1919] 



Ml LLER—C YCA S 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 fig. 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; cb, cambium; 

 ph, secondary phloem of second cylinder; sp, split 

 in cortex caused by expansion lower down of third 

 cylinder; X85. 



