294 THE ANATOMY OF WOODY PLANTS 



We may now turn our attention to more complicated stems 

 belonging to the general group of the Cycadofilicales. The genus 

 Medullosa is of particular importance, since it possesses a type of 

 stelar organization presented by several or many concentric strands 

 which are best considered as belonging to a siphonostelic cylinder. 

 The general organization of the axis in the genus under discussion 

 is well shown in Fig. 208, which portrays in a somewhat diagram- 

 matic fashion the structures involved. There are three large 

 concentric fibrovascular strands present which consist of a central 

 core of primary wood indicated by cross-hatching, surrounded 



sc 



_ _____ be 



ab , 



It. an j>^~r"\ }' o'.c, 

 st P d an It 



(-*.. 

 FIG. 208. Transverse section of the stem of Medullosa anglica (after Scott) 



with a cordon of secondary xylem, represented by radiating lines. 

 The outline of the stem is irregular, the salients being due to the 

 presence of the bases of large leaves. The surface is covered with 

 the reticulated sclerenchymatous strands already described in the 

 case of Heterangium. The fundamental tissues are occupied by 

 secretory canals, which probably contained mucilage, as do those 

 of the living Cycadales. In the cortical region are accommodated 

 larger and smaller strands ordinarily lacking secondary growth. 

 The bundles of greater dimensions are mostly the larger foliar 

 bundles and possess concentric organization. The small strands 

 which are found characteristically in the actual leaf bases are 

 collateral in their organization and exarch in the structure of their 

 xylem. Fig. 209 illustrates two of the smaller strands together 

 with fundamental tissues and contained mucilage canals. The 

 general situation in the genus Medullosa is complicated frequently 



