30 BORTHWICK—ADVENTITIOUS ROOTS AND RELATION 
bundles was carefully studied in the specimens in order to make 
quite sure that there should be no chance of mistaking any of 
these several bundles for those of the adventitious roots. 
THE ADVENTITIOUS Roots.—The microscopical structure of 
these roots agrees very closely with those in the other species. 
The transverse series, in fact, presents no new features. 
If the axis-cylinder be examined under a high power, Plate X., 
Fig. 38, the various tissues are found to present very character- 
istic root-like features. The xylem-strands stand out conspicu- 
ously, and between them lie the smaller fine-walled cells of 
phloem. A well-marked endodermis and pericycle may also be 
seen. The conspicuous ring of dark-walled celis belongs to the 
endodermis. The ring just inside it is the pericycle, whose cells, 
especially those opposite the intervals between the xylem-strands, 
are in an active state of division. Immediately outside the 
endodermis ring occur groups of cells with distinctly-marked 
dot-like cuticularisations on their radial walls. 
Those roots originate from medullary rays, which become 
much broadened out and swollen at their ends (Plate XI., Fig. 42). 
The cambium of the stem becomes arched in a vaulted manner, 
and runs round the periphery of the club-headed medullary ray. As 
the internal protuberance increases in size, the cortex of the twig 
is raised up into a papilla. The phellogen-layer does not appear 
to take any part in the formation of this papilla. Ultimately the 
periderm is ruptured and the root is seen protruding through the 
torn ragged collar just like an ordinary soil-root(Plate XI., Fig.43). 
It will be seen from this figure that the cambium of the stem 
is directly continuous with that of the root; further, the continua- 
tion of the root back into a wedge-shaped medullary ray is quite 
apparent. To the left of the old root may be seen a younger one 
in course of development. 
In the apple-stem the endcdermis is by no means a well- 
defined layer, but the pericycle may be recognised by the patches 
of thick-walled sclerenchyma-fibres, which are developed in it. 
The cells of the pericycle between those patches are of a paren- 
chymatous nature. The hard sclerenchyma-strands offer con- 
siderable resistance to the outgrowing root. Those on the 
flanks of the root become bent out of their course and displaced 
toone side. Those directly in front are forced outwards to some 
