GEORGE K. K. LINK 605 



and also along the axis of the plant by rapid elongation of the stem. It has, however, 

 been well established that the legume nodule organisms pass from cell to cell in the 

 form of zoogleal strands. Nixon's' studies of fire blight indicate that penetration of 

 tissues by zoogleal strands may be more general in bacterial diseases than has been 

 realized. 



Effects upon plant tissues. — The cellular responses of the host to the presence of 

 bacteria may be necrotic or hyperplastic or hypoplaspic or a combination of these. 

 The latter type generally is evidenced by suppressed differentiation or suppressed 

 enlargement of cells. 



In crown-gall there is a development of xylem elements from cells which usually 

 do not develop xylem. It is doubtful, however, whether this is a real case of meta- 

 plasia, because it is not a case of direct change of a permanent cell of one type into 

 another type, but rather an instance of changing tissues, which are more or less per- 

 manent, into a meristematic state, and rendering these totipotent whereas the cells 

 in that location usually are not totipotent. 



It is contended that the bacteria present in the leaf teeth of Ardisia crispa, and 

 to a lesser extent in the entire plant, even the seed, are real mutualists.^ It is assumed 

 that the local injury done in the form of chlorosis of the leaf serratum is more than off- 

 set by the nitrogen which they are assumed to contribute. A similar claim is made for 

 the Pavettas by von Faber.^ Fred, Whiting, and Hastings-' report conclusive proof 

 that the legume-nodule bacteria alone can assimilate atmospheric nitrogen and thus 

 contribute nitrogen to the host plant, thereby offsetting their local injury to the roots. 



"extra-vegetal" life of bacterial plant pathogens 

 This topic is of interest because it involves the phenomena of parasitism and sapro- 

 phytism and also that of sources of inocula. Every one of the described plant patho- 

 gens can be grown on culture media that are not at all specialized. In other words, 

 these organisms are not in the main highly specialized so far as food requirements are 

 concerned. Many, however, lose their pathogenicity after long culture on artificial 

 media. Some of them are known to live in the soil, such as Bad. tumefaciens and the 

 legume-nodule organisms, while it is suspected that many others also are capable of 

 soil life. Others, although they are not spore-formers, can live and endure on the pro- 

 tective surfaces of plants or in or on plant debris even under extremely trying condi- 

 tions of temperature, desiccation, and illumination. Bact. campestre, for example, can 

 live on the seed coat of cabbage seed and consequently is readily introduced by this 

 route into the seed bed. 



insect dissemination 



Insects not only disseminate bacteria mechanically^ but there are also two and 

 possibly three experimentally demonstrated cases of internal biological dissemination 

 of various pathogens by insects. The olive fly, Dacus oleae, carries the pathogen of 

 olive knot, Bact. savastanoi, in its salivary glands and intestinal diverticula as a regu- 



' NLxon, E. L.: "Fire Blight," Pa. State Coll. Bull. 203, 3-22. 1926. 

 ^ Mielie, H.: Ber. d. Deutsch. Bot. Gesell., 34, 576-80. 1916. 

 3 von Faber, F. C: Jahrb.f. wiss. Bot., 54, 243. 1914. 



■• Fred, E. B., Whiting, A. L., and Hastings, E. G.: "Root Nodule Bacteria of Leguminosae," 

 Wisconsin Agr. Exper. Sta. Res. Bull. y2. 1926. 



5 Waite, M. B.: "Results from Recent Investigations in Fire Blight," Bot. Gaz., 16, 259. 1891. 



