WINTER BUDS, SHOOTS, ETC. 3°5 



If we happened to examine quite old portions of these stems, we would have 

 observed that a large part or the entire portion of the thin-walled tissue, sep- 

 arating the woody portions of adjacent bundles, had changed to thick-walled 

 or woody tissue, so that there is here in the older portions of the sunflower 

 plant a continuous ring of xylem, This is the case also to some extent with 

 the bast tissue. We already have noticed that the cambium ring in these 

 stems is a continuous one, although the cambium between the bundles of the 

 sunflower plant was not so active as that in the bundle proper. There is, 

 however, a difference between the tissue lying between adjacent bundles and 

 that of the bundle itself. 



574. The bundles in the ash stem and in other woody stems lie very 

 closely side by side, so that at first it might appear as if they were continu- 

 ous. We note, however, that there are radiating lines which extend from the 

 pith out toward the bast. These run between the bundles. These radiat- 

 ing lines are formed by the tissue lying between the bundles becoming 

 squeezed into thin plates, which extend up and down between the bundles. 

 They are termed the medullary rays,* since they radiate from the pith or 

 medulla. These are shown well in a section of an oak stem. 



575. Difference in the firmness of the woody ring. — We have already 

 noted that the inner portion of the wood zone contains more and larger ducts, 

 than the outer zone, and that in the outer portion of the same zone the woody 

 fibres predominate. The ducts are formed during the early spring growth, 

 and later in the season the development of the fibres predominates. 



576. Annual rings in woody stems. — If we now cut across a . shoot of the 

 ash which is several years old, we will note, as shown in fig. 400, that there 

 are successive rings which have a similar appearance to the woody ring in 

 the one-year-old stem. This can well be seen without any magnification. 

 The larger size of the woody ducts which are developed each spring, and 

 the preponderance of the fibres at the close of each season's growth, mark 

 well the growth in diameter which takes place each year. 



577. While the thickened walls of all the cells give strength to the wood, 

 the different kinds of cells vary in the percentage of strength which they 

 give. Thus the bast cells which have very thick walls are yet more flexible 

 than the wood fibres, as can be seen if one strips off some of the bark of the 

 basswood tree. Again, the woody fibres give more strength to wood than 

 the same diameter of wood vessels, because they are much more firmly 

 bound together, and the ends are long and tapering, and are spliced over 

 each other where cells below and above meet. In the case of the wood 

 vessels the ends do not taper out so much, or in some cases they meet ad- 

 jacent cells below or above squarely. 



578. Wood then which has a large number of wood vessels compared with 

 the fibres, or in which the size of the vessels is great, is not so strong as 



* Rays, or radiating plates, of tissue appear also in the bundle. 



