K.—BOTANY. 235 
of the two sets of processes must produce in the host. With improvements 
in our methods of biophysical and biochemical analysis we may anticipate 
a time when these hidden reactions may be revealed and a new basis for 
the classification of plant diseases established. 
Another striking difference between animal and plant pathology which 
is worth insisting upon is that relating to disease resistance. Disease 
resistance is shown both in plants and animals, but the particular type of 
immunity which has been most clearly studied by the workers on the 
animal side is acquired immunity, ¢.e. that type of specific resistance which 
is the result of one attack of a specific disease. Such immunity must 
have forced itself on man’s attention from very early times, and it is by a 
study of such resistance that animal bacteriologists—building firmly on 
the work of Pasteur—have developed the modern treatment of disease by 
the injection of dead organisms and of the blood fluid of animals containing 
suitable antibodies. The development of such vaccine and serum therapy 
should, I think, be rightly considered as one of the most remarkable achieve- 
ments of modern biology. 
On the other hand, the problem of immunity in plants is a far more 
difficult one than that with which the animal pathologist is faced. The 
acquired immunity due to one attack of a disease which is so common 
in animals is unfortunately quite unknown in plants, at least in relation 
to definite disease. The modern view of recovery from infectious bacterial 
disease in animals is that it is due to a very well-marked and highly 
specialised reaction of the invaded organism. Part at least of the reaction 
is the development of antibodies which neutralise the toxins produced by 
the invading bacteria and help to bring about their death. It is true 
that in the Erysiphacee and the Uredinez and in certain cases of 
endotrophic mycorhiza, and in the well-known orchid fungus, the invaded 
cells show a very marked reaction which may lead to the death, and 
sometimes to the digestion later, of the invading hyphe. These, however 
are not cases of ordinary disease and the cells show no acquired resistance. 
Again, whatever may be the behaviour of individual plant cells when 
attacked, one never finds that general bodily reaction which is so marked 
and characteristic of many infectious diseases in the higher animals. The 
parts of the plants are, of course, much less highly correlated than those 
of the animal body; there is no circulating blood stream by which the 
most distant cells of the body can with great rapidity be brought into 
physiological relationship. Even in the case of the highly specialised 
parasitism of the Rust Fungi, where there are obvious complex physio- 
logical reactions between host and parasite, we find no general reaction 
by the plant, but cells or small groups of cells carry on a struggle with the 
invading bacteria and hyphe apparently in complete independence. It 
follows that in the absence of any suitable reservoir—such as the blood 
stream of animals supplies—in which toxins and antitoxins may be sought, 
the likelihood of their demonstrations, should they be produced, is very 
slight. The absence in plants of a general bodily reaction to disease would 
seem also to preclude the possibility of the application to them of serum 
therapy. If, in spite of the absence from plants of the acquired resistance 
which is the basis of serum therapy in animals, such sera could be 
prepared, there would be the great difficulty of distributing such substances 
throughout the plant. Another and apparently insuperable barrier to 
