320 Robinson and Walkden.—A Critical Study of Crown Gall . 
internodes of the stem of Nicotiana ajfinis and of the apex of Chrysanthemum 
frntescens are in greater or less part due to the different potentialities of 
development and growth in the two cases. In the internodes of tobacco 
practically no growth in length took place after preparing the surfaces for 
inoculation, while in the Chrysanthemum the immature leaves and stem of 
the inoculated bud continued their growth and development, although this 
took place under the influence of modifications introduced by the presence 
of the bacteria. We have thus throughout our work found it necessary and 
instructive to consider the resulting growths in the light of the subsequent 
development of the host plant after inoculation, and it has been seen that 
this development plays an important part in determining the form and dis¬ 
tribution of the structures which arise. This continued development of 
plants, as contrasted with animals, must be borne in mind when comparisons 
such as we have been considering are instituted, and our results show that 
for this reason alone superficially similar structures, in the two cases, may 
have quite different methods of origin. 
Whilst thus compelled by the facts we have described for crown gall 
to dissent from the close comparisons with cancer referred to above, we 
would, nevertheless, agree that there are features in the plant disease which 
suggest more general comparisons with malignant disease. In both cases, 
for example, cells and tissues are stimulated to active atypical proliferation, 
and it is possible that a real insight into the nature of the changes taking 
place under the influence of Bacterium tumefaciens might throw light on the 
changes taking place as a result of unknown causes in the cells of the animal 
tumours. We have as yet few facts relating to such features of crown gall, 
but we regard the cell-wall changes referred to in the body of our paper, 
and certain precipitation effects we have observed in the gall-tissues, as 
indicating the direction of further study in this connexion. Similarly we 
have made some preliminary steps in the investigation of the metabolic 
changes and the respiratory activity of the plant cells under the influence of 
B. tumefaciens , and we are not without hope that future investigation of the 
sequence of changes resulting from the activity of this organism upon the 
tissues of higher plants will throw light upon little-understood problems of 
tissue differentiation and of development in the healthy plant. 
Summary. 
i. The development of the galls produced by the inoculation of cut 
surfaces of Chrysanthemum frntescens with B. tumefaciens has been traced 
from the earliest stages. 
The effect of the bacterial stimulus is to produce a growth very similar 
in form, structure, and general appearance to callus growths on woody 
shoots arising as a result of wounding. At first the bacteria are located on 
