26 H. L. Russell. 



the tissues become so mature that the germ is not able to pass 

 from cell to cell. It is also particularly prominent in the blighting 

 stem, where the disease is usually confined to the youngest, most 

 succulent tissue. This is characteristic of bacterial plant-diseases in 

 general, that the most pronounced lesions are always found in tissue 

 which has not yet lost its power of growth. 1 



We turn now to consider the chemical sources of protection which 

 the plant may possess against its enemies. First of all we have the 

 chemical reaction of the plant-tissues. As was noted in the intro- 

 duction, it is to this source that most authors ascribe the general 

 freedom of plants from bacteria. This was before it was thoroughly 

 proven that vegetable structures were affected to any extent by bac- 

 terial diseases, and was probably based upon the then prevailing 

 idea that bacteria required alkaline and fungi acid substrata for 

 their development. So many exceptions to this law are now known 

 that this statement has lost much of its original force. Most of those 

 forms which we know to be able to cause bacterial plant-maladies 

 are usually more or less indifferent to the reaction of the medium, 

 growing in either weakly acid or alkaline nutrient media. Arthur 

 succeeded in raising B. amylovorus in 2 per cent malic-acid bouillon. 



The experiments already detailed indicate that saprophytes, and 

 even some animal pathogenic forms, are not destroyed by the plant 

 juices either within or outside of the plant. Forms like B. prodi- 

 giosus, B. lac. aerogenes, B. megaterium, B. coli commune, Kiel 

 water-bacillus and blue pus germ grew in the expressed plant juices of 

 various kinds and produced a considerable turbidity in 24-48 hours. 



It would seem, then, that the importance of this factor as the 

 prime cause of immunity and resistance has been considerably over- 

 estimated. Very much more stress is to be laid upon the mechan- 

 ical barriers which the plant possesses against the entrance of germs, 

 as well as the activity of the living protoplasm. 



The nutritive conditions offered by the plant may also be con- 

 sidered in this connection. While the dead tissues of many plants 



1 The observation which Prillieux noted in the case of B. Vuillemini, which causes 

 the tumors in the old wood of the Aleppo pine (Pinus halapensis), is only apparently 

 contradictory. The fact that a local hypertrophy of tissue was produced by means 

 of the bacterial stimulus showed that the tissues were yet in a secondary meri- 

 stematic condition. 



2 Zopf: DiePilze, S. 173. 



