NATURE OF PLANTS 



73 



the cells at the angles or on all sides and by the silvery luster of 

 the walls. The walls, though very elastic and tough, are capable 

 of growth and so especially adapted to the support of the young 

 elongating stems. In older stems, where elongation has ceased, 

 greatly elongated cells, called stereome fibers or sclerenchyma 

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

 ends which interlock and bind the cells very firmly together 

 (Fig. 41, B). These strengthening cells form a compact zone 

 about the stem or they may be arranged in separate bundles, 



B 



Fig. 41. The stereome or strengthening cells of the cortex: /I , collenchyma 

 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 stereome of mature 

 flax stem. 



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

 bands extending along the surface of the stems of many herb- 

 aceous plants. The central region of the stem is characterized 

 by a mass of parenchyma with several vascular bundles arranged 

 in a circle (Fig. 40). The central mass of parenchyma is called 

 the pith. 



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

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

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

 two regions a delicate layer of cells, the cambium (Fig. 42). 

 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 



