A careful examination of a cross-section of the bark shows first, if taken from a branch not more than two or three years old, the epidermis composed of cells not unlike those of the leaf, but whose walls are usually browner. Underneath are cells with brownish walls, and often more or less dry and dead. These cells give the brown color to the bark, and later both epidermis and outer ground tissue become entirely dead and disappear. The bulk of the ground tissue is made up of rather large, loose cells, the outer ones containing a good deal of chlorophyll. Here and there are large resin ducts (Fig. 76, _H_), appearing in cross-section as oval openings surrounded by several concentric rows of cells, the innermost smaller and with denser contents. These secrete the resin that fills the duct and oozes out when the stem is cut. All of the cells of the bark contain more or less starch. The phloem, when strongly magnified, is seen to be made up of cells arranged in nearly regular radiating rows. Their walls are not very thick and the cells are usually somewhat flattened in a radial direction. Some of the cells are larger than the others, and these are found to be, when examined in longitudinal section, sieve tubes (Fig. 76, _E_) with numerous lateral sieve plates quite similar to those found in the stems of ferns. [Illustration: FIG. 76.--Scotch pine. _A_, cross-section of a two-year-old branch, × 3. _p_, pith. _c_, bark. The radiating lines are medullary rays. _r_, resin ducts. _B_, part of the same, × 150. _cam._ cambium cells. _x_, tracheids. _C_, cross-section of a two-year-old branch at the point where the two growth rings join: _I_, the cells of the first year's growth; _II_, those of the second year. _m_, a medullary ray, × 150. _D_, longitudinal section of a branch, showing the form of the tracheids and the bordered pits upon their walls. _m_, medullary ray, × 150. _E_, part of a sieve tube, × 300. _F_, cross-section of a tracheid passing through two of the pits in the wall (_p_), × 300. _G_, longitudinal section of a branch, at right angles to the medullary rays (_m_). At _y_, the section has passed through the wall of a tracheid, bearing a row of pits, × 150. _H_, cross-section of a resin duct, × 150. _I_, cross-section of a leaf, × 20. _fb._ fibro-vascular bundle. _r_, resin duct. _J_, section of a breathing pore, × 150. _i_, the air space below it.]