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circle of bundles surround a relatively large pith ( Fig. 97 ) . Ridges of the 

 pith that extend between the bundles may be visible in cross sections as 

 pith rays. In old herbaceous stems, as those of sunflower, there may be a 

 conspicuous ring of xylem with narrow xylem rays. Chlorenchyma is 

 usually present in the cortex of herbaceous stems and sometimes in the 

 phloem and pith. In a monocot like corn, the bundles are scattered 



Fig. 97. Photomicrograph of a cross section of a young sunflower stem. 



throughout the stem (Fig. 98). In many species of both monocots and 

 dicots the cells of the pith disintegrate at an early stage of development, 

 with the result that the stem is hollow. If an herbaceous stem bearing 

 leaves is cut off near the surface of the soil and the cut end of the stem 

 is placed in a weak solution of eosin or other dye that stains the 

 xylem, the dye is pulled up the stem with the water, and the stained 

 bundles in both stems and leaves may be seen more readily. 



The stems of most monocots differ from those of dicots also in the 

 absence of a vascular cambium. Hence, stems of most monocot plants, 

 even though they are perennial, do not increase in diameter from year 

 to year. For example, no slender bamboo stem ever becomes a big 



