CHAPTER III. ORGANS OF PLANTS. 



189 



In woody stems of this kind, every year during the season of 

 growth, and for a time even in herbaceous stems, the cells of the 



cambium-layer divide by fission in a 

 tangential direction, and those inte- 

 rior and next the wood are developed 

 into xylem, and so add to the thick- 

 ness of the woody region, while those 

 exterior are developed into phloem 

 elements, and increase the thickness 

 of the liber. The newest wood, 

 therefore, is its outermost layer, 

 while the newest bark is its inner- 

 most layer. 



The interior and older xylem 

 tissues of woody stems commonly 

 become strongly lignified and con- 



Fig. 450. 



Fig. 451. 



Fig. 450, A and B.— Diagrams of fibro-vascular system of stem of Clematis vitalba, a 

 Dicotyledon. 



A. Longitudinal view of upper part of stem, rendered transparent to show course of 

 bundles, a, central cylinder, showing six fibro-vascular bundles arranged in a circle at its 

 periphery: b b' , bundles passing off to leaves ; c c' , rudimentary leaves near apex of stem. 



B. Transverse section of young mternode. a, a bundle with an outer, phloem part, an 

 inner, xylem part and an intervening meristem area; b, pith; c, medullary ray; d, primary 

 cortex. Both reduced from figures by DeBary. 



Fig. 451. — Transverse section of a stem of Menispermum Canadense. in the third year 

 of its growth, a. fascicular cambium: b, interfascicular cambium; c, a crescentic mass of 

 bast fibers in the outer phloem: d, a ring of growth; e, a medullary ray; _/*, pith; g, the so- 

 called medullary sheath; //.collapsed cells of the soft bast in the phloem. Magnification 

 14 diameters. 



