NATURE'S WOODCRAFT: A CHAPTER ON STEMS 



209 



FIG. 261. DICOTYLEDONOUS STEM. 



Diagram of transverse section. The eight nbro-vascular 



bundles are seen embedded in the ground tissue (?/). (m) 



Medulla or pith, (cr) Cambium ring. 



which the inner (A) is called the 

 wood or xylem (Greek xfdon, wood 

 or timber), and the outer (B) the 

 liber* or phloem (Greek phloios, 

 bark) ; and it is to these growing 

 cells that all increase of the woody 

 bundle is due. They are filled, 

 indeed, with protoplasm, and in 

 the growing season are constantly 

 undergoing division to form new 

 cells, by which means new wood is 

 added to the outside of the xylem, 

 and new liber to the inside of the 

 phloem. All the woody bundles of 

 the stem are, in a way, united by 

 the cambium, which forms an un- 

 broken ring in the stem, those 

 portions of the ring which lie be- 

 tween the bundles being known 



as interfascicular cambium (fig. 261, cr). As the cambium remains dormant 

 during the winter, and the cells which it forms in the spring are larger 

 than those of the autumn, the extent of its work each year may be easily 

 traced indeed, the concentric rings of wood in the trunk of a dicotyledo- 

 nous tree are the abiding records of its annual and annular labours, and 

 furnish means of forming a fairly accurate computation of the age of the 

 tree. The interfascicular cambium 

 serves the double purpose of 

 lengthening the medullary rays (see 

 fig. 258; and adding fresh phloem and 

 xylem between the original bundles. 

 In fact, it assists in completing the 

 circles of the liber and wood, thus 

 making the stem one solid whole. 



It should be added that all 

 Flowering Plants do not have the 

 fibro-vascular bundles arranged in 

 the manner above described. In 

 Monocotyledons Palms, Lilies, 

 Grasses, etc. they are scattered 

 irregularly in the stem; nor are 



* The name liber was applied by the 

 Romans to the inner bark or rind of a tree FlG " ^- 



used for paper. Our word "library" traces A trangverse section of thfa MonTCOtvledoa showin? th 



back tO It. closed fibro-vascular bundles embedded in ground tissue. 



17 



