222 MINUTE ANATOMY OF FLOWERIXG PLANTS 



373. Fibrovascular bundle of a Di- 

 cotyledon : ph , phloem ; c, 

 cambium ; (?, duct, and /, 

 fibers of the xylem. 



It is called the xyle^n or wood portion. The outer con- 

 tains more rounded cells, but typically possesses bast 



fibers in groups, and scat- 

 tered tubes. It is called 

 the phloem. Between xylem 

 and phloem is a region occu- 

 pied by thin-walled formative 

 tissue, from which, by cell 

 division, growth, and modi- 

 lication, all the elements of 

 both xylem and phloem are 

 derived. It is called tlie cam- 

 Inum. The cambium, during 

 the active growth of the stem., 

 continuou,sly forms xylem on 

 one side, phloem on the other. 

 The outside of the xylem is thus tlie newest, while tlie 

 innermost parts of phloem are the newest. In old, woody 

 stems, where the number of bundles is increased, and they 

 are crowded together, the cambiums of the several bundles 

 are continuous around the 

 stem, forming a thin sheath 

 outside the wood. It is at 

 the cambium that the Ijark 

 of twigs, especially in spring 

 when growth is most active, 

 may easily be separated from 

 the wood. The phloem is 

 then, of course, removed with 

 the bark, of which it forms 

 the inner part. 



515. Fibrovascular bun- 

 dles of the sort described in- 

 crease in thickness from year 

 to year, if the plant is a 

 perennial. They are found in dicotjdedons. The cliarac- 

 teristic bundle of the monocotyledons lacks tlie caml)ium 

 (^Fig. 374:). The xylem also is much reduced. Eacli 



.Monocotjiedoiious fibrovascu- 

 lar bundle : ph, i^hlocm : (?, 

 duct (xylem) ; p, pith cell. 



