PTERIDOPHYTES 



'57 



Vascular system. At least four kinds of vascular cylinder are rep- 

 resented in the stems of this great group, and there is much discussion 

 as to their historical sequence. They are as follows: (i) the protostek 

 (see p. 125), recognized as being the most primitive type (as in Gleichenia, 

 fig. 378); (2) 'the amphiphloic siphonostcle, in which the phloem occurs 

 on both sides of the xylem in the pith-containing cylinder (as in Adian- 

 tum, fig. 379); (3) the polystele, in which several concentric bundles 

 traverse the stele without organization into a definite cylinder (as in 

 Pteris, fig. 380) ; and (4) the ecto- 

 phloic siphonostele, in which the 

 phloem occurs only on the outer 

 side of the xylem in the pith- 

 containing cylinder (as in Os- 

 munda, fig. 381), which is thus 

 composed of collateral bundles 

 (p. 146). This last type of cylinder 

 is regarded as the most advanced, 

 since it is the characteristic cylinder 

 of the majority of seed plants. 



An important variation in the 

 character of the xylem must be 



noted. The first xylem elements 



. , , p FIGS. 172-176. Types of annulus: 



to appear are spiral vessels of ^ spOT ^ gi ^ of Lygodium with apical 



small Caliber. This initial group annulus (after PRANTL); 373, sporangia of 



of vessels is called the protoxylem, Lygodium in position; some indusia shown 



, ., . . . , -. (after SADEBECK); 774, sporangia of A neimia, 



and its position with reference to ^^ apical ann 3 ulus L and the verticalde . 



the subsequent xylem (metaxylem) hiscence (after SADEBECK); 375, 376, two 

 is important to note. In a Stele views ' of sporangium of Hemitelia, showing 



the complete vertical annulus and the trans- 

 usually several protoxylem regions yerse dehiscence (after KARSTEN). 



appear, and if all the metaxylem 



develops centripetally (towards the center of the stele), the xylem is 

 exarch, which means that the protoxylem regions are external to the 

 metaxylem. Exarch bundles are regarded as the most primitive, and 

 are characteristic of all roots, of the protostele of Lycopodium, etc. If 

 the metaxylem develops in all directions from the protoxylem, the xylem 

 is mesarch (figs. 380, 381), which means that the protoxylem is sur- 

 rounded by metaxylem. Mesarch bundles are very characteristic of 

 ferns, and all of the four kinds of bundles described above as occurring 

 among ferns are prevailingly mesarch. If all the metaxylem develops 



