[Illustration: FIG. 94.--_A_, cross-section of the stem of the shepherd's-purse, including a fibro-vascular bundle, × 150. _ep._ epidermis. _m_, ground tissue. _sh._ bundle sheath. _ph._ phloem. _xy._ xylem. _tr._ a vessel. _B_, a young root seen in optical section, × 150. _r_, root cap. _d_, young epidermis. _pb._ ground. _pl._ young fibro-vascular bundle. _C_ cross section of a small root, × 150. _fb._ fibro-vascular bundle. _D_, epidermis from the lower side of the leaf, × 150. _E_, a star-shaped hair from the surface of the leaf, × 150. _F_, cross-section of a leaf, × 150. _ep._ epidermis. _m_, ground tissue. _fb._ section of a vein.] A microscopic examination of a cross-section of the older root shows that the central portion is made up of radiating lines of thick-walled cells (fibres) interspersed with lines of larger, round openings (vessels). There is a ring of small cambium cells around this merging into the phloem, which is composed of irregular cells, with pretty thick, but soft walls. The ground tissue is composed of large, loose cells, which in the older roots are often ruptured and partly dried up. The epidermis is usually indistinguishable in the older roots. To understand the early structure of the roots, the smallest rootlets obtainable should be selected. The smallest are so transparent that the tips may be mounted whole in water, and will show very satisfactorily the arrangement of the young tissues. The tissues do not here arise from a single, apical cell, as we found in the pteridophytes, but from a group of cells (the shaded cells in Fig. 94, _B_). The end of the root, as in the fern, is covered with a root cap (_r_) composed of successive layers of cells cut off from the growing point. The rest of the root shows the same division of the tissues into the primary epidermis (dermatogen) (_d_), young fibro-vascular cylinder (plerome) (_pl._), and young ground tissue (periblem) (_pb._). The structure of the older portions of such a root is not very easy to study, owing to difficulty in making good cross-sections of so small an object. By using a very sharp razor, and holding perfectly straight between pieces of pith, however, satisfactory sections can be made. The cells contain so much starch as to make them almost opaque, and potash should be used to clear them. The fibro-vascular bundle is of the radial type, there being two masses of xylem (_xy._) joined in the middle, and separating the two phloem masses (_ph._), some of whose cells are rather thicker walled than the others. The bundle sheath is not so plain here as in the fern. The ground tissue is composed of comparatively large cells with thickish, soft walls, that contain much starch. The epidermis usually dies while the root is still young. In the larger roots the early formation of the cambium ring, and the irregular arrangement of the tissues derived from its