THE BACTERIA AND VIRUSES 355 



treatment, and are supposed by some to be evidence of a complex and lengthy 

 life-cycle, but they obviously create uncertainties in identification. The 

 great majoritv of the supposed species described more than thirty years 

 ago are so uncertain as to be valueless, and at the present day it is recognized 

 that onlv prolonged and detailed study of a particular organism, including a 

 knowledge of its powers of variation, can enable us to say with certainty 

 whether it is a distinct species or not. 



The older idea that Bacteria were so variable that the discrimination of 

 species was impossible cannot now be upheld. Most of the important disease- 

 producing organisms are so fully known that their specific character can be 

 accepted with reasonable certainty. Variation is nevertheless a factor of 

 great importance. Some organisms like the nitrogen-fixing bacterium 

 Azotobacter and the root-nodule bacillus Rhizobiwn show a definite cycle of 

 changes in the course of their natural lives, so that a knowledge of the full 

 life history is essential to understand them. Many other Bacteria may be 

 in a like case, but all too little is known on the subject. 



/ 



Bacteria in Relation to Higher Plants * 



Although we generally associate Bacteria with animal and human diseases, 

 a great many plants are also attacked by them, and many important plant 

 diseases are produced in this way. Bacterial infection most generally takes 

 place through wounds in the host plant. For example, the Olive tubercle 

 caused by Pseudomonas savastanoi is frequently found to have been started 

 by wounds caused by hailstones. Certain Bacteria enter the plant through 

 broken roots. In other instances the entrance of the Bacteria is made through 

 natural openings in the tissues, such as the stomata of leaves. 



Various agents assist in the transference of disease from one plant to 

 another. Insects, worms and slugs carry Bacteria from diseased to healthy 

 plants, and apart from these the plants themselves may harbour parasites 

 from one year to another in the roots and stems, which re-infect the fresh 

 growth produced in the following spring. 



Black Rot of cabbages is transmitted through the seeds, and the so-called 

 " Yellows " disease of Hvacinths is carried in the bulbs. The soil around 

 the plants may be a source of infection, and healthy plants grown in infected 

 soil will in all probability develop disease. ^lan and animals, especially 

 through the agency of dung heaps, help to spread certain diseases, and birds 

 have been suspected of transmitting some bacterial plant pathogens. 



Parasitic Bacteria are frequently followed by saprophytic ones which 

 complete the destruction of the tissues, and the wounds caused by one 

 bacterial disease may enable another and more virulent one to gain admission. 

 In general. Bacteria are unable to enter a plant except through some break 

 in the cell tissue of the host ; they cannot bore their way through a healthy 

 cell wall. 



The efi^ect on the host plant varies according to the particular disease. 



* A fuller treatment of bacterial plant pathogens will be found in Volume IV. 



