PILIFEROUS LAYER AND EXODERMIS 67 



appear as tubular unbranched outgrowths with bluntly rounded 

 tips. The greater part of any root-hair is occupied by a large 

 vacuole continuous with that of the epidermal cell and filled 

 with sap. There is consequently only a thin lining layer of 

 cytoplasm, which is best seen near the tip where the single 

 nucleus usually lies embedded. 



Beneath the withered piliferous layer, in the older part of 

 the root, lies the exodermis (Fig. 28, Ex.), a layer of protective cells 

 which are on the whole rather smaller than the adjacent cells 

 of the cortex. Their brownish, slightly thickened walls are 

 chemically changed (i.e. suberised, cf . p. 136) in such a way that 



Co. 



FIG. 28. Piliferous layer (in part withered) and exodermis of the root of 

 the Creeping Buttercup (Ranunculus repens). Co., cortex ; Ex., exo- 

 dermis ; r., root-hairs arising from cells of piliferous layer ; St., starch. 



they are almost impermeable to water ; but this alteration does 

 not usually take place till the root-hairs begin to wither, so that 

 the water they absorb can readily pass inwards to the vascular 

 cylinder. In some cases, however (e.g. in most Monocotyledons), 

 where the exodermis differentiates at an early stage, thin-walled 

 passage-cells (Fig. 33 P.), through which the water travels, occur 

 at regular intervals. 



The cell-walls of the cortex become thinner towards the centre, 

 and intercellular spaces (Fig. 27, i.p.) are abundant. The latter 

 form a continuous system' which permits of gaseous exchange 

 with the aerial parts of the plant. The vascular strand is de* 



