7 BACTERIA IN RELATION TO PLANT DISEASES. 



the parts still alive and growing. Others reach the surface of the living plant, if at all, 

 principally through natural openings, i. c, stomata or lenticels. 



I do not know of any bacteria confined to the sieve-tubes, or particularly at home 

 therein, but probably this only amounts to saying that the whole field has not been surveyed. 



Parasitic bacteria, limited to the woody tissues of trees and shrubs or supposed to be 

 peculiarly at home therein, have been described but are unknown to the writer. Examples 

 cited in literature are Mai nero of the vine, and Janse's disease of dadap trees, both abund- 

 antly doubtful etiologically. The writer has seen bacteria-like bodies in fossil woods, but 

 these woods may have been occupied by them after submersion in some swamp. He has 

 also seen a yellow Schizomycete very abundent in the wood of pear trees attacked by pear- 

 blight, but this had no pathogenic properties. Viala and Ravaz found a very abundant 

 multiplication of a Schizomycete in the vessels of vine cuttings buried in sand to be used 

 later as grafts, but the organism was unable to propagate itself in the living, growing plant 

 when these cuttings were used either as grafts or scions, and cultures made from these 

 bacteria had no pathogenic power when inoculated into the vine. There is no apparent 

 reason, however, why the wood of living trees should be wholly exempt from the attacks 

 of bacteria, and cases will probably be discovered in which bacteria are confined pretty 

 closely to the woody tissues. They must of course be sorts able to live on a minimum 

 quantity of water. 



The highest type of bacterial disease, and the most interesting from many standpoints, 

 is that in which all the growing tissues, pith, wood, cambium, and bark are involved, and 

 are stimulated into abnormal, excessive growth, death occurring only after extensive 

 hyperplasia. These overgrowths may attack roots or shoots. Good examples are olive- 

 tubercle, pine-tubercle, beet-tubercle, the daisy knot, and crown-gall of the peach. In 

 the olive, oleander and daisy they often occur on the leaves. Metastatic tubercles and 

 secondary tumors occur. This type will be discussed more at length under Reactions of the 

 Plant. There appear to be two forms of these overgrowths. In the olive tubercle, bacterial 

 cavities occur and the organism is abundant in them, and is easily observed wedging its 

 way between cells. In the crown-gall no such cavities have been observed, the causal 

 organism is difficult to detect with the microscope, and its location in the tumor tissue 

 appears to be unlike that of the olive-tubercle organism. Moreover, plate cultures show 

 that it is not very abundant in the tissues, at least in a viable form. The olive tubercle 

 organism occupies intercellular spaces. The crown-gall organism occurs within the rapidly 

 dividing cells, as in case of the root-nodule organism of Legumes, but less abundantly 

 and does not form a bacterial strand. 



MASS-ACTION OF BACTERIA. 



A few words are necessary on the mass-action of bacteria. It is a common observation, 

 one made by the writer at least a hundred times, that in culture-media not exactly adapted 

 to the needs of an organism, a scanty inoculation may not give any growth not even 

 after a long time whereas a copious one will lead to a growth which gradually clouds the 

 fluid or covers the solid. The penetration of bacterial strands from cell to cell in the root- 

 nodules of Leguminosae is another example (figs. 21, 22). The only explanation I can 

 think of is that a multitude of the bacteria are stronger than a few, and thus by union are 

 able to overcome obstacles too great for the few. The same fact comes repeatedly to the 

 attention of the animal pathologist as a result of his inoculations. The animal body, we 

 must assume, is often able to overcome and destroy a few hostile organisms, where it would 

 not be able to defend itself against many; otherwise whole races would be exterminated 

 by natural infections. The same is undoubtedly true in plants. The modus operandi in 

 plants is not altogether clear. We may advance several hypotheses: (1) The formation 

 of a resistant cork-layer before the bacteria have multiplied to such an extent as to prevent 



