246 CHANDLER—AERIAL ROOTS OF 
a forcing frame. After a short time, the aerial roots developed 
as ordinary roots, giving off lateral rootlets, and becoming quite 
white. In this respect, the roots examined differed in no way 
in their development from ordinary aerial roots which reach 
the soil. 
The structure of the unbranched portion of the aerial root is 
normal, The pith is large, a considerable quantity of wood 
tissue is present, the whole vascular area forming a normal 
complete ring (Fig. 11), A section of an older portion of the 
root, as compared with the young one, shows a somewhat 
smaller pith, more wood tissue, and a very marked cork 
cambium, with a large development of cork (Fig. 12). 
It is noteworthy that in the sections of the older portion the 
cork cambium consists of a layer of cells rounded, but lengthened 
in a transverse direction, with two layers of clear cells on either 
side, more squarish in outline (Fig. 12 cc.). 
The most interesting feature of these aerial roots is their 
terminal arrest in growth, and the consequent branching that 
takes place. The root tip appears to die off after the roots have 
attained a short length, and the tissues around the apex 
continuing development, cause a swelling encircling the dead 
tip after the fashion of a callus cushion (Figs. 3 and 7), and the 
whole end of the root has a club-like shape with a depression at 
the end a mm. or more in depth, at the bottom of which is 
the dead growing point of the root. From the tissues forming 
the margin of this depression there may sprout a circlet of 
branch rootlets (Figs. 3 and 7). In other cases the death of the 
tip of the primary root is followed by what is really a fission of 
the root. The tissues around the dead tip do not develop 
symmetrically to form a circular cushion, but on two sides of a 
median plane, and thus two usually flattened out-growths, equal 
or unequal in size, develop with the rooted root tip between 
them at their point of separation (Figs. 6, 10). Or again, from 
the point where the dead tip was, the whole root may become 
flattened—opened out, as it were—as a fasciated structure 
divided at its end into lobes of varying number (Figs. 4, 8)- 
We have two distinct things to look at here :— 
L. The lobes, as I have called them, which arise by the 
splitting of the end of the original mother root, as is illustrated 
by Figs. 4, 6, 8, ro. 
