6 4 



STEM STRUCTURE 



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

 tapering ends which interlock and bind the cells very firmly 

 together (Fig. 40, B). These strengthening cells form a com- 

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



FIG. 40. The stereome or strengthening cells of the cortex: A, 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- 

 walled 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 bands 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 bundles 

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

 is 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 

 these two regions a delicate layer of cells, the cambium (Fig. 41). 

 The transport of all substances is largely confined to the vas- 

 cular bundles, the xylem conducting principally the crude ma- 

 terials while the bulk of the organic substances passes through 

 the phloem. We are now interested to study the character of 

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

 the xylem occur various large spaces, the vessels or ducts (Fig. 

 41, v), and smaller spaces, wood cells of different kinds.. The 

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

 numerous than those of the xylem. Therefore very thin sec- 



