=712. Origin of root tissues.=—A similar apical meristem exists in roots, but there is in addition a fourth region of formative tissue in front of the meristem called _calyptrogen_ (fig. 420). This gives rise to the “root cap” which serves to protect the meristem. The vascular cylinder in roots is very different from that of the stem. There is a solid central cylinder in which the groups of xylem radiate from the center and groups of phloem alternate with them but do not extend so near the center (fig. 421). As the root ages, changes take place which obscure this arrangement more or less. Branches of the roots arise from the central cylinder. In fern roots the apical meristem is replaced by a single four-sided (tetrahedral) apical cell, the root cap being cut off by successive divisions of the outer face, while the primary root tissues are derived from the three lateral faces. [Illustration: Fig. 420. Median longitudinal section of the apex of a root of the barley, Hordeum vulgare. _k_, calyptrogen; _d_, dermatogen; _c_, its thickened wall; _pr_, periblem; _pl_, plerome; _en_, endodermis; _i_, intercellular air-space in process of formation; _a_, cell row destined to form a vessel; _r_, exfoliated cells of the root cap. (After Strasburger.)] [Illustration: Fig. 421. Cross-section of fibrovascular bundle in adventitious root of Ranunculus repens. _w_, pericycle; _g_, four radial plates of xylem; alternating with them are groups of phloem. This is a radial bundle. (After De Bary.)] =Function of the root cap.=—The root cap serves an important function in protecting the delicate meristem or cambium at the tip of the root. These cells are, of course, very thin-walled, and while there is not so much danger that they would be injured from dryness, since the soil is usually moist enough to prevent evaporation, they would be liable to injury from friction with the rough particles of soil. No similar cap is developed on the end of the stem, but the meristem here is protected by the overlapping bud-scales. One of the most striking illustrations of a root cap may be seen in the case of the Pandanus, or screw-pine, often grown in conservatories (see fig. 447). On the prop roots which have not yet reached the ground the root caps can readily be seen, since they are so large that they fit over the end of the root like a thimble on the finger. =713. Descriptive Classification of Tissues.=