THE STEM AND THE LEAF 



63 



groups: one with no continuous woody cylinder, like the stein 

 of such a climber as the Dutchman's-pipe (fig. 42, A), and one 

 with a continuous woody cylinder, like that of the sunflower 

 (fig. 42, j5). The real difference between the two kinds of stem 

 is that, like most climbers, the stem of the Dutchman's-pipe 

 begins the season's growth with a set of separate fibro vascular 

 bundles which remain separate, while in the sunflower the 

 bundles are at first sepa- 

 rate but soon join each other. 

 The boundary between bark 

 and wood is a layer of thin- 

 walled cells (c, fig. 42, A), 

 the cambium layer. It is 

 this cambium, its cells filled 

 with mucilaginous proto- 

 plasm, that makes up the 

 slimy layer just outside 

 the wood, from which the 

 bark peels so readily in 

 early spring, when boys are 

 making whistles or strip- 

 ping off sheets of slippery- 

 elm bark. It is important 

 to notice that each fibro- 

 vascular bundle consists of 

 an outer portion, o, which belongs to the bark, and an inner 

 portion, iv, which belongs to the wood. 



A much better idea of the details of structure of the several 

 regions of the stem can be gained from a lengthwise section, 

 like that shown in figure 43, than from cross sections, like 

 those of figure 42. 1 The uses of some of the parts shown 

 in figure 43 are briefly stated on the following page. 



1 Since the type of stem structure shown in figure 43 is not exactly like 

 that of the other two figures, it will not be possible to identify all the kinds 

 of cells shown in figure 43 with those of the other two. Note especially that 

 in figure 48 the cambium is not readily distinguished from the overlying 

 tissues, and that no distinct layer of heavy-walled bast fibers is found. 



Fu;. 43. Lengthwise section of a young 

 dicotyledonous stem 



<>p, epidermis; cort, cortex; 6, bast; v, ves- 

 sels; tth, sheath surrounding pith ; p, pith; 

 si, sieve cells of the bast. Magnified about 

 90 diameters. After Bonnier and Sablon 



