. A manual of botany. Botany. 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 mo
. A manual of botany. Botany. 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. Fi'/. 764. Endodermis and perioycle of root of Iris florentina. en,I. Bnilo- tlermis. Passage cell. pe. 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, ). 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 im
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