Robinson and Walkden. — A Critical Study of Crown Gall. 321 
the wounded surface, and to some extent also in the vessels, and in inter¬ 
cellular spaces of the cortex near to this. The later development of the 
gall is, for the most part, due to the active presence and multiplication of 
B. tumefaciens on the rough external surface of the gall. 
2. Most of the work of Erwin F. Smith regarding the production of 
secondary tumours has been repeated, but additional results have been 
obtained which show that the facts bear very different interpretations from 
those adopted by Smith. Both in Chrysanthemum frutescens and in Nico- 
tiana affinis we have definitely demonstrated, by staining, zoogloeal 
strands of B. tumefaciens intruding along intercellular spaces and proto- 
xylem vessels forming centres for pathological disturbance and .gall-pro¬ 
duction along the tract. This migration of the causal bacteria in experi¬ 
mentally inoculated shoots of Nicotiana affinis , in which little growth 
extension was possible, fully accounted for the secondary galls obtained. 
*On Chrysanthemum frutescens secondary galls have only been produced by 
inoculating the meristematic tissues near the apices of shoots. In these 
cases, and also in the galls resulting from the inoculation of the growing- 
point of Nicotiana affinis , the part played by the growth and extension of 
the wounded and inoculated tissues has been shown to be a very important 
additional factor in determining the form and distribution of the galls 
which arise. 
3. The primary and secondary galls and tumour-strands arise by 
a subdivision and subsequent proliferation of pre-existing cells of the host 
plant in the presence of the bacterial stimulus. There is no invasive growth 
of tumour-tissue over any considerable distance. The intrusive growth of 
the bacteria in the intercellular spaces and in protoxylem vessels, together 
with growth extension of inoculated tissues, fully account for the strands of 
tumour-tissue connecting the secondary galls with those arising at the points 
of inoculation. 
4. It is held that the results obtained regarding the distribution of the 
bacteria in the galls invalidate most of the close comparisons which have 
previously been made between crown gall and malignant tumours. 
Barker Cryptogamic Research Laboratory, 
University of Manchester. 
