THE STEM AND THE LEAF 



47 



One-year-old stems of dicotyledonous plants which are not 

 climbers usually differ in structure from the type shown in 

 Fig. 29 mainly in having the bundles more or less completely 

 joined into a continuous cylinder (shown in the cross section 

 as a ring, Fig. 30). 



45. Strengthening cells. There are several kinds of cells 

 which give either toughness, stiffness (Sect. 46), or both of 

 these qualities to the parts of the plant body 

 where they occur. Only four of these kinds 

 need be mentioned in this place. The two 

 shown in Fig. 32 are commonly found in the 

 cortex of dicotyledons. Collenchyma cells (A) 

 are like the thin-walled cells of the pith, but 

 are reenf orced at the 

 angles, just as some 

 packing boxes have 

 strips of board nailed 

 fast on the inside of 

 the box at the junc- 

 tions of the sides. 

 Bast filers (^) are 

 extremely slender 

 tubes, with closed 

 and pointed ends, 

 much like a piece of 

 thermometer tubing 

 drawn to a point in 

 a gas flame and thus 

 closed. Collenchyma 

 gives moderate stiff- 

 ness to the parts in 

 which it occurs, and is highly elastic, so that it does not 

 hinder the growth of the stem which it incloses. Bast fibers 

 are flexible but very tough, and therefore enable the parts 

 of the root, stem, or leaf in which they occur to resist being 

 pulled apart. 



FIG. 32. A, collenchymatous and other tissue 



from stem of balsam (Impatiens) ; J?, a group of 



hard-bast fibers 



e, epidermis ; c, Collenchyma ; i, intercellular spaces 



between large parenchyma cells; a, cut-off ends; 



b, lengthwise section of fibers. Greatly magnified. 



A, after Strasburger; B, after Tschirch 



