344 Boyle. — Studies in the Physiology of Parasitism. VI. 
closely clustered together, are divided by numerous transverse walls into 
short segments with membranes that become dark brown with time.’ 
De Bary so arranged one of his infection experiments that the 
actively growing mycelium of a culture of Sclevotinia Liber tiana came into 
contact with the stem of the bean plant after a short passage through 
damp air. He noticed that under these conditions the fungus did not 
penetrate into the host directly, but proceeded to form appressoria (Haft- 
biischel), and while these were being formed he observed that the underlying 
cells collapsed and became blackened before penetration of the cuticle took 
place : ‘ . beginnt das Absterben der vom Buschel beriihrten Zellen bevor 
der Pilz in und durch die Epidermis . . . gedrungen ist.’ 1 When the fungus 
was applied to the stem in a nutrient fluid he observed that penetration 
took place without the formation of attachment organs. Believing, as de 
Bary did, in the softening and dissolution of the cuticle, he concluded that 
the omission of the formation of attachment organs in the later case was 
due to the fact that the mycelium in the nutrient liquid was better nourished 
than that which had grown a short distance through the air and consequently 
secreted the necessary toxin for the softening and dissolving of the cuticle 
more rapidly, so that the necessary stimulus for the formation of attachment 
organs was not forthcoming. 
Similar conditions of infection were obtained in the present investigation 
by placing freshly cut bean leaves on the surface of an actively growing 
culture of the fungus so that the hyphal tips came into contact with the host 
surface after a short passage through the air. Another method adopted was 
to infect drops of potato-mush agar on the surface of the bean leaf with the 
mycelium of the fungus as already described. In the latter case the fungus 
grew vigorously in the nutrient agar, but was not observed to penetrate the 
cuticle underneath the drop. Aerial branches were sent out, and when 
these came into contact with the host surface through the air, penetration 
was observed to take place. In neither of these experiments were the 
observations of de Bary on the formation of attachment organs confirmed. 
Although appressoria are very abundantly formed under conditions of 
aerial infection (Figs, io, u, 12, 14, 18), several instances of penetration 
by single hyphae without the formation of the characteristic attachment 
organs were also observed (Figs. 8, 9, 13, 15). It is difficult, therefore, 
to accept the views of de Bary and Hasselbring that the nutrition of the 
fungal hyphae controls the reaction to contact stimulus. 
As noted by de Bary, penetration from characteristic appressoria has 
not been observed when infections are made in nutrient liquid on the surface 
of the leaf. If the absence of appressoria be found to be a constant feature 
in the case of infection from liquid nutrient drops, the difference in 
behaviour compared with aerial infection may possibly be due to differences 
1 (5) 1. c., p. 4 1 3- 
