method of Brown. 1 The prune juice agar was prepared by adding 
to a 2*5 per cent, decoction of prune juice an equal volume of 3 per cent, 
agar solution. The malt extract agar used was made by dissolving 20 gm. 
extract of malt in a litre of water and adding 20 grm. of agar. The medium 
was then steamed for half an hour to dissolve the agar, ‘ tubed ’, and 
sterilized. 
The fungus grows much more quickly on prune juice agar than on 
potato-mush, or malt extract, agar. A Petri dish of prune juice agar 
infected at the centre with the hyphae of the fungus and incubated at 25 0 C. 
is covered by a continuous coating of mycelium in about a week. After 
about ten days aerial branches arise, beginning round the edge of the 
culture and extending inwards, converting the original sparse growth into 
a white felted mass. Dense white aggregations of the mycelium soon 
become apparent round the edges of the culture, and after about a fortnight 
these turn black, forming the characteristic sclerotia of the fungus. 
1 1. c., p. 318. 
