698 
Journal of Agricultural Research voi. xxvn, no. 9 
The root projections on the stem-tumors carrying darkened and 
usually infected tissue deep into the tumors, and the hard woody nature 
of the outgrowths themselves, make isolations difficult. When Bac¬ 
terium tumefaciens did not appear on the isolation plates, it was thought 
due to the dry or old condition of the material used, and it was not 
until 1923, when young, tender stem-tumors were studied along with 
older tumors, that the conclusions given in this paper were reached. 
Whether or not the woolly apple aphis is the primary cause of the 
stem-tumors on mature apple stems herein described has not been within 
the scope of this investigation. There is a possibility, of course, that 
an organism may be transmitted by the aphids into the stems of apple 
trees—an organism which undergoes a change through insect transmis¬ 
sion and which on that account does not retain its ability to infect after 
it has been isolated. This might explain the appearance on agar plates 
of colonies isolated from apple stem-tumor which at first so strikingly 
resemble the crowngall colonies, but lose this resemblance when cultured 
artificially and which also fail to infect when inoculated into apple trees 
and other plants susceptible to Bacterium tumefaciens . Had crowngall- 
infected apple trees been in the neighborhood of the apple trees affected 
by stem-tumor, the writer would be inclined to give attention to this 
possibility. But she found none. It seems, therefore, that Bad . tume¬ 
faciens is eliminated from the consideration. 
