194 SCIENCE PROGRESS 



Wakker's extended investigations (1883-89) into the nature of 

 the Yellow-stripe of hyacinths led the way in the study of an 

 interesting type of bacterial parasitism, involving a destruction of 

 the tissues which advances along the vascular bundles. A feature 

 of this disease is a blocking of the xylem vessels by a yellow, 

 gummy substance, followed by the dissolution of the cellulose 

 walls. Frequent inoculations always produced a recurrence of 

 the same symptomatic characters in healthy plants ; but these 

 were the result of direct infection experiments without the 

 intervention of culture media. 



A knowledge of good cultural conditions was not wanting 

 even at this early date. Klebs' and Brefeld's gelatine methods 

 of preparing culture media were already in operation ; the 

 introduction, in 188 1, of Koch's methods of isolation by means 

 of plate cultures simplified the preparation of pure cultures and 

 afforded further facilities for bacteriological research. Koch's 

 dicta, in 1883, established the recognised procedure necessary 

 for the definite determination of a disease due to a specific 

 organism, viz. : (i) it is essential that the organism be present 

 in the diseased tissues ; (2) that it be grown artificially in 

 suitable media for several successive generations ; (3) that 

 inoculations from the pure cultures so obtained should pro- 

 duce the same manifestations of the disease in healthy tissues ; 

 and (4) the same organism must be again isolated from the 

 artificially infected tissues. 



The knowledge which was available was, however, not 

 generally applied and unfortunately much of this early work 

 rested upon evidence which could not be regarded as con- 

 clusive, owing to imperfect methods of experiment and the 

 absence of proper precautions to ensure pure cultures. The 

 unsatisfactory character of some of the work in this sphere no 

 doubt contributed to maintain a disbelief in the part played by 

 bacteria pathogenic to plants. But when many careful inves- 

 tigations of a later date and an exact study of the life-history 

 left nothing to be desired, great reluctance was still shown to 

 admit the truth of these conclusions. Much doubt and opposi- 

 tion continued to be expressed towards the whole principle of 

 bacterial plant-diseases, by reason of established preconceptions 

 which were founded upon a misapprehension of the true nature 

 of the problem. 



Hartig considered the plant-organism protected from bac- 



