suspend his researches because his fundamental conceptions and assumptions are disputable and disputed; nor does he turn to some other branch of science in order to borrow from it others that have more prestige. Let us follow his example.

Let us gather our facts of human nature by objective and by introspective observation. Let us make our empirical generalisations and correlations of these facts, building up our own science in our own way. Let us boldly affirm that, just as the physical sciences do not proceed deductively from any system of exact abstract propositions, so also psychology, the most concrete of the sciences, is not required by any higher authority to accept or formulate any abstract propositions as an unchanging deductive basis.

It may be that eventually men of science will agree that there are in the universe two ultimately different kinds of process, the mechanistic and the purposive, the strictly determined and the creative, the physical and the mental. Or it may be that, eventually, one of these may be shown to be merely an appearance of the other, an appearance due to the present limitations and imperfections of our understanding. At present we cannot decide this issue.

But, if I attempt to guess at the future development of Science, I incline to follow the lead of the most powerful intellects of all ages, and to predict that, if such resolution of the two types of process into one shall ever be achieved, the purposive type that we regard as the expression of Mind will be found to be more real than the other.

SECTION K.-BOTANY.

PHYSIOLOGICAL ASPECTS OF

PARASITISM.

ADDRESS BY

PROFESSOR V. H. BLACKMAN, Sc.D., F.R.S.,

PRESIDENT OF THE SECTION.

THE President of the Association will have expressed the satisfaction which all the Sections feel in meeting for the fourth time in the history of the Association in the great Dominion of Canada. To Section K the almost overwhelming size of the country and the great, diversity of vegetation, both natural and artificial, must have an especial appeal.

Last year the President of Section K had to deplore the loss of three prominent botanists. I am less unfortunate in that our loss this year is far lighter. We have, however, to regret the death of Thomas Frederick Cheeseman, a distinguished worker in systematic botany who devoted himself to the study of the flora of New Zealand.

In deciding on the subject of a Presidential Address, the vastness of Canada's agricultural and sylvicultural interests can hardly be overlooked, even in a section the interests of whose members are in the main those of pure botany. It appeared to me appropriate that if possible some aspect of pure botany should be chosen which would have at least implications in applied botany. The subject of disease is, of course, one of great moment wherever plants are massed together in artificial cultivation. Some aspect, therefore, of plant pathology seemed a fit subject for an address on such an occasion, since in it we have a branch of botany securely based on scientific interest and firmly buttressed by economic importance. Some consideration of disease in plants seemed peculiarly apposite also when it is recalled that at the last meeting of the British Association at Toronto, in 1897, the President of this Section was Professor Marshall Ward, the first English plant pathologist of the modern school. The value of his contributions to our knowledge of disease in plants is recognised by all; that he should have been cut off in his prime, British botanists will long deplore.

It is significant of the growth of botany in all its branches that Marshall Ward set himself as his presidential task a wide survey of the fields of mycology, parasitism, and fermentation. Needless to say, the task that any President of Section K can at the present time essay must be one of much smaller compass.

In the field of plant pathology which has been so assiduously cultivated of late years, attention has been mainly focussed on the study of the lifehistory and mode of infection of fungal and bacterial parasites, and on the methods of controlling infection. The relationship of host and parasite

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. 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 Uredineæ.

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 Uredineæ and Erysiphaceae (and probably the Ustilaginales, and possibly also the Exoascaceae) 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 interaction of the physiological processes of the host and parasite. This interaction 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 6 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 (rapà σîros) means etymologically 'beside the victuals.' As Sir Ray Lankester has pointed out, it was the Greek term applied to those attending sacrifices to obtain food. It had no suggestion of meanness till rich men for purposes of display cultivated hangers-on.' In its primary sense it can be used for any co-liver' whether or no it does harm.

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, i.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 achievements 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 Erysiphaceae and the Uredineæ 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 hypha. 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 physiological 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 hyphæ 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

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 natural immunity. 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 penetration 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 Uredineæ, 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 acidiospores 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, but it 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.