22 TRANSACTIONS OF THE [ Sess. Lx xn. 
The epidermis is interesting. The cells are markedly 
thickened to the outside, at the pomt where they touch 
the root-cap, while the other sides of the walls remain 
unthickened (fig. 3, @ and b). These epidermal cells are 
concave to the outside, but gradually take on a more 
regular, flat-shaped form. Owing to the direction in which 
the epidermal cells lie, a longitudinal section of the root 
shows a transverse view of the epidermal cells; a transverse 
section shows. a longitudinal view of the cells. A special 
median section would show the epidermis as a layer quite 
distinct from the rest of the meristem tissue right to the 
apex, thus dividing the root-cap from the root itself. The 
cells of the epidermis increase through radial division, and 
the thickening of the outer part of the walls can be watched 
from very early stages upwards. 
In the second stage of the life-history of these aerial 
roots internal changes have also taken place. In a longi- 
tudinal section the root-cap is seen to be only slightly 
adhering to the root (fig. 4). The outer thicker layer has 
noticeably peeled off in layers, forming the tufted mass 
round the root, which can easily be detected with the naked 
eye (fig. 1, b). The inner layer, too, gradually loosens from 
the epidermis, the cells of which become gradually thickened 
to the outside (fig. 4, a). 
In the final or thorn stage the metamorphosis is complete ; 
the root-cap, if present at all, is so loose that it is more like 
a thimble on the root, like the root-pocket of the Lemnacez, 
and can easily be detached, leaving behind a bristle or thorn 
(fig. 1,c). The transverse sections of aerial roots at various 
stages in their life-history show more distinctly the process 
of hardening (figs. 5-9). The essential features shown in a 
transverse section are, the epidermis, the protective sheath 
(Schiitzhiille), consisting of thick cells (Dr. Friedrich says 
that he has observed this tissue in no other aerial root, but 
it is observable to some extent in palm pneumathodes, 
roots of Bromeliads, ete.), an outer cork tissue, an inner cork 
tissue with larger cells to the outside, small to the inside, a 
distinct endodermis, a typically palm vascular system xylem 
alternating with the phloem, and the pith. Transverse 
sections of the upper portion of the axis of the thorns show 
well-marked aerenchyma in the inner cortex. The figures 
