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B A C T E R I O L O G Y
[general
be that Aphides or other piercing insects infect the plants, much as insects convey pollen from plant to plant, or (though in a different way) as mosquitoes infect man with malaria. If the most recent work on the cabbage disease may be accepted, the bacteria make their entry at the water pores at the margins of the leaf, and thence via the glandular cells to the tracheids. Nothing is known of the mode of action of bacteria on these plants, but it may be assumed with great confidence that they excrete enzymes and poisons (toxins), which diffuse into the cells and kill them, and that the effects are in principle the same as those of parasitic fungi. Support is found for this opinion in Beyerinck’s discovery that the juices of tobacco plants affected with the disease known as “leaf mosaic,” will induce this disease after filtration through porcelain. In addition to such cases as the Kephir and Ginger-beer plants (Figs. 9, 10), where anaerobic bacteria are associated with yeasts, several interesting examples of sym^ ^ biosis among bacteria are now known. Bacillus bfosis. chauvoei ferments cane-sugar solutions in such a way that normal butyric acid, inactive lactic acid, carbondioxide, and hydrogen result; Micrococcus acidi-paralactici, on the other hand, ferments such solutions to optically active paralactic acid. Nencki showed, however, that if both these organisms occur together, the resulting products contain large quantities of normal butyl alcohol, a substance neither bacterium can produce alone. Other observers have brought forward other cases. Thus neither B. coli nor the B. denitrificans of Burri and Stutzer can reduce nitrates, but if acting together they so completely undo the structure of sodium nitrate that the nitrogen passes off in the free state. Van Senus showed that the concurrence of two bacteria is necessary before his B. amylobacter can ferment cellulose, and the case of mud bacteria which evolve sulphuretted hydrogen below which is utilized by sulphur bacteria above has already been quoted, as also that of Winogradsky’s Clostridium pasteurianum, which is anaerobic, and can fix nitrogen only if protected from oxygen by aerobic species. It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, e.g., Tetanus. Astonishment has been frequently expressed at the powerful activities of bacteria—their rapid growth and dissemination, the extensive and profound Actjvity of decompositions and fermentations induced by bacteria. them, the resistance of their spores to desiccation, heat, &c.—but it is worth while to ask how far these properties are really remarkable when all the data Fig. 10.—The Ginger-beer plant. A, one of the brain-like gelatinous masses for comparison with other organisms are considered. In into which the mature “plant” condenses; B, the bacterium with and without its gelatinous sheaths (cf. Fig. 9); C, typical filaments and the first place, the extremely small size and isolation of rod lets in the slimy sheaths ; D, stages of growth of a sheathed filament— a at 9 a.m., & at 3 p.m., c at 9 p.m., d at 11 a.m. next day, e at 3 p.m., /at the vegetative cells place the protoplasmic contents in 9 p.m., <7 at 10.30 a.m. next day, h at 24 houi's later. {Original.) peculiarly favourable circumstances for action, and we by other agents. Saprophytic bacteria can readily make may safely conclude that, weight for weight and molecule their way down the dead hypha of an invading fungus, for molecule, the protoplasm of bacteria is brought into or into the punctures made by insects, and Aphides have contact with the environment at far more points and over been credited with the bacterial infection of carnations, a far larger surface than is that of higher organisms, though more recent researches by Woods go to show the whether—as in plants—it is distributed in thin layers correctness of his conclusion that Aphides alone are re- round the sap-vacuoles, or—as in animals—is bathed in sponsible for the carnation disease. On the other hand, fluids brought by special mechanisms to irrigate it. Not recent investigation has brought to light cases in which only so, the isolation of the cells facilitates the exchange bacteria appear to be the primary agents in certain of liquids and gases, the passage in of food materials and diseases of plants. The principal features are the stop- out of enzymes and products of metabolism, and thus page of the vessels and consequent wilting of the shoots; each unit of protoplasm obtains opportunities of imas a rule the cut vessels on transverse sections of the mediate action, the results of which are removed with shoots appear brown and choked with a dark yellowish equal rapidity, not attainable in more complex multislime in which bacteria may be detected, e.f/., cabbages, cellular organisms. To put the matter in another way, if cucumbers, potatoes, Ac. In the carnation disease and in we could imagine all the living cells of a large oak or of a certain diseases of tobacco and other plants the seat of horse, having given up the specializations of function bacterial action appears to be the parenchyma, and it may impressed on them during evolution and simply carrying
access through the stomata, they could not penetrate the cell-walls bounding the intercellular spaces, for although certain anaerobic forms are known to ferment cellulose, no undoubted bacterium has been shown to possess the power of penetrating the cell-walls of living cells, unless we except the bacteroids of Leguminoseae first described by Marshall Ward in 1887, and confirmed by Miss Dawson in 1898. On the other hand a long list of plant-diseases have been of late years attributed to bacterial action. Some, e.y., wet rot of potatoes, the Sereh disease of the sugar-cane, the slime fluxes of oaks and other trees, are not only very doubtful cases, in which other organisms such as yeasts and fungi play their parts, but it may be regarded as extremely improbable that the bacteria are the primary agents at all; they are doubtless saprophytic forms which have gained access to rotting tissues injured
