158 MORPHOLOGY 



centrifugal! y (away from the center of the stele), the xylem is endarch, 

 which means that the protoxylem is internal with reference to the 

 metaxylem. Endarch bundles are characteristic of seed plants, and 

 are attained by some pteridophytes, as in Equisetales and Ophioglos- 

 sales. A fern whose vascular cylinder is composed of collateral bundles 

 resembles seed plants in this feature ; but in such a case the bundle of 



FIG. 377. Marattia : growing on a steep 

 slope, and showing the short tuberous stem 

 covered by an armor of persistent leaf bases. 



the fern is mesarch collateral, while that of the seed plant is endarch 

 collateral. It must be remembered that to all such general statements 

 in reference to great groups there are exceptions, and that the state- 

 ment refers only to the prevailing condition in a group. Metaxylem 

 must not be confused with the secondary xylem, which is formed by 

 the cambium. 



Another feature of the vascular cylinder of ferns is noteworthy. Where 

 the leaf traces leave the cylinder to pass through the cortex, a gap im- 

 mediately above the trace is left in the cylinder/ which closes again fur- 

 ther up. These leaf gaps are characteristic of the vascular cylinder of 



