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man and animals. No explanation for these interesting morpholog- ical differences has hitherto been advanced, and if any significance is to be attached to them it has yet to be discovered.

Exactly what it is that constitutes virulence in an organism and makes the distinction between parasitic and saprophytic forms is entirely unknown. One naturally asks whether an organism may possess virulence for both plants and animals: so far as ex- perience goes this seems not to be the case; one may perhaps as the result of eating bacterially diseased fruits experience a tem- porary disturbance of the alimentary system, but nothing of a more serious nature need be feared. On the face of it such a phenom- enon as a general occurrence would seem to be unlikely from the fact that the reaction of the medium in the two cases is very differ- ent; an organism which is favoured by the alkalinity of the animal serum can hardly be expected to grow strongly in the sap of a plant where the reaction is often strongly acid, and vice versa. On the injection of plant parasites into laboratory animals nothing more than a slight local disturbance an abscess results or occasionally the animal may show a disinclination to move and take food, a malaise for a brief period from which it quickly recovers. In the serum of such inoculated animals antibodies, specific agglutinins, are produced but this probably has nothing whatever to do with virulence since it follows also upon the injection of the common saprophytes.

The effects of bacteria upon the attacked plant are by no means so helpful towards a diagnosis of the disease as are the effects of bac- teria upon the human being. The specific symptoms of disease in man by which the general practitioner is enabled to diagnose his case with more or less certainty, although he may have only a rudimentary knowledge of bacteriological technique, have no coun- terpart in the diseases of plants. The plant pathologist must first isolate and identify the causal organism, often a task of considerable difficulty, before he can arrive at a true diagnosis, the symptoms of disease produced by a number of different organisms being almost exactly similar. According to these group symptoms the bacterial diseases of plants may be divided into four main types, namely: Soft Rots, Wilts, Intumescences and Local Lesions.

Soft Rots. The plants most attacked by rot-producing organisms are the root vegetables and potatoes. A certain amount of disease may occur while the plants are still in the ground, but the greatest losses take place during storage of the roots through winter. The rot results through the solution of the cementing substance, the middle lamella, which holds the cells of the plant tissue together just as mortar holds together the bricks in a building. This cementing substance consists of pectin material and its solution is effected through the agency of an enzyme, a pectinase, produced by the bacteria. The removal of this substance causes the tissue to lose all coherence and the cells to become reduced to a wet pulpy mass. Diseases of this type are the " White Rot " of turnips, the " Soft Rot " of carrots and other vegetables, the " Heart Rot " of celery, and the " Blackleg " of potatoes.

Wilts. A number of very destructive diseases is included under this head. The symptoms are almost identical in all cases and are the result of the blocking up of the conducting system of the plant by bacterial growth in the vessels, so that those parts of the plant, whose natural supply of watery sap is thereby cut off, die from wilting, and become the prey of all kinds of bacteria from the soil and air, and finally either dry up or become reduced to a wet rotting mass. Other symptoms are striping of the leaves, a general dwarfing of the attacked plants, and a one-sided growth of the plants resulting from a one-sided localization of the infection. The striping of the leaves is due to pigments, either in the bacteria themselves or in the wood of the invaded vessels, making the course of these vessels apparent from the outside as streaks usually of a yellow, red, or brown colour.

Many of these wilts are caused by organisms which are extraor- dinarily similar in many of their characters. They belong to the genus Pseudomonas, are strongly yellow in colour and are indis- tinguishable under the microscope. They might be considered to be only varying strains of one and the same species except that they show constant differences in degree of pigmentation and in certain of their physiological characters; moreover they seem to be quite specific for the diseases in the plants or orders of plants in which they have been found, and all attempts to produce disease in one kind of host by inoculation with the specific organism of disease in another kind have so far been quite unsuccessful.

Included in this group of diseases are the very troublesome " Black Rot " of cabbage and other members of the family Brassica; Wakker's disease of hyacinths, which has been responsible for the entire disappearance of some of the most beautiful varieties of hyacinths from the beds of the Netherlands with serious financial loss to the Dutch growers; and a disease of sugar-cane known as Cobb's disease which produces heavy losses in seedling canes and also much difficulty and loss in extraction of the sugar by reason of the gummy slime which the bacteria produce, causing trouble in the crushing machinery and in the evaporating pans. Other serious wilt diseases are the wilt of cucumbers, the wilt of tomatoes, pota- toes and other solanaceous plants, including tobacco whose cultiva- tion in parts of Malay and other districts has had to be entirely abandoned as the result of this disease.

Intumescence Diseases. Here the disease takes the form of large warty or pseudo-cancerous growths on the stems and leaves of the attacked plant caused by hypertrophy of the cortical tissues and mesophyll under the irritating stimulus of the presence of the invad- ing organism. Crown Gall, a destructive disease of roses, grape- vines, hops and a large number of other hosts, belongs here. In this case the trouble is largely confined to the crown of the root where it extends from year to year, eventually growing to such a size that death of the tree results through destruction of the con- ducting tissue of the root. Another disease of this type is the Olive Knot, a well-known pest wherever olives are in cultivation.

Local Lesions. Local lesions or cankers result through destruction of the external tissues of plants in localized areas upon the stems, leaves and fruits. Stripe disease of tomatoes is well known to growers in Great Britain ; the Citrus canker is a serious disease in S. Africa and S. Florida, and in the tropics generally Leaf Spot diseases of beans and of cotton have been shown to be caused by bacterial parasites.

Control Measures. At present there is no means of control for bacterial diseases in plants which can be of general application. Obviously the prophylactic and curative methods of injection so successfully used against disease in animals cannot be of use for plants. The use of sprays which are often most effective against fungal diseases of plants is of no avail against the bacterial ones. Sterilization of the soil might be of service against such parasites as are infective of the plant through the soil, but it is clear that, in order to kill the parasitic form, one would at the same time necessar- ily interfere with the normal soil flora upon the functions of which the fertility of the soil depends.

In certain instances where the disease is carried by some biting insect, attacks upon this carrier have resulted in more or less suc- cessful control. A case in point is that of the wilt of cucumbers, where the organism is introduced on the mandibles of a beetle. In this case it is found that the beetle has a special predilection for the wild squash, and by growing these in drills between the rows of cucumbers almost all the beetles can be collected upon them, where they can be periodically annihilated by spraying with kerosene. Another means of control is found in the manurial treatment of the soil whereby a more hardy and resistant plant is produced. In this way by the increase of potash it has been possible to effect a con- siderable reduction of the Stripe disease in tomatoes. The rotation of crops, so that several years elapse before a crop which has been diseased is again grown on the infected soil, is for the majority of bacterial diseases the only means of control known at the present time. In this way the parasite, not finding its particular host for some time, may die out or may become so altered physiologically as no longer to possess the power of attack upon the plant.

REFERENCES. Gurney-Dixon, Transmutations of Bacteria; J. A. Arkwright, " Variation in Bacteria in Relation to Agglutination both by Salts and by Specific Serum," Jour. Path, and Bad. (1921); P. H. De Kruif, " Dissociation of Microbic Species," Jour. Amer. Med. Assn. (1921); E. J. Russell, Soil Conditions and Plant Growth; Journal of Agricultural Science; Journal of Agricultural Research; Soil Science; G. J. Fowler, " The Conservation of Nitrogen with Special Reference to Activated Sludge," Journal of the Indian Institute of Science (1920) ; E. F. Smith, Bacteria in Relation to Plant Diseases; E. F. Smith, Bacterial Diseases of Plants; Phyto- pathology; Annals of Applied Biology; Bulletins of Experimental Stations, U.S.A. ' (S. G. P.)

II. MEDICAL BACTERIOLOGY

It has been more and more recognized by the epidemiologist that one of the chief structural units in the bridge which connects one outbreak of a disease with another is the carrier. By the term " carrier " is meant an individual who, though healthy and thus unsuspected of infectivity, still harbours in his body pathogenic bacteria which, passed in the various excretions, constitute, when given favouring circumstances, a danger to those about him. These favouring circumstances may be with- held for long periods, but the individual, on the other hand, may continue to conserve and distribute the microorganisms for still longer periods even many years.

Disease Carriers. A certain number of bacteria pathogenic to man find carriers among animals, for instance the virus of Malta fever, which multiplies and is distributed in the milk of infected goats, but mainly man is himself responsible.

The carrier may be a person who has survived an attack of the disease in question and failed to rid himself of the causative or- ganisms, which, lodging themselves in the respiratory, genito-urin- ary or intestinal tract, continue an existence much as do the sapro- phytic organisms normally found in those regions. He may, on the other hand, be an individual who entertains the bacterium without ever having displayed any symptoms of the disease. A chain of such carriers, recording no history of illness, but passing on the virus in secret, as it were, would be the explanation of sporadic cases, say, of cerebros'pinal meningitis, occurring in non-epidemic times, at