320 Plant Physiology 



xylem, or wood, in which wood-cells, pitted and spiral 

 vessels occur. The outer portion develops the phloem, 

 or sieve-tube and soft-bast part of the bundle. Between 

 these two there remains a growing meristem, the cambium, 

 which is most important in secondary growth, or thicken- 

 ing, as subsequently stated. In wood}' plants this cam- 

 bium becomes a continuous ring, being formed between the 

 bundles by the differentiation of the ground meristem. 



In the root, however, the procambial areas originating 

 the phloem and the xylem of the several bundles alternate 

 radially (radial arrangement) ; and when a ring of cam- 

 bium is formed, it is between phloem and xylem, as well 

 as on the periphery of the xylem, thus constituting an 

 irregular or lobulate ring. 



187. Secondary thickening. Secondary vascular growth 

 normally arises by the development of new bundles from 

 the ring of cambium. Ultimately so many of these second- 

 ary bundles may be interposed as to produce in woody 

 or semi-woody plants a complete wood-cylinder. The 

 bundles may be more or less completely separated one 

 from the other by thin bands of ground-tissue, known as 

 medullary rays. In perennials, the cambium differ- 

 entiates each year (or season) new xylem within; thus 

 the cambium is carried farther and farther from the 

 center. It develops new phloem without, so that both 

 wood and bark are annually or seasonally augmented. 

 However, the xylem vessels produced during the first 

 flush of growth, that is, in the season of greatest activity, 

 are larger and more important, and they have thinner 

 walls than those produced later, so that a definite ring 

 formation results. 



