8oo 



A TEXTBOOK OF THEORETICAL BOTANY 



that perhaps absorbed water is carried through the cell walls rather than 

 through the cells, and that the endodermis is the real limiting layer in this 

 operation. If that be so, increase of the outer surface is not of great signifi- 

 cance in water absorption, but it may nevertheless play an important part in 

 the absorption of dissolved salts, which penetrate by diffusion through the 

 cell surfaces. This would be especially the case in a watery medium, where 

 the concentration of salts is often lower than in the soil. 



The Cortex. 



The cortex in roots is usually wide and the cells are rounded, so that the 

 intercellular air spaces are well developed even in land plants. The cortical 

 cells in many roots, especially in Monocotyledons, are arranged in regular 

 radial rows, and the air spaces when formed are usually also radially arranged 

 (Fig. 795). In water plants the air spaces of the inner portion of the cortex 

 become very extensive and a large aerating system is formed. 







Piliferous layer 

 Outer cortex 



Intercellular space 



Inner cortex 

 Endodermis 



,< .T;V 



^gf, Xylem vessel 

 Phloen) 



v V^'*' 



Fig. 795. Phragjnites communis (Great Reed). Trans- 

 verse section of the root showing lacunar cortex and 

 reduced vascular system. 



The endodermis is morphologically the innermost layer of the cortex, 

 not the outermost layer of the stele. The demarcation of the stele at the 

 growing point depends on the formation of the pericycle layer ; the 

 endodermis is differentiated later. 



The first and most characteristic stage in the development of endodermal 

 cells is the appearance, at a very early period, of the Casparian band, which 

 surrounds each cell like a belt, on the radial and horizontal walls. This belt 



