240 



MORPHOLOGY 



Fig. 538. — Stem tip of Hippuris (a di- 

 cotyledon), showing dermatogen (outermost 

 layer), periblem (the five layers beneath 

 the dermatogen), and the plerome (central 

 region of more elongated cells). — After 

 DeBary. 



Epidermis. — The dermatogen passes into the epidermis, which is 

 usually a single layer of protective cells closely interlocked and with 



relatively impervious walls, but 

 pierced by stomata. In case the 

 stem increases in diameter, as in 

 dicotyledon* >us trees, the epidermis 

 is usually ruptured and destroyed, 

 and another protective layer is 

 developed by the cortex, as de- 

 scribed below. 



Cortex. — The periblem pro- 

 duces the cortex (figs. 539, 541), 

 which is a hollow cylinder of 

 living cells. It is exceedingly 

 variable in structure, but is char- 

 acterized by containing chloro- 

 phyll tissue abutting against the 

 epidermis; and if it is thick enough, there is also a deeper region of 

 the cortex free from chlorophyll. The layer of cortical cells abutting 

 against the stele often forms a very distinct bounding layer, like an 

 inner epidermis, and is called the endodermis (fig. 379). In the cortex, 

 strands of fibrous cells may be de- 

 veloped, and cavities or canals of 

 various kinds may occur. 



In case the stem increases in 

 diameter, the cortex develops a 

 meristematic layer known as the 

 phellogen or cork cambium (fig. 

 539), which forms cork cells, a very 

 impervious kind of cell (see p. 318). 

 Continuous activity of the phellogen 

 within results in an increasing thick- 

 ness of the sheet of cork cells with- 

 out, and such sheets form a most 



efficient protection. The chloro- Fig. 539. -Transverse section of outer 

 phyll tissue beneath the cork cells portion of cortex of a geranium, showing 



maintains connection with the air thc cork cambium or phellogen (0 cutting 



,.,,., off layers of cork cells (four such layers 



for a time through special structures, beneath epidermk) . below arc chlorophyll- 

 called Icnticels (fig. 540) , interrupting containing cells of the cortex. 



