78 



STEM STRUCTURE 



receiving stations for the carbohydrates during their transport 

 through the stem. The cells in the outer portion of the cortex 

 frequently become thickened and more or less elongated to give 

 strength to the stem. One of the most common modifications of 

 this kind is shown in Fig. 41, A. This tissue, collenchyma, is 

 characterized by the thickening of 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 

 they are especially adapted to the support of the young elongating 

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

 greatly elongated cells, called stereome fibers or sclerenchyma 



B 



Fig. 41. The stereome or strengthening cells of the cortex: A, 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. 



fibers, are often formed. These cells have thick walls and taper- 

 ing 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. 

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

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

 ceous 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- 



