458 



A TEXTBOOK OF THEORETICAL BOTANY 



"1 "-"" iiiii -uimx. 



Cuticle 



Cutinized wall 



intercellular 

 wedges 



EpiderrDis 



Fig. 445. — Ficus elastica. Transverse section of leaf 

 showing cutinized epidermis. 



There are two main types of endodermis, which are thin or thick walled. 

 In both types the cells are somewhat elongated vertically and are at first thin 

 walled. In the type common to Pteridophyta and most Dicotyledons the 

 tangential walls remain thin, but a differentiated band forms on the radial 

 walls, completely encircling the cells. This band, which is impregnated 

 with lignin and with fatty acids, is called the Casparian band (Fig. 446). 

 It is usually much narrower than the cell and appears in transverse sections 

 as if it were a thickened zone on the radial walls, by which this type of 

 endodermis is easily recognized. Broad Casparian bands are usually 

 perforated by numerous pits. 



The thick-walled type of endodermis, characteristic of monocotyledonous 

 roots, is thickened with cutin, and sometimes partially lignified, either all 

 over, or only on the inner tangential and radial walls. Such a layer is naturally 

 impervious to water. Certain cells, however, called passage cells, remain 

 unthickened (Fig. 446). They lie opposite the protoxylem cells (see p. 803). 

 In cases where an endodermis becomes completely cutinized, as in old roots, 

 the cortex is cut off from its water supply and disappears, leaving the endo- 

 dermis as the external layer of the root. 



The Cork 



Corky or suberized cells form the phellem, a thick layer of impervious 

 cells which make up the outer bark of old stems and roots (Fig. 447). The 

 cells appear square or oblong in outline and often rather narrow in transverse 

 section, but in tangential view they usually show suturings like those of 

 epidermal cells. The cell wall contains a definite layer of suberin, next to 

 the middle lamella. Suberin is a mixture of fatty materials closely allied to 



