234 SECTIONAL ADDRESSES. 
and their mutual reactions have until recently secured but scant attention. 
It is some of these physiological aspects of parasitism that I propose to 
take as the subject of my address. 
In dealing with any aspect of this branch of Botany one is faced by 
the fluidity of our conception of parasitism.1 It may range from the simple 
relationship to its host of a Sooty Mould or of Botrytis cinerea to the 
complicated relationship found in the Uredinee. 
The physiological aspects of parasitism in the case of a fungus like 
Botrytis cinerea are apparently of the simplest when once it has entered the 
host. The cells of the host plant are killed in advance by the secretion of 
an enzyme of a pectinase type and the dead tissues serve as food for the 
parasite. On the other hand, in the case of the parasitism of fungi belonging 
to the Uredinee and Erysiphaceze (and probably the Ustilaginales, and 
possibly also the Exoascaceze) we have a complicated relationship in 
which there is a definite physiological resistance of the host cells to the 
attack of the fungal organism. There is action and reaction, the balance of 
forces sways this way and that—in favour of the host or the invader—and 
there may for a time be an equilibrium in which the fungus is held in check 
but not vanquished. 
The existence of this reaction between the host and parasite which we 
find in the Rust Fungi, and which I shall discuss more in detail later, 
has only been realised comparatively recently, and thus, on the botanical 
side, the physiological aspect of disease has been largely overlooked. 
Disease is abnormal physiology, and it is necessarily the result of the inter- 
action of the physiological processes of the host and parasite. This inter- 
action between the physiological processes of the two organisms has long 
been recognised in animal disease ; it exhibits itself in the specific symptoms 
which are characteristic of disease in man and the higher animals generally. 
The specific symptoms of such diseases were recognised long before the 
‘ germ basis ’ of disease was substantiated, and thus the attention of animal 
pathologists was inevitably turned towards a study of the physiological 
response of the affected organism. These special reactions are in general 
so clearly marked that the nature of an infectious disease in man can 
generally be determined without reference to the invading organism. 
In plants, on the other hand, the symptoms of parasitic disease are highly 
generalised, a large number of infectious diseases displaying the same 
symptoms. It is thus often very difficult, and sometimes impossible, to 
determine the nature of a plant disease without knowledge of the nature 
of the parasite. This distinction between diseases of plants and animals 
is, however, not a fundamental one. The point must be stressed that 
although the symptoms of different parasitic diseases may be superficially 
similar, yet the existence of physiological reactions of the host specific for 
each infection can hardly be doubted when once it is recognised that disease 
is abnormal physiology, the physiological processes of the host being 
modified by the physiological processes of the parasite. At the present 
time we are unable to distinguish the special reactions which the clash 
1 Parasite (mapa oiros) means etymologically ‘ beside the victuals.’ As Sir Ray 
Lankester has pointed out, it was the Greek term applied to those attending sacrifices 
to obtainfood. It had no suggestion of meanness till rich men for purposes of display 
cultivated ‘ hangers-on.’ Inits primary sense it can be used for any ‘ co-liver’ whether 
or no it does harm, 
