lO 



MORPHOLOGY OF GYMNOSPERMS 



in amount and break up into a loose network, and these hold iio 

 necessary relation to the vascular strands of leaves or branches (80). 

 Vascular bundles have also been characterized by Scott (24) 

 according to the relation of the protoxylem to subsequent xylem 

 elements. The procambium strand that appears in the plerome does 

 not develop vascular tissue simultaneously throughout its whole 



transverse section, but be- 

 gins at one or more definite 

 points, and from each of 

 these points the subsequent 

 xylem extends in one or 

 more directions, 'This first 

 group of vessels (spiral and 

 annular) constitutes the 

 protoxylem, and the subse- 

 quently formed xylem of 

 the primary bundle consti- 

 tutes the metaxylem. If 

 the protoxylem is in con- 

 tact with the phloem, the 

 metaxylem thus being all 

 centripetal, the bundle is 

 called exarch (fig. 7); if 

 the protoxylem is sur- 

 rounded by metaxylem, the 

 bundle is called mesarch 

 (fig. 8) ; if the protoxylem 

 is in contact with the pith 

 (or at the center of a solid 

 xylem strand), the metaxylem thus being all centrifugal, the bundle 

 is called endarch (fig. 9). These types are not rigidly restricted to 

 difierent great groups, but they are very serviceable in combination 

 with other characters. In general, exarch wood is considered to be 

 the most primitive type, characteristic of roots, common in the 

 stems of lycopods, and occasional in the stems of ferns. Mesarch 

 wood is very characteristic of ferns, so much so that when it occurs 

 in any part of the vascular system of gymnosperms, it is spoken of 



Fig. 6. — Adiantum pedalum: transverse sec- 

 tion of stem; the smaller bundle is a leaf trace, 

 opposite the leaf gap in the central cylinder; 

 X16. — After Jefpeey (37). 



