400 Brown.— Studies in the Physiology of Parasitism. III. 
now invigorated, commenced an active growth, sending out hyphae in 
all directions, and in particular through the cuticle of the bean stem into 
the internal tissue. When the fungus was applied to the bean stem in 
a drop of nutrient material, de Bary found a similar sequence of pheno- 
mena, with the exception that the formation of attachment organs was 
omitted. De Bary’s argument on this subject is as follows : ‘The cause of 
the omission of attachment organ formation (in the latter case) can hardly 
be other than that the hyphae which are directly surrounded with nutrient 
secrete the toxin necessary for the softening (Erweichung) of the substrate 
more rapidly than those which have grown through the air, and to which 
nourishment must first be drawn from the distant assimilating mycelium ; 
that therefore the softening of the host tissue which makes penetration 
possible takes place immediately after contact of the fungus with the 
epidermis, so that the resistance which constitutes the stimulus for the 
formation of attachment organs is not forthcoming.’ 
On the basis of these observations, de Bary postulates a toxic sub- 
stance which is capable of diffusing through the cuticle. In examining his 
extracts from this point of view he was unable to arrive at any definite con- 
clusion. In some cases he found that the fungal extract when placed 
on the epidermis had no action whatever on the underlying tissue ; in 
others he found that some action did take place. Action of the extract 
through the cuticle was as a rule more definite with the liquid obtained from 
sclerotia than with the extracts obtained from ordinary mycelium. The 
failure to obtain positive results in all cases was ascribed by de Bary 
to the extract losing its activity before it could get through the cuticle. 
Nordhausen 1 accepted de Bary’s conclusions as a working hypothesis 
in his experiments on Botrytis. He states that while it is difficult to obtain 
infection when the spores are sown in large drops of liquid, infection is 
readily produced when the drops are small. Failure of infection when the 
drops are large he considers to be the result of undue dilution of the toxic 
substance, which therefore fails to reach a sufficiently high concentra- 
tion to be effective. This effective concentration, on the other hand, is 
readily attained when sowings are made in small drops. As a result 
the underlying cells are killed and infection is thereby assured. Two 
experiments of Nordhausen may be specially mentioned : 
i. In order to see if the fungus in process of penetrating a membrane 
excreted any acid substance (such as oxalic acid), he stained pieces of 
epidermis of Allium with Congo red, and allowed the fungus to grow 
through these. Any excretion of acid would be shown by formation of the 
blue acid dye in the neighbourhood of the penetrating hyphae. No such 
effect was however observed. 
1 Nordhausen, M. : Beitrage zur Biologie parasitarer Pilze. Jahrb. f. wiss. Bot., vol. xxxiii, 
1899. 
