^8 BOTANY OF CROP PLANTS 



4. Wood Fibers.— Give strength to the stem. 



Growth in Thickness of Dicot Stem. — Medullary ray cells 

 give rise to cambium that joins with the cambium in the 

 vascular bundles. Thus there is formed a continuous cam- 

 bium ring (Fig. 15). At the end of the first year of growth 

 or the beginning of the second, another sort of cambium, 

 called cork cambium, is differentiated in the outer cortex. 

 Growth in thickness of the stem consists then in the produc- 

 tion and growth of new cells from: (i) cambium of vascular 

 ring, and (2) cork cambium. The cambium cells of the 

 vascular ring may differentiate into xylem, or phloem, or 

 remain cambium. Each cambium cell divides by a wall 

 which is parallel with a tangent to the outside of the stem. 

 If the inner cell resulting from the division becomes a xylem 

 element, the outer usually remains cambium. On the other 

 hand, if the outer cell resulting from the division becomes a 

 phloem element, the inner remains cambium. Hence, by a 

 division of cambium cells of the vascular ring, new xylem 

 is laid down on the outside of the old xylem, and new phloem 

 is laid down on the inside of the old phloem. Not only 

 do the vascular bundles grow in a radial direction, but also 

 somewhat laterally. This lateral growth of existing vascular 

 bundles, together with the formation of new ones between 

 the old ones, brings about a narrowing of the medullary rays, 

 so that in an old stem they appear as mere lines or rays radiat- 

 ing from the pith or medulla. And, furthermore, the wood 

 comes to form quite a solid ring, as does also the phloem. 



In addition to the increase in stem thickness by the pro- 

 duction of more xylem and phloem^ the cork cambium cells 

 aid in this process. Cork cambium cells which divide by a 

 wall that is parallel to a tangent of the stem, give rise to 

 cork tissue, and to secondary cortex. Hence, each year, there 

 are produced in the dicot stem: 



