STRUCTURE OF HERBACEOUS DICOTYLEDONOUS STEMS 195 



portant strengthening tissue common to all dicotyledonous stems. 

 Centerward and matching each mass of phloem, is a mass of xylem, 

 wedge-shaped in outline with point towards the center of the 

 stem. This opposite arrangement of phloem and xylem contrasts 

 with the arrangement in roots, where phloem and xylem alternate. 

 Between the phloem and xylem is the cambium, the meristematic 

 tissue whereby the vascular tissues can be increased indefinitely. 

 Vascular bundles provided with cambium are called open bundles, 

 and are characteristic of Dicotyledons and Gymnosperms, 

 whether herbaceous or woody. During further development, 

 the cambiums of the different vascular bundles extend through 



FIG. 174. Lengthwise section through a vascular bundle of a herba- 

 ceous dicotyledonous stem, x, xylem showing pitted, annular, spiral and 

 scalariform vessels; p, phloem showing sieve vessels and companion cells; 

 c, cambium. Highly magnified. Modified from Hanson. 



the intervening pith and connect to form the continuous cam- 

 bium ring. Then due to the activity of the cambium ring in the 

 formation of other vascular bundles between those first formed 

 and in the enlargement of all, the intervening pith, excepting 

 narrow strands of it called pith rays, is crowded out, and finally 

 a compact vascular cylinder as shown in Figure 173 is formed. 

 In many herbaceous Dicotyledons, such as the Giant Ragweed 

 and others that grow rapidly, the cambium is so active in adding 

 new xylem on its inner side and new phloem on its outer side 

 that both phloem and xylem constitute zones of considerable 

 thickness at the end of one summer's growth. The zone of 

 xylem is often so prominent that the basal portions of such stems 

 are considered woody. 



