126 ANATOMY OF STEM, ROOT AND LEAF 



of the primary wood, namely, tracheae or vessels, tracheids, fibres, 

 fibrous cells and wood-parenchyma; the vessels and tracheids, 

 however, are never spirally or annularly thickened, but usually 

 marked with bordered pits and reticulate thickenings. 



All these elements may be present or only a few ; for example, 

 the wood of the yew consists of tracheids only, that of the bulk 

 of coniferous trees of tracheids and wood-parenchyma, while 

 the wood of most dicotyledons contains all the above-mentioned 

 elements. 



The elements of the secondary bast are similar to those of 

 the primary bast, namely, sieve-tubes with their companion-cells 

 and parenchyma; bast-fibres and living fibrous cells are also 

 present in some cases. After functioning for a short time as 

 conductors of food, the sieve-tubes, companion-cells and most of 

 the bast-parenchyma become empty, and in the older parts are 

 compressed into an irregular mass in which no cell cavities are 

 visible. When firm thick-walled bast-fibres are abundant, as 

 in lime and other trees, the bast in transverse sections appears 

 in the form of thin, ring-like bands. 



Besides the production of wood and bast, certain cells of the 

 cambium-ring become changed into medullary ray cells {m, 

 Fig. 56); the primary medullary rays existing between the 

 first-formed vascular bundles of the unthickened stem are con- 

 tinued by the interfascicular cambium when thickening begins 

 and therefore always extend right through from the pith to 

 beyond the bast. Totally new secondary medullary rays are 

 subsequently started by certain cells of the cambium ring at 

 successive irregular intervals during the growth in thickness. 

 These new medullary rays extend from the annual rings of 

 wood in which they first appear to the corresponding bast 

 rings on the opposite side of the cambium ; they are therefore 

 of variable length. 



The medullary rays are of variable width even in the same 

 stem. Sometimes they are only one cell thick and in trans- 



