THE STEUCTURK OF THE ROOT 



377 



later. The apex of the root consists of a mass of merismatio 

 tissue in which the dermatogen, periblem, and plerome can be 

 distinguished. The separate histogenic layers are frequently 

 more distinct than in the case of the stem. The dermatogen 

 divides into a number of layers, the outermost of which form 

 a many-layered epidermis known as the root-cap (fig. 763, li). 

 The innermost layer is in most Dicotyledons continued back- 

 wards over the surface, and forms the so-called ^?7i/ero((s layer. 

 This is not, however, invariably the case. In most Mono- 

 cotyledons the dermatogen does not persist far from the apex, 

 and the external layer above the point of its disappearance is 

 the outer layer of the periblem. In either case, from the external 

 layer the special absorbent structures of the root, the root-hmrs, 

 are developed {fig. 711). 



The periblem of the root gives rise to a cortex which persists 



Fig. 764. 



Fig. 765. 



Fi'/. 764. Endodermis and perioycle of root of Iris florentina. en, I. Bnilo- 



tlermis. p.c. Passage cell. pe. Perioycle.. Fig. 765. Endodermis of 



root witli cogged tliickening. 



for a longer or shorter time. Its cells are generally parenchy- 

 matous with thin walls, and there are many intercellular spaces 

 among them. The hypodermal layer of this is known as the 

 exodermis, and its cells are frequently thickened as already 

 described (fig. 718). The endodermis is well marked, and its cells 

 are usually thickened considerably. "When, as in Iris florentina, 

 the thickening is regular round the internal and radial walls, there 

 are certain cells left unthickened { fig. 764, p.c). Very frequently 

 the thickening is deposited in the form of the band described at 

 page 338, the separate thick portions somewhat resembling the 

 projections of a cogged wheel {fig. 765). The thickenings are 

 cutioularised in both eases. In Equisetum the endodermis is 

 two laj ers of cells thick. 



The stele or central cylinder differs in many important 

 particulars from that of the stem. The pericycle is usually only 

 one layer of cells in thickness, though in the Gymnosperms and 



