26 BORTHWICK—-ADVENTITIOUS ROOTS AND RELATION 
the cortex which have been pushed aside and displaced. The 
line of displaced sclerenchyma-fibres on the right and the white 
streak formed by splitting to the left indicate approximately 
where the protruding parenchy ma-cone abuts on the cortex. At 
its apex is seen a patch of disorganised tissue which is again 
caused by sclerenchyma. The prolongation of the wood- 
cambium around the protruding medullary ray is very clearly 
seen. This cambium-mantle lays down tissue on both sides. 
The cells deposited on the inside become thick-walled and 
lignified, while the tissue on the outside retains its parenchymatic 
character and appears as a light-coloured zone. 
A transverse section near the apex of a mature outgrowth 
shows a central and a cortical mass of tissue surrounded by a 
peripheral layer of cork. In sections taken lower down it is 
quite easy to make out endodermis, pericycle, phloem, xylem, 
and pith. The arrangement of the vascular elements is that 
characteristic of roots as may be seen in Plate X., Fig. 30, 
which illustrates the appearance of a transverse section taken 
through the base of the protruding portion ofa root. In it the 
first traces of xylem may be distinguished, between which lie 
the three patches of phloem rendered conspicuous by the large 
white cells which one usually finds associated with the phloem 
in maples. The pith runs out into three arms, at the ends of 
which occur the xylem-patches. Outside the pericycle and 
endodermis comes a relatively broad cortex with its outer layer 
of cork-cells. Plate X., Fig. 28, shows a transverse section 
through the basal portion of an older and more mature root. 
The pith has become thick-walled and sclerosed. The three 
patches of xylem have become more elongated and the phloem 
is more pronounced. In other cases, not figured here, however, 
secondary growth in thickness was found to have taken place. 
In Plate X., Fig. 28, the outer cork-layer is also more marked 
than in Plate X., Fig. 30. Although the figures given here 
show tetrarch construction, diarch and triarch bundles were of 
quite as frequent occurrence. 
A longitudinal section of a mature aes ray-root with its 
central cylinder, cortex, and outer cork-layers is shown in Plate 
X., Fig. 26, but the features of interest to be noted here are the 
two lenticular patches placed right and left at the base of the 
