462 Harvey -Gib son. — Contributions to Knowledge of 
but not at all of them. The roots are developed at each fork, 
the ventral one becomes functional but the other develops 
into a leaf-bearing shoot. The functional root forks at 
a variable distance from the stem in a plane at right angles 
to the long axis of the stem, the next forking taking place 
at right angles to the previous one, and so on. The dorsal 
papilla develops in most cases into a leafy shoot from which 
at its base a functional root arises (PI. XX, Fig. 1 1). A section 
taken between the origin of the root and the main axis shows 
a central cylinder with an extended protoxylem and a slight 
lacuna. Just before the origin of the root the protoxylem 
breaks up and is distributed round the metaxylem. Beyond 
the point of origin of the root the stele differentiates into 
the usual three steles of the ordinary branch. A trans- 
verse section of an old aerial root shows a well-marked 
epidermis, a slightly sclerotic external cortex, followed by 
a thick cortex of large elongated cells. The stele has an 
endodermis with radial cuticularization, a pericycle three or 
four cells deep, then one or two rows of sieve-tubes followed 
by three or four layers of phloem-parenchyma. The xylem 
forms a crescentic band of scalariform and sometimes reticulate 
fibres, the largest elements being on the convex side, viz. that 
towards the apex of the shoot from which the root arose. 
The phloem-parenchyma turns the horns of the crescent and 
is continuous with the parenchyma, which entirely lines the 
concave side and separates the proto- and metaxylem by three 
or four layers of cells. The protoxylem is thus aggregated in 
a strap-like mass in the hollow of the crescent. The sieve- 
tubes stop abruptly at the horns of the crescent, but the 
pericycle is continuous round the horns and outside the 
protoxylem. In sections of smaller roots taken close to 
the stem the endodermis is not so well marked, the pericycle 
is thinner and the sieve-tubes are more numerous. The 
xylem is horse-shoe shaped with a break at the convexity 
and with an isolated protoxylem. Previous to the forking 
of the root the protoxylem-band also divides, and the two 
protoxylems take up positions one-third and two-thirds of 
