240 



MORPHOLOGY 



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 

 dicotyledonous 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), 



FIG. 538. -Stem tip of Hippuris (z di- which j s a hollow cy l m der of 



cotyledon), showing dermatogen (outermost .. 



layer), periblem (the five layers beneath llvm g cells - !t 1S exceedingly 



the dermatogen), and the plerome (central variable in Structure, but is char- 



region of more elongated cells). After acter ized by containing chloro- 

 DEBARY. 



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- FlG 53Q _ Transverse section of outer 



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



maintains connection with the air the cork cambium or phellogen (c) cutting 



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



for a time through special structures, beneath epidermis); be low are chlorophyll- 

 balled lenticels (fig. 540) Interrupting containing cells of the cortex. 



