Observations on Squamosis and Exanthema of the Citrus . 133 
mosis develops freely in actively growing shoots, but less readily and 
extensively when growth is no longer active. 
The relation of growth to gummosis is also brought out by a study of 
the anatomy of the disease. The gum pockets are formed, it will be 
remembered, in a sickle of susceptible tissue laid down symmetrically 
on both sides of the wound. This tissue will be found, upon close examina- 
tion, to show all the characters, especially noticeable in the medullary rays, 
of having been rapidly formed. The greatest growth will have taken place 
in the immediate neighbourhood of the wound, the least at the edge of the 
sickle. The most extensive gum pockets occur necessarily in the area 
of greatest growth. If now one studies sections taken from diseased shoots 
that have shown various degrees of growth activity, he will observe that the 
size of the sickle of susceptible tissue formed is directly proportional to the 
rapidity of growth ; he will also observe that the gum is the more fluid the 
more sapid the tissues. 
The question will be asked, Why is the growth of the susceptible tissues 
more marked in the neighbourhood of the wound, and less and less exten- 
sive as one proceeds nearer to the apices of the sickle ? It is generally 
supposed that this is due to a response of the plant to traumatic stimuli 
which are necessarily more marked near the place of origin, but an explana- 
tion less vague, I think, can also be offered. It is a matter of common 
observation that, when a ligneous branch is pruned in the proper position 
with respect to a bud, a callous tissue forms and the bark becomes slightly 
raised ; again, when the trunk of a vigorously growing tree is slit longitu- 
dinally, marked growth occurs, and the two lips of the bark are pushed 
apart. In these two cases, what, in reality, takes place ? It appears to me 
simply this. The cambium is capable of laying down new tissue elements 
with extreme rapidity under favourable conditions of growth, but, owing to 
the pressure exerted by the cortex, the number of cells laid down in a given 
time are fewer in number than would be the case if the pressure were 
removed. When, therefore, through any kind of a wound, the pressure 
exerted by the cortex is more or less greatly reduced the genetic power of 
the cambium proportionally increases. As the pressure of the ruptured 
bark from approximately zero will increase gradually as one passes away 
from the wound, finally reaching the height it had attained before the 
release was effected, it must necessarily follow that the activity of the 
cambium will suffer a gradual reduction, and we are prepared to understand 
the fusoid development of the susceptible tissues. In a similar manner we 
will be able to explain the greater length attained by the gum pockets 
upwards from the point of initiation than downwards. The growth of the 
cambium depends on the amount of elaborated material that the phloem is 
able to supply, and it must necessarily follow that when it is destroyed 
the cambium below the lesion must decrease in activity. The amount of 
