70 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, 7. e., 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 Mal 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 
