—" Fs” - 
10 
The outer part of the cortex, immediately underlying the epidermis 
to a depth of three to six layers, is composed of cells smaller than the 
deeper-lying ones. The walls of these, while they are in young and active 
parts of the roots, are very thin, with a notably dense protoplasm. Sub- 
sequently the walls thicken, those of the outermost cells first, until the 
lumen is almost obliterated; they acquire a stony hardness and dark 
color (figs. 3 and 4), thus forming a closed shell around the root, 
protecting it against animals or fungi and having a mechanical value 
already mentioned. ‘The imperviousness of this shell to water is shown 
‘by its effect on the epidermis and on the formation of pneumathodes. 
The zone in which the hypodermis forms is that at which the root ceases 
to absorb water from the soil. 
The larger the root, the farther from the tip is this likely to be. In very 
active ones the distance is as much as 5 centimeters; in those less active, but by 
no means inert, having a diameter of 7.5 millimeters, it is found to be 2 centimeters 
from the tip, while during drought it advances to a position well within the 
firmly adherent part of the cap. 
Between the hypodermis and endodermis the cortex is composed of 
rather large cells, isodiametric or somewhat elongate longitudinally, with 
thin, colorless walls, watery contents, and numerous intercellular spaces 
(fig. 5). After the layers bounding it reach their final state, parts of the 
interlying cortex become unequally thick walled and lignified. At the 
basal end of old roots this intermediate cortex breaks, probably as a result 
of tension between the elastic stele and nonelastic shell, leaving the 
former loose inside of the latter. 
The dermatogen is questionably distinguishable around the growing 
point, even in most favorable sections. ‘The epidermis is a transitory 
tissue, dying when its connection with the inner part of the root is inter- 
rupted by the development of the hypodermis. Its most conspicuous 
feature is that the least diameter of its cells is the longitudinal (figs. 6-8). 
No root hairs are ever formed, but the superficial area is slightly increased 
by the breaking apart of the outer ends of the cells—a process which is 
most evident in longitudinal sections. In a soil where the supply of 
water is even moderately constant and ample the coconut root, with its 
short absorbing zone and absence of hairs, would be regarded as but a 
poor water gatherer, but when water is abundant, hairs are not needed ; 
and in a dry time their sacrifice is spared to the coconut. A tree whose 
normal economy is planned on the absence of root hairs is comparatively 
well able to survive periods of abnormal difficulty in obtaining water. 
Pneumathodes (figs. 9-14).—The development of the hypodermal shell 
so completely cuts off the interior of the root from all communication 
with the outside that it can not carry on the limited exchange of gases 
necessary for its respiration, and therefore it is obliged to develop special 
breathing organs, or “pneumathodes.” These are specialized roots which 
quickly grow to a length of from 3 to 6 millimeters and then abruptly 
