64 



STEM STRUCTURE 



chyma fibers, are formed. These cells have thick walls and 

 tapering ends which interlock and hind the cells very firmly 

 ther i Fig. 40. B). These strengthening cells form a com- 

 pact zone about the stem or they may be arranged in separate 





5 



P 







Fig. 40. The stereome or strengthening cells of the cortex: .-/, collen- 

 chyma cells in growing stem of Begonia, showing above the cells in cross- 

 section and below in longitudinal section, B, a group of elongated thick- 

 I cells, called sclerenchyma fibers or stereome fibers, from the stere- 

 ome of mature flax Stem. 



bundles, in this latter case they often appear to the eye as light 

 colored hands extending along the surface of the stems of many 

 herbaceous plants. The central region of the stem is charac- 

 terized by a mass of parenchyma with several vascular hundles 

 arranged in a circle ( Fig. 39). The central mass of parenchyma 

 i- called the pith. 



33. The Vascular Bundle.— The vascular bundle contains three 

 distinct regions; an inner thick-walled portion (the wood or 

 xylem), an outer thin-walled portion (phloem), and between 

 th. se two n gions a delicate layer of cells, the cambium < Fig. 41 1. 

 The transport of all substances is largely confined to tin 

 cular hundles. the xylem conducting principally the crude ma- 

 terials while the hulk of the organic Bubstances passes through 

 the phloem. We are now interested to study the characi 



these cells and note their adaptation to the work in hand. In 



cur various large spaces, tin vessels or ducts (Fig. 

 ;i. . 1. and smaller spaces, w 1 cells of different kinds. The 



cells of the phloem are much smaller, thinner walled and less 



numerous than those of the : re ven thin 



