THE STEM 197 



In the stems of lower types these present themselves under two 

 general conditions: namely, the protostelic and the siphonostelic. 

 In the former the xylem constitutes a solid mass without any central 

 pith, while in the latter the fibrovascular complex is thrown into a 

 hollow cylinder in which gaps are frequently present. These are 

 generally related to certain of the appendages, but may not have 

 any topographical connection with the lateral parts. In the 

 siphonostelic condition internal phloem is often seen, particularly 

 in the lower forms, and the stele or central cylinder is clearly 

 separated from the central pith or medulla by the presence of an 

 internal endodermis. In higher types the internal surface of the 

 tubular stele tends to degenerate, with the resulting disappearance 

 of internal phloem and endodermis. The original state of the wall 

 of the hollow cylindrical stele can often be inferred from the 

 character of the foliar strands which, in accordance with a general 

 principle to be enlarged upon in a later chapter, often perpetuate 

 a situation which has disappeared in the stem. Where the foliar 

 traces are concentric and separated from the fundamental tissues 

 by a well-marked endodermis, it may be assumed that a similar 

 condition was formerly present in the organization of the central 

 cylinder of the axis. 



A further interesting phase in the evolutionary development 

 of the vascular cylinder is to be noted in connection with the 

 progressive degeneracy of the primary wood. In the lower forms 

 this region of the xylem is well developed, and both by its massive 

 character and by the lack of seriation of its elements it is clearly 

 distinguishable from the secondary cylinder which surrounds it. 

 The stem of higher groups presents the primary structures in an 

 obsolete condition, which results in the bringing of the rays of the 

 secondary wood into apparent contact with the pith or medulla. 

 This advanced state of modification of the central cylinder is 

 responsible for the extremely inappropriate name medullary rays, 

 which is applied to the radial parenchyma of the secondary wood. 

 These structures are best called wood rays or simply rays, since 

 primitively in the stem and always in the root they are quite 

 divorced from any connection with the pith. 



