BY R. GREIG SMITH. 247 



the organism in question into the tissues of the host. By the 

 reproduction of the symptoms of disease the etiological relation- 

 ship is established. This method serves very well in the case of 

 animals in which toxic symptoms can be readily observed, and 

 especially as the toxic products of the bacteria cannot as yet be 

 identified chemically. But with plants it is different. The toxic 

 effects are not as a rule decided enough to convince the sceptic, 

 and not unfrequently control plants develop disease as readily as 

 the inoculated. 



In my work with plant diseases I have always endeavoured to 

 produce the typical bacterial product in the laboratory and to 

 compare it chemically with the substance formed in the host 

 plant. This is, of course, not always capable of being done, but 

 in the present instance the production of arabinin the laboratory 

 by Bact. acacice is a much more decided proof that the bacterium 

 produces arabin in the plant than would be the production in the 

 plant after an infection with the bacterium, for the simple reason 

 that we could not be absolutely certain that the tree would not 

 have developed gum independently of the infection, possibly as 

 the result of a previous or a subsequent accidental infection with 

 the same or another organism. 



Still I made an infection experiment, but it was done more 

 with the idea of testing whether the bacterium of the wattle 

 could produce gum in the peach, the only susceptible tree which 

 chanced to be in my garden. The trees were about 5 feet high, 

 and were infected at places about 2J feet from the ground. 

 Three trees were selected. One of them forked at 2 feet, and 

 one fork was infected with J3act. acacice, the other with Bact. 

 metarahinum. A second tree was inoculated with Bact. acacice^ 

 and a third with Bact. metarahinum. 



Upon returning after the summer vacation the trees were care- 

 fully examined and ^um was found in two cases. One was the 

 tree infected with both bacteria, the other that which had been 

 inoculated with Bact. acacice. The third, infected with Bact. 

 metarahinum only, had no gum. The forked tree had been 

 inoculated with the two bacteria to see if different sjums would be 



