THE STRUCTURE AND GROWTH OF STEMS 173 



develops in stems with scattered bundles, the bundles 

 gradually become limited, of course, to that hollow cylinder 

 which forms all that there is of the stem. This is not to 

 be confused, however, with a cylinder which is composed 

 exclusively of the bundles. All stems are solid at their 

 growing tips ; that is, at the point where the primary meri- 

 stem lies. It is in the region of elongation that hollowness 

 first appears. An old-fashioned bamboo fishing pole is an 

 excellent example of a hollow stem whose bundles have the 

 scattered arrangement. As you may have noted, however, 

 and as the bamboo shows very well, hollow stems are solid 

 at their nodes. 



F. Cells of the Bundles. It is not important that you 

 should know many of the very many kinds of cells 

 which may be found in the vascular bundles of dif- 

 ferent plants ; special cells which appear in connection 

 with the bundles of one plant may not appear at all in 

 connection with the bundles of another, and with such 

 special cells we are not now concerned. It is important, 

 however, that you know something of the few principal 

 kinds of cells found in wood and bast. Already you know 

 something of the difference in appearance between phloem 

 cells and xylem cells, at least as they are seen in cross 

 section. When viewed lengthwise, however, cells show the 

 real nature of their structure far better than when viewed 

 crosswise. (See Figure 60.} 



The cells of the wood that conduct water are called 

 tracheary vessels. Wood also includes cells of smaller 

 diameter called fibers; the fibers have pointed ends and 

 are usually very strong and tough ; they, more than the 

 vessels, give toughness to timber. The tracheary vessels 



