236 SECTIONAL ADDRESSES. 
success would be the continued development exhibited by the plant, which 
would necessitate the endowment of the plant body not only with acquired 
immunity to the disease in question, but an immunity of such a type as 
would be passed on to the newly developing organs. A reaction of the 
nature of inherited, acquired immunity would have to be attained, and 
this in view of the experience of animal bacteriologists is unlikely of 
realisation. 
Immunity and resistance to diseases are, of course, well known in 
plants, but they are of the nature of naturalimmunity. Plant pathologists 
need not, I think, reproach themselves for the small progress that has been 
made in the elucidation of the nature of this resistance, for the basis of 
natural immunity in animals remains still very obscure, although the 
physiological field has been worked for a much longer term of years by 
animal than by plant pathologists. 
Some of the processes concerned in the achievement of parasitism in 
plants may now be considered. The question of the mode of entry of a 
parasitic organism into a host plant is one of great physiological interest 
and importance; for a barrier which the would-be invader cannot pass is 
one of the most obvious means of defence against fungal attack. Apart 
from entry through wounds, there are two chief modes of entry of the 
aerial parts of plants, either through a stomatal pore or by actual pene- 
tration of the superficial cells of the host. The entry through the stoma, 
at least in the case of a germ-tube, is clearly the most facile one, and it 
is somewhat of a biological puzzle that any germ-tubes should follow the 
hard road of epidermal-cell-penetration rather than the easy path of 
stomatal invasion where moisture and food material can so easily be 
obtained.? Yet the germ-tubes of Botrytis, Colletotrichum, and Fusicladium, 
for example, and the germ-tubes of the sporidia of Uredinez, apparently 
never enter the open stoma but proceed to bore their way laboriously 
through the epidermis. The case of the Rust Fungi just mentioned is 
particularly striking, for the germ-tubes of the uredospores and ecidio- 
spores on the other hand invariably enter through the stomata. 
The nature of the reaction which brings about the stomatal type of 
entry is still very obscure. It is frequently assumed that the entry is in 
response to some hydrotropic reaction, that the germ-tube passing over 
the stomata finds itself exposed to a stream of water vapour diffusing 
out of the pore and thus a tropistic reaction is produced. Balls, some 
years ago, showed that the uredospores of Rust Fungi when placed on a thin 
perforated sheet of rubber above a water surface developed germ-tubes 
which passed through the perforations towards the water. This interesting 
experiment demonstrates that the germ-tubes in question are capable of 
hydrotropic curvature, butit does not show that the entry into the stoma 
is due to such a reaction. In the experiment with the rubber sheet there 
must have been marked differences in the concentration of water vapour 
on the sides of the membrane. In the case of a germinating spore on the 
surface of a leaf and under the conditions in which infection usually occurs, 
the differences in concentration on the two sides must be very slight. 
The surface of the leaf would be covered with layers of air very nearly 
2 A germ-tube without the capacity for penetration of the epidermis would be at 
a disadvantage on a non-stomatal surface. 
