MORPHOLOGY OF HIGHER PLANTS. 197 



joining cortical parenchyma. The exodermis takes the place of 

 the epidermis when the latter is worn oflF, except in the few cases 

 where hypodermal cork becomes developed, as in Cephalanthus, 

 Solidago, and Bignoniacese. The root bark is parenchymatic ; 

 being commonly referred to as the cortex, and is either homcn 

 geneous or divided into two zones, the outer or peripheral being 

 composed of thick-walled cells which naturally belong to the 

 exodermis and an inner or internal strata made up of thin-walled 

 cells. The cells of the cortical parenchyma may contain starch, 

 calcium oxalate, calcium carbonate and there may be associated 

 with them secretory cells, frequently referred to as " ducts," as 

 " resin ducts," etc. The innermost layer of cells of the cortex 

 is quite distinct and known as the endodermis (EN). It consists 

 always of a single layer of cells, without any intercellular spaces, 

 and the radial walls show in transverse section Casparyan spots,' 

 depending upon a local folding of the cell-wall, which is here 

 suberized. In the course of time the cell-walls of the endodermis 

 frequently become thickened, either all around, or only on the 

 inner or radial walls, so that we might speak of an O-endodermis 

 as in Honduras sarsaparilla (Fig. 194) or an U-endodermis as 

 in Mexican sarsaparilla (Fig. 194), according to the manner of 

 thickening. This is especially the case in the monocotyledons 

 where the walls of the endodermal cells become completely suber- 

 ized and impermeable to water. In some roots the cells of 

 the endodermis may be uniformly thick-walled throughout, while 

 in others some of the cells may remain thin-walled, and these cells, 

 the so-called " transition cells " or " passage cells," form channels 

 of communication between the cortical parenchyma and the vessels 

 of the stele ; they are therefore located just outside the peripheral 

 vessels of each ray of the hadrome (or xylem). 



Inside the endodermis is the stele, formerly called the central- 

 cylinder. In this the peripheral stratum, sometimes composed 

 of two or three layers of cells, represents the peri-cambium. 

 The cells are generally thin-walled and in Dicotyledons and 

 Gymnosperms are able by cell-division to form cork and secondary 

 cortex, but in all vascular plants it is capable of giving rise to 



' Physiologische Pfianzenanatomie. By Dr. G. Haberlandt, p. 245. 



