1384 ELEMENTS OF PLANT ANATOMY. 
suberized, and in this way corky layers are formed. Those within 
the phellogen layer, if any, grow to resemble the ground cells in 
form, and remain living, their walls never becoming suberized. 
Thus the outer or corky layers are suited to perform the functions 
of the primary epidermis, their suberized walls: rendering them 
almost impervious to air 
and water; while the inner 
layers add to the radial 
diameter of the rind. The 
increase in circumference is 
supplied in both cases by 
the formation of radial walls 
and the subsequent growth 
of cells so formed in the 
phellogen layer. 
= 
\ eae 
FiG. 57. 
Fia. 58. 
1. athe cuticle sprung off by the extension of 
the cellulose part of the wall below it. bthe a shows the cuticle; b epidermis with 
epidermal cells in process of division. c¢ the thickened outer wall ; c the outermost 
outer wall of the outer rind layer. cork cells which have originated from 
2. The corky layers, bb, have been developed. the layer of rind cells just under the 
The cells in the lower layer, 6, which have a epidermis ; d the phellogen cells, and 
nucleus, are the phellogen cells. ec the rind next to these the phelloderm contain- 
cells. —(Th. Hartig.) ing chlorophyll. — (Th. Hartig.) 
In all of the three classes described, there are suberized walls 
between the primary epidermis and the inner cells of the rind, 
upon which it depends for food. Lacking this it soon dies, and 
