146 
Notes . 
when an aerial root gives off a secondary root which dips back into 
the soil, the latter at once becomes thicker, but does not form tertiary 
roots, as do the secondary roots arising near the same spot on the 
aerial root. Another peculiarity is that a developing aerial root is 
longer than a subterranean root of the same age. Thus the aerial 
roots, while in actual process of growth, increase in length at a greater 
pace and branch more copiously than subterranean roots ; but the 
aerial roots are thinner and their growth only lasts for a definite 
period. 
Histology of the Subterranean Roots . — These roots possess a well- 
developed velamen, consisting of about eleven layers of tracheides ; 
as also an epidermoidal layer (or exodermis), and about seventeen 
layers of cortical cells. The cortical cells, with the exception of the 
exodermis and one or two external layers of small cells, are struc- 
turally remarkable. Their walls have coarse broad reticulate thick- 
enings which are lignified ; the thin portions of the wall consist of 
pure cellulose and have small pits in them. Here and there in the 
cortex are large idioblasts, elongated in the direction of the long axis 
of the root, and possessing thick suberised walls ; in their youth these 
cells contain raphides, which are subsequently dissolved. 
In the vascular cylinder there are about fifteen radial series of large 
wood-vessels separated from the strands of phloem by thick-walled 
parenchyma. 
Histology of the Aerial Roots . — An aerial root, having a central 
cylinder not quite as large as that of the subterranean root described, 
has the following structure. Externally is an epidermal layer which is 
only one cell in thickness, excepting here and there where a cell is 
divided into an outer and an inner half. Each cell is flattened radially, 
but is elongated in the direction of the long axis of the root ; its walls 
have delicate reticulate thickenings, which are directed at right angles 
to the long axis of the cell. The exodermis has thinner walls than in 
the subterranean root. The rest of the cortex is comprised of eight 
layers of cells, which are longer in the direction of the long axis of the 
root, but narrower than those of the subterranean root. In these 
cortical cells there is a tendency to form broad prominent bars of 
lignified membrane continuing from cell to cell, rather than to form 
the coarse irregular fenestrated thickenings seen in the underground 
root. The idioblasts are fewer in number and have thinner walls. In 
the vascular cylinder there are no radial lines of wide wood-vessels, 
