THE STRUCTURE AND GROWTH OF STEMS 163 



to see are always more difficult to understand than facts 

 which may be seen with the naked eye. 



It will not be so easy to understand the structure of 

 stems as it was to understand the structure of roots. The 

 structure of roots is of just one general kind ; the structure 

 of stems is of two general kinds. The arrangement of 

 tissues in the root is simple ; the arrangement of tissues in 

 the stem is complex. 



So you are warned to give this section particularly care- 

 ful attention in order to get it straight ; the structure of 

 stems is a matter which high school students are in the 

 habit of getting mixed. Before reading ahead, you should 

 review the section on the structure and growth of roots. 

 {Section 43.) 



A. Contrast with Roots. — Epidermis, cortex, and stele 

 are the three great divisions of the tissues of roots. They 

 are also the three great divisions of the tissues of stems. 

 The stele of stems often includes a central tissue called 

 pith. Roots do not have pith. 



Meristem is tissue whose cells retain the power to divide 

 into new cells. You learned of it in connection with roots. 

 In stems it is found principally in two places : in the center 

 just behind the growing tip, and in older parts near the 

 edge of the stele. The meristem near the growing tip is 

 called primary meristem ; that in older parts is secondary 

 meristem. Primary growth is growth due to the enlarge- 

 ment of cells formed by the primary meristem ; secondary 

 growth is due to the enlargement of cells formed by second- 

 ary meristem. Secondary meristem in stems is also called 

 cambium. 



Roots have the radial arrangement of xylem and phloem. 



