238 SECTIONAL ADDRESSES. 
this kind seems, however, insufficient to explain the reaction of germ-tubes 
towards the surface ofa host plant. The germ-tubes of Botrytis developing 
in a drop of turnip-juice will penetrate the surface of a bean leaf; and as 
the turnip contains substances which strongly attract germ-tubes (at least 
those of Rhizopus),it seems unlikely that the concentration of any attractive 
substances which may diffuse through the cuticle would be sufficient to 
produce a stronger response than that due to the comparatively high 
concentration of the active substances in the drop. Again, Dr. Brown 
has shown in an unpublished observation that germ-tubes of Botrytis 
growing in turnip-juice will penetrate a thin sheet of paraffin (about 10p 
in thickness) which is floating on the same fluid. In such cases where a 
positive chemotropism appears very unlikely, the only other possible 
reactions which might be at work seem to be a negative chemotropism 
of the germ-tubes towards its own waste products, or a positive reaction 
towards the surface with which the germ-tube is in contact. If such a 
negative reaction were the main factor in penetration, one would expect 
the germ-tubes of any fungus, such as Penicilliwm or Rhizopus, to enter a 
bean leaf from turnip-juice ; this, however, does not occur. Furthermore, 
Fie.) 2: 
it may be argued § that there will be a higher rather than a lower con- 
centration of waste products on the side of the germ-tube towards the 
substratum (fig. 1), owing to the difficulty of the escape of such products 
in this direction. The question really resolves itself into that of the 
nature of the waste products and their relative rate of diffusion through 
the water of the drop on the one hand, and through the epidermal cell- 
wall on the other. If the waste products can diffuse with fair rapidity 
through the cuticle and epidermal cell-wall and so escape into the general 
body of the leaf, or if they are taken up in some way (possibly by 
adsorption) by the host cells, then it is quite possible that the concen- 
tration on the lower side towards the host tissue may be such as to lead 
to a growth towards that host surface. The probability of a negative 
chemotropism of this k nd playing any considerable part in the responses 
of the germ-tube which lead to penetration does not, however, seem very 
strong. Such a chemotropism certainly does not prevent the germ-tubes 
of such fungi as Botrytis and Colletotrichum fixing themselves firmly to an 
impermeable glass surface. 
If both positive and negative chemotropism are excluded it would seem 
that a contact stimulus must play the major part in the entry of a parasite 
8 As Dr. W. Brown has suggested to me. 
