SECONDARY THICKENING 61 



thickening differs usually quite materially from the pri- 

 mary xylem. It contains much more fibrous tissue, is 

 more compact, and forms a true wood. The phloem 

 also is interspersed with more bundles of bast, and may 

 by its formation soon crush out of recognizable shape the 

 primary phloem. In addition, the tissues forming the 

 primary medullary rays become active. The layer of 

 parenchyma cells that connects the edge of the cambium 

 of one bundle with that of the next bundle becomes 

 itself converted into cambium by the accumulation of 

 large amounts of cytoplasm in the cells, and the formation 

 of periclinal walls. Part of this interfascicular cambium 

 thus formed gives rise only to cortical and medullary 

 parenchyma, but at intervals new bundles arise by the 

 formation of xylem and phloem, respectively, on the 

 inner and outer faces of the cambium layer. Thus, sec- 

 ondary bundles are formed, which divide the medullary 

 rays longitudinally, and as the bundles become more and 

 more numerous, these primary rays may 

 eventually be reduced to thin plates of paren- 

 chyma, only one or two cells thick, and per- 

 haps only a few cells wide (measured in ver- 

 tical direction), but still extending from the 

 pith to the cortex. Additional ("second- 

 ary") medullary rays are formed within the Fiq. 29 — 

 bundles when certain cambium cells cease growth of a vas- 

 to form xylem elements and from that time 

 forward produce parenchyma cells. These secondary 

 medullary rays usually arise at varying distances from 

 the center, a certain number of new ones being laid down 

 each year. 



92. Where the growth is continuous and equal, the 

 wood is usually of fine grain and uniform. Most woody 

 plants of the temperate zones, however, and of those 



