THE STEM 169 



and modern, a subsequent ligneous development made up of cells 

 regularly seriate radially and including masses of radial parenchyma. 

 This regularized and later-appearing xylem is known as the second- 

 ary wood. In older types the primary wood is distinct from the 

 secondary ligneous structure, and its conspicuousness under these 

 conditions is due largely to the more abundant development of the 

 primary elements and their frequent exarch or mesarch configura- 

 tion. In the higher gymnosperms and in the angiosperms they 

 can be recognized only with difficulty except in the root. This 

 situation is the result of the slight development of the primary 

 elements and of the fact that they are in series with, and formed in, 

 the same direction as the secondary wood. 



It will be apparent from the statements made in the preceding 

 paragraphs that the organization of the stem is characterized by 

 a considerable degree of variety as regards the general topography 

 and microscopic organization of the primary structures. When 

 the special consideration of the ferns and their allies is reached in 

 later chapters, it will be clear that the possible complexities of the 

 gross fibrovascular structures have been by no means exhausted. 



It is now convenient to turn our attention to the secondary 

 fibrovascular structures of the stem. In the older vascular 

 plants the secondary wood was extremely simple in its organi- 

 zation and consisted merely of radial parenchyma and longitudinal 

 tracheary elements. The radial parenchyma in the lowest forms 

 was definitely separated from the pith, as is well illustrated in 

 Fig. 122, by the presence of a clearly developed zone of primary 

 wood. With the progress of geological time came a progressive 

 reduction of the primary wood and important modifications in 

 the structure of the secondary woody cylinder. The gradual 

 reduction of the primary structures of the woody cylinder has 

 involved interesting conditions which must now occupy our 

 attention. The progressive reduction of the region of the wood 

 known as primary brought with it certain important topographical 

 changes. The primary wood was at first continuous and in most 

 instances constituted in the stem a siphonostele interrupted only 

 by the passing off of the traces of branches or leaves. The breaks 

 in the continuity of the primary cylinder thus determined are 



