FORMATION OF NODULES. 
263 
with the central vascular system. The cells of the strand are 
carried sideways in both directions, and become scattered in 
small fragments along the whole length of the leaf scar. These 
cells are functionless, and may contain easily decomposable 
substances ; the decomposition products would set up a state of 
irritation in the surrounding cortical cells. These would begin 
to divide and a cambium would arise, which would form cells 
round the point of irritation so as to isolate it from the adjacent 
healthy cells, and so a nodule would be formed. Bateson 
points out that this theory accounts only for nodules occurring 
on leaf scars, and suggests that for the formation of a nodule 
it is probably necessary to have only a small point of irritation ; 
thus, local death of cortical cells, from various causes, might 
give rise to nodules in areas outside the leaf scars. He states 
further that this theory does not account for nodules occurring 
on tapped surfaces, where the cortex containing remains of leaf 
bundles is pared away. 
A short note by Bateson (6) in a later Bulletin announces his 
discovery that the irritant present in the cortex is the coagu- 
lated latex in old latex vessels. This causes burrs to originate 
in both untapped and renewing bark. 
Kuijper in 1913 gave a detailed account of the structure of 
nodules (16) ; he mentions the presence of a brown point or 
line in the centre of the nodule, consisting of ordinary cortical 
parenchyma cells, and occasionally a single sclerenchyma cell. 
These are surrounded b}^ wood elements arranged radially 
round the brown centre, consisting of wood parenchyma with 
tracheidal elements and libriform fibres. Towards the peri- 
phery of the nodule the wood parenchyma is disposed in groups 
between other wood elements. Cells resembling wood vessels 
occur. The whole is enclosed by a cambium. The wood 
fibres are strongly curved and of irregular outline. In the 
cortex are found brown points and lines consisting of dead 
cell groups round which cambial activity sets in ; this represents 
the first stage in the formation of a nodule, but the origin of 
the dead cell groups is obscure. Kuijper, after close exami- 
nation, concludes that plant and animal parasites play no part, 
and that nodule formation is induced by tapping or otherwise 
wounding the tree. 
