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ting 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, x 3. _p_, pith. _c_, bark. The radiating lines are medullary rays. _r_, resin ducts. _B_, part of the same, x 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, x 150. _D_, longitudinal section of a branch, showing the form of the tracheids and the bordered pits upon their walls. _m_, medullary ray, x 150. _E_, part of a sieve tube, x 300. _F_, cross-section of a tracheid passing through two of the pits in the wall (_p_), x 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, x 150. _H_, cross-section of a resin duct, x 150. _I_, cross-section of a leaf, x 20. _fb._ fibro-vascular bundle. _r_, resin duct. _J_, section of a breathing pore, x 150. _i_, the air space below it.] The growing tissue (cambium), separating the phloem from the wood, is made up of cells quite like those of the phloem, into which they insensibly merge, except that their walls are much thinner, as is always the case with rapidly growing cells. These cells (_B_, _cam._) are arranged in radial rows and divide, mainly by walls, at right angles to the radii of the stem. If we examine the inner side of the ring, the change the cells undergo is more marked. They become of nearly equal diameter in all directions, and the walls become woody, showing at the same time distinct stratification (_B_, _x_). On examining the xylem, where two growth rings are in contact, the reason of the sharply marked line seen when the stem is examined with the naked eye is obvious. On the inner side of this line (_I_), the wood cells are comparatively small and much flattened, while the walls are quite as heavy as those of the much larger cells (_II_) lying on the outer side of the line. The small cells show the point
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