ADJUSTMENT TO WATER 53 



or sieve area of the bundle consists of sieve tubes, companion cells, 

 and thin-walled fibers. The sunflower and similar woody herbs 

 possess in addition bundles of thick- walled fibers, or bast fibers, 

 located on the outside of the sieve tissue. 



Between the phloem and the xylem lie several or many layers 

 of clear, thin-walled seriate cells, usually referred to as cambium. 

 Properly speaking, the true cambium is but a single layer of active 

 meristem, each cell of which by division alternately forms a xylem 

 and a phloem cell. Since the latter are modified but slowly into 

 the various parts of the bundle, there are always a number of 

 layers which closely resemble the mother cells. The xylem com- 

 prises wood fibers and tracheary vessels, the latter of various 

 types, ringed, spiral, reticulated, and pitted. Occasionally, also, 

 large, pitted wood fibers occur here and there: these are called 

 tracheids. The tracheary vessels are usually found in the middle 

 of the xylem, surrounded and separated by the wood fibers. In 

 the longisection it should be noticed that the vessels are continuous 

 tubes of great length, while the wood fibers are single closed cells. 

 Within the fibrovascular ring, and derived like it from the plerome, 

 lies the pith, consisting usually of inactive parenchyma, some- 

 times used for storage. 



67. Stems of monocotyledons. A section of the stem of an 

 herbaceous monocotyledon shows essentially the tissues found in 

 the dicotyledon. The cambium, however, is found only near the 

 periphery, and the bast is replaced by a sheath of stone fibers 

 which surrounds each bundle. The irregular arrangement of the 

 bundles likewise makes it impossible to distinguish a cortical region 

 or the pith. 



68. Structure of woody stems. A woody dicotyledon has 

 essentially the grouping of tissues found in the herbaceous type. 

 Wood and bast have been greatly emphasized at the expense of 

 other tissues, and a new tissue, the cork, has been added. A 

 section of a typical woody stem shows on the outside the cork, 

 together with certain cambium cells which produce it. Beneath 

 the cork are a few layers of collenchyma, followed by a broader 

 area of cortical parenchyma. Within the latter follow the bast 

 bundles, parenchymatous cells, sieve tubes, and companion cells 

 of the phloem. Beyond the latter is the cambium, and the rings 

 of wood surrounding the pith. The wood consists chiefly of 

 tracheids and wood fibers, together with a larger or smaller numl)cr 



