THE RELATIONS OF CROWN-GALL TO OTHER 
OVERGROWTHS IN PLANTS 
ERWIN F. SMITH 
Bureau of Plant Industry, U. S. Department of Agriculture 
In the time assigned the most I can hope to do is to give the 
barest outline of the suggested relationships. Some of these are well 
determined; others are only suspected and are mentioned here as 
hopeful lines of research rather than as definite conclusions. Indeed, 
I am quite willing to admit that our work on crown gall has opened up 
more problems than it has settled, but, one way or another, all of this 
present uncertainty will make for progress and an eventual better 
understanding of the whole mechanism of overgrowth. My own 
belief is that all overgrowths are correlated phenomena, are the 
response of the organism to essentially similar (but not necessarily 
identical) stimuli, the visible difference in response when brought 
about by parasites being due to number and location of the parasites, 
age and kind of tissues invaded, and volume, direction, and velocity 
of the stimulus exerted. In other words, in every case, I think the 
stimulus is primarily a physical stimulus due to changed osmotic 
pressures rather than a direct chemical stimulus. Overgrowths, 
therefore, do not always involve the presence of a parasite although 
as observed in nature parasites are probably responsible for most of 
them. 
I. Factors governing type of overgrowth in crown gall. 
A. The host reaction depends on the type of tissue infected. 
(1) Vascular vs. parenchymatic. For example, depend- 
ing on the tissue in which it originates the vessels 
in a tumor may be numerous or few, the paren- 
chyma abundant or scanty. 
(2) Nexus of cells stimulated, 7. e., unipotent, multi- 
potent, or totipotent cells. Thus, according to the 
tissues infected by the crown gall schizomycete, we 
have it causing either organoid galls or histioid 
galls. 
(3) Rate of growth. The rate of growth depends on the 
readily available supply of food and water, on the 
age of the tissues when infected and on the species 
448 
