158 MORPHOLOGY 
centrifugally (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 
