IQI91 



MILLER—CYC AS MEDIA 



217 



walls. Fig. 9 shows characteristics of the xylem in that region 

 of the cylinder where protoxylem would be expected if there were 

 any. All cells are uniformly thickened and, without exception, 

 equipped with bordered 

 pits, which is always a 

 mark of secondary 

 origin. In lig. 10 the 

 same region is seen in 

 radial section. Here the 

 xylem element nearest 

 the stem center, and 

 even bordering on pro- 

 tophloem of the normal 

 cylinder, is pitted. This 

 one drawing illustrates 

 tracheids of the first 

 cortical cylinder which 

 are as nearly scalariform 

 as could be found; they 

 are as rare as spiral 

 tracheids are in the nor- 

 mal cylinder. By far the 

 greater number of xylem 

 elements in this centrip- 

 etal region of the cylin- 

 der are pitted in exactly 

 the same fashion as 

 ordinary secondary tra- 

 cheids of the normal 

 cylinder, and they must 

 in turn be considered as 

 of secondary origin. 



In fig. 8 it will be seen 

 that the region between 



the normal cylinder and the first cortical one is composed of purely 

 cortical cells. Also the region between the first cortical cylinder and 

 the split in the cortex, which marks the place where the second 



Fig. 8. — Cycas media: transverse section of stem, 

 showing entire second cylinder as it appears near 

 tip of stem; ph', protophloem of first cylinder 

 crushed; ph", secondary phloem of first cylinder; 

 c, cortical cells; x", secondary xylem of second 

 cylinder; /, unthickened xylem cells; ch, cambium; 

 ph, secondarj' phloem of second cylinder; sp, split 

 in cortex caused by expansion lower down of third 

 cylinder; X85. 



