240 



MORPHOLOGY 



Fig. 53S. — 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 

 DeBaey. 



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), 

 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 

 ■\'arious 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 ^'^^ ""'^ cambium or phellogen (c) cutting 



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



for a time through special structures, beneath epidermis); below are chlorophyll, 

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



