FORMATION OF NODULES. 
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round latex vessels belonging to the same latex vessel cylinder. 
Some latex vessels are surrounded by cortical cells, whose 
walls show the yellow colouration only. Round adjacent 
latex vessels the cortical cells have yellow coloured walls and, 
in addition, an abundant tannin content. Further, some of 
these last show cell division beginning in the neighbouring 
cortical cells. The diffusion of this yellow colouring matter 
appears to stimulate the cells to the production of tannin ; 
possibly the tannin is secreted as a means of protection against 
the poisonous effects of the yellow colouring matter. 
The cell walls of the latex vessels and other elements in the 
central core are thickened, often strongly thickened. The 
cell walls are lignified and take on all the lignin stains. The 
fact that the latex vessel walls are lignified indicates that the 
lignification occurs subsequently to the inception of the nodule. 
The other lignified cells are ordinary cortical cells, and have 
no resemblance to the stone cells of the cortex ; their thickened 
walls are not striated and pitted as are the walls of stone cells. 
Stone-cell groups occasionally occur in the core as accidental 
inclusions. 
Starch is found in the core in varying quantity, and crystals 
of calcium oxalate may also be seen. 
(6) Wood Elements. 
These consist of cubical wood parenchyma cells, medullary 
rays, tracheides, short tortuous vessels, and fibres. 
The cubical wood parenchyma cells are the first productions 
of the nodule cambium, and immediately surround the central 
core. They are disposed in a regular manner radiating out 
from the centre. 
The medullary rays are similar to normal medullary rays, 
and also radiate out from the centre. They can often be 
traced continuously from the cortex mto the nodule, right 
across the nodule, and into the cortex beyond again. The 
nodule cambium thus lays down medullary ray cells conti- 
nuous with those in the cortex. This facilitates the transport 
of food material into the nodule and its storage as starch, 
which is sometimes very abundant. A little tannin may 
occur in the medullary rays. 
