BACTERIA 



665 



gained access to rotting tissues, injured 

 by other agents. 



"Saprophytic bacteria can readily make 

 their way down the dead hyph (branch) 

 of an invading fungus, or into the punc- 

 tures made by insects, and aphides have 

 been accredited with the bacterial infec- 

 tion of carnations, though more recent 

 researches by Woods go to show the cor- 

 rectness of his conclusions that aphides 

 alone are responsible for the carnation 

 disease. On the other hand, recent in- 

 vestigations have brought to light cases 

 in which bacteria are certainly the prim- 

 ary agents in the diseases of plants. The 

 principal features are the stoppage of the 

 vessels, and the consequent wilting of the 

 shoots: as a rule, the cut vessels on the 

 transverse sections of the shoots appear 

 brown and choked with a dark yellowish 

 slime in which bacteria may be detected; 

 examples in cabbages, cucumbers and po- 

 tatoes." 



A familiar example is that of the com- 

 mon blight of apple and pear. See article 

 on this subject under Pear. 



Prof. Ward observes further: 



"In the carnation disease and in certain 

 diseases of tobacco and other plants the 

 seat of bacterial action appears to be the 

 parenchyma, and it may be that aphides 

 and other piercing insects infect the 

 plants, much as insects convey pollen 

 from plant to plant, or (though in a dif- 

 ferent way) as mosquitoes infect man 

 with malaria. If the recent work on 

 cabbage diseases may be accepted, the 

 bacteria make their entry at the water 

 pores at the margins of the leaf, and 

 thence by way of the glandular cells of 

 the tracheids. Little 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 

 a disease known as 'leaf mosaic,' will 

 induce this disease after filtration 

 through porcelain." 



Method of Study and Discovery 



How to discover the specific cause of 

 any disease, is a question of the greatest 

 importance. This is done generally by 

 three steps; 



First. The discovery of disease in the 

 affected tissues. 



Second. Obtaining the bacterium of 

 this disease, in pure culture. 



Third. The production of the disease, 

 by inoculation, with a pure culture. 



By means of microscopic examination 

 more than one organism may sometimes 

 be observed in the tissues, but one single 

 organism by its constant presence and 

 special relation to tissue changes, can 

 usually be selected as the probable cause 

 of the disease, and attempts towards its 

 cultivation can then be made. 



In cultivating bacteria, outside the 

 body from which it is originally taken, 

 the food material in which it is cultivat- 

 ed, must first of all be sterilized by heat. 

 This food material should be as nearly 

 like that of the body from which it is 

 taken, as possible. The media are used 

 either in a fluid or solid condition, and 

 are placed in glass tubes or flasks plugged 

 with cotton-wool. 



InornlatJon 



In testing the effects of bacteria by in- 

 oculation, young and vigorous cultures 

 must be used, and in the case of plants, 

 injected into the sap at some point where 

 the cells are vigorous, or in the case of 

 animals, by means of a hypodermic 

 syringe into the subcutaneous tissue into 

 the vein, into one of the serus sacs, or 

 more rarely into some special part of the 

 body. 



Imiuiinity 



By immunity from disease, we mean, 

 non-susceptibility; or not being suscept- 

 ible by means of infection, contagion or 

 inoculation. The entrance of a bacter- 

 ium or any number of bacteria into the 

 tissues, does not mean disease, necessari- 

 ly. Even though the bacteria be virulent 

 or poisonous, the plant or the person may 

 have resisting power sufficient to neutral- 

 ize its effects. With regard to diseases 

 in persons, it has been shown that cer- 

 tain races are practically Immune from 



