2 75 
in the Root and Stem of Dicotyledons . 
The root-tubers grow to an enormous size. They form 
huge irregularly rounded swellings, which are often between 
two and three inches (5 to 7*5 cm.) in diameter. The root above 
and below them is cylindrical and not more than about 5 
mm. thick. The root-tubers are deeply furrowed in the 
longitudinal direction. The furrows, which are usually four 
in number, correspond in position to the four primary xylem- 
groups of the root, the ridges between them to the primary 
phloem. As compared with the ordinary root, the tubers 
show most interesting changes of structure consisting (1) in 
increased centrifugal activity of the normal cambium, with 
production of an excessive amount of parenchyma, the rays 
bearing a large proportion to the wood ; (2) in enormous 
dilatation of the pre-existing parenchyma, especially in the 
region immediately surrounding the primary xylem. This 
dilatation results in the breaking up of the inner lignified 
portions of the secondary wood into isolated strands im- 
bedded in parenchyma. Often the dilatation of the tissues 
bordering on the primary xylem nips off, as it were, the outer 
ends of the xylem-plates. Where this happens the protoxy- 
lem may be carried by the continuous extension of the tissues 
behind it, far out into the surrounding parenchyma. The 
result is very curious. In one case two out of the four xylem- 
plates were unaffected and remained in their original position. 
In a third xylem-plate the dilatation had separated off the 
outer part at a rather late stage, so that the protoxylem was 
only carried a little way out. The fourth protoxylem-group 
however had been nipped off quite early, and was now 
stranded in the parenchyma at a distance from the rest of the 
primary xylem equal to many times the diameter of the 
original vascular cylinder. During the dilatation every 
isolated group of lignified tissue forms a centre around which 
active cell-division goes on, and owing to these local meri- 
stematic regions the dilated parenchyma loses all trace of its 
former regular radial arrangement. The growth of the tuber 
is accompanied by a great increase in the amount of its inter- 
xylary phloem. The tertiary phloem-bundles are especially 
