12 EFFECT OF BLACK ROT ON TURNIPS. 



common name. Frequently there is no external indication of tlie 

 cavity. For a painting- in perspective of such a root see Centralblatt 

 fiir Bakteriologic, 2 Abt., Ill Bd., PI. VI, tig. 1. 



THE METHOD OF INOCULiATION, ETC. 



On the frontispiece will be found a diagrannnatic representation of 

 a turnip plant inoculated with a pure culture of Pseudomonaii cain- 

 pestris. This figure is intended to represent plant No. 53, which was 

 inoculated on the blades of two leaves by means of delicate needle 

 punctures. The plant, was then some weeks old and about 9 or 10 

 or possibly 12 inches high. The material used for inoculation con- 

 sisted of a well clouded, moderately turbid bouillon culture (eleven 

 days old), which had been used for control in thermal-death point 

 experiments, and which was just beginning to throw down a small 

 amount of yellow precipitate. In other words, the culture was still in 

 active growth and in excellent condition for purposes of inoculation. 

 The original source of the organism was the interior of a turnip 

 such as that which furnished the cross section shown on PI. I, fig. 1. 

 The inoculations were made in the following manner: Selecting two 

 leaves five or six removes from the lowest 'leaf, some of the germ- 

 laden fluid was first removed from the tube on the end of a sterile 

 platinum loop and placed on the clean surface of the leaf blades, 

 and then from 75 to 100 delicate pricks were made through this fluid 

 into the leaf, by means of a fine-pointed steel needle, which was passed 

 through the'flame before and after use on each plant. Finally, a fresh 

 loop of the bacterial liquid was lifted out of the tube and spread 

 over these punctures. The punctures of themselves did not do the plant 

 any serious injury. The inoculated leaves were covered for an hour or 

 two after the punctures by means of clean white paper — i. e., until 

 sunset. This was done partly to avoid insolation and partly to prevent 

 a too rapid evaporation of the fluid from the surface of the leaf. The 

 leaf surfaces were not sterilized before inoculation for three reasons: 

 (1) Because it was desired to have the inoculations made under condi- 

 tions simulating as nearly as possible those occurring naturally; (2) 

 because numerous experiments had already shown that with a proper 

 selection of plants such as those used for this series of inoculations, 

 needle punctures unaccompanied by bacteria did not lead to disease; 

 (3) because exposure of the delicate leaves to mercuric chloride or 

 other strong germicides for a time suflicient to destroy all surface 

 spores would, probably, in spite of subsequent washings, have left 

 enough poison on the leaves to inhibit the growth of the parasite, if 

 not to seriously injure the plant. The pricked area on each leaf 

 included perhaps 2 to 3 square centimeters. 



