1919] 



MILLER— CYC AS MEDIA 



15 



they meander back and forth tangentially, so that only short 

 patches can be caught here and there in a single radial section. 

 The meandering habit is not so pronounced in the traces, and 

 consequently longer stretches of primitive xylem elements can be 

 seen and identified as such. 



Secondary xylem of the normal cylinder is composed of tra- 

 cheids which are characteristically pitted, the pits being confined 

 largely to the radial 

 walls, as described by 

 both Chamberlain and 

 Jeffrey. 



The phloem situation 

 of the normal cylinder 

 adds emphasis to the fact 

 of the latter's procam- 

 bium origin, for proto- 

 phloem is as distinct here 

 as it is in any of the 

 typical monoxylic cycad 

 stems. Fig. 7 illustrates 

 the upper phloem region 

 of this cylinder, showing 

 the crushed cellular sub- 

 stance which once was 

 organized protophloem. 

 This dark crushed mass 

 has the appearance of a 

 thick irregular ring in transverse section, entirely surrounding the 

 normal cylinder and immediately inside the centripetal limits of 

 the first cortical cylinder (fig. 8). The ring of course is inter- 

 rupted here and there by medullary rays, but in. many cases it 

 extends unbroken across them, being squeezed in between the 

 cells of the pith or cortical medulla. From this protophloem 

 center primary and secondary phloem extend, fanlike, outward 

 and downward in typical fashion. The rather startling character 

 of the secondary phloem is its large number of suberized bast 

 fibers compared to the number of sieve tubes. The former far 



Fig. 6.— Cycas media: radial section of stem, 

 showing centripetal end of bundle of innermost 

 cylinder; m, pith; px, protoxylem distinctly spi- 

 ral; s, scalariform tracheids of primary xylem, 

 left one also having spiral thickenings; X400. 



