222 MINUTE ANATOMY OF FLOWERING PLANTS 



373. Fibrovascular bundle of a Di- 

 cotyledon : ph, phloem ; c, 

 cambium ; d, duct, and /, 

 fibers of the xylem. 



It is called the xylem 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- 

 fication, all the elements of 

 both xylem and phloem are 

 derived. It is called the cam- 

 bium. The cambium, during 

 the active growth of the stem, 

 continuously forms xylem on 

 one side, phloem on the other. 

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

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

 stems, where the number of bundlesjsancr eased, and they 

 are crowded together, the cam^ji^^S^ffie^sev^ral bundles 

 are continuous around th 

 stem, forming a thin sheat 

 outside the wood. It is at 

 the cambium that the bark 

 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 dicotyledons. The charac- 

 teristic bundle of the monocotyledons lacks the cambium 

 (Fig. 374). The xylem also is much reduced. Each 



374. Monocotyledonous fibrovascu- 

 lar bundle : ph, phloem ; d, 

 duct (xylem) ; p, pith cell. 



