=Microscopic Structure of Roots.=--Near the end of any young root or shoot the cells are found to differ from each other more or less, according to the distance from the point. _This differentiation takes place in the region just back of the growing point._ To study growing points, use the hypocotyl of Indian corn which has grown about one half inch. Make a longitudinal section. Note these points (Fig. 47): (_a_) the tapering root-cap beyond the growing point; (_b_) the blunt end of the root proper and the rectangular shape of the cells found there; (_c_) the group of cells in the middle of the first layers beneath the root-cap,--this group is the growing point; (_d_) study the slight differences in the tissues a short distance back of the growing point. There are four regions: the =central cylinder=, made up of several rows of cells in the center (_pl_); the =endodermis=, (_e_) composed of a single layer on each side which separates the central cylinder from the bark; the =cortex=, or inner bark, (_e_) of several layers outside the endodermis; and the =epidermis=, or outer layer of bark on the outer edges (_d_). Make a drawing of the section. If a series of the cross-sections of the hypocotyl should be made and studied, beginning near the growing point and going upward, it would be found that these four tissues become more distinctly marked, for at the tip the tissues have not yet assumed their characteristic form. The central cylinder contains the ducts and vessels which convey the sap. [Illustration: FIG. 47.--GROWING POINT OF ROOT OF INDIAN CORN. _d_, _d_, cells which will form the epidermis; _p_, _p_, cells that will form bark; _e_, _e_, endodermis; _pl_, cells which will form the axis cylinder; _i_, initial group of cells, or growing point proper; _c_, root-cap.] =The Root-cap.=--Note the form of the root-cap shown in the microscopic section drawn in Fig. 47. Growing cells, and especially those which are forming tissue by subdividing, are very delicate and are easily injured. The cells forming the root-cap are older and tougher and are suited for pushing aside the soil that the root may penetrate it. =Region of most Rapid Growth.=--The roots of a seedling bean may be marked at equal distances by waterproof ink or by bits of black thread tied moderately tight. The seedling is then replanted and left undisturbed for two days. When it is dug up, the region of most rapid growth in the root can be determined. Give a reason why _a root cannot elongate throughout its length_,--whether there is anything to prevent a young root from doing so. [Illustration: FIG. 48.--THE MARKING OF THE STEM AND ROOT.] In Fig. 48 is shown a germinating scarlet runner bean with a short root upon which are marks made with waterproof ink; and the same root (Fig. 49) is shown after it has grown longer. Which part of it did not lengthen at all? Which part lengthened slightly? Where is the region of most rapid growth? [Illustration: FIG. 49.--THE RESULT.]