PTERIDOPHYTES 



157 



Vascular system. — At least four kinds of \'ascular 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 protostele 

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

 fig. 378) ; (2) the amphiphloic si phono stele, 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 

 to appear are spiral vessels of 



small caliber. This initial group annulus (after Prantl); 373, sporangia of 



Lygodium in position; some indusia shown 



(after Sadebeck); 374, sporangia of j4neimja, 



showing apical annulus and the vertical de- 



the subsequent xylem (metoxy/em) hiscence (after Sadebeck); 375, 376, two 



is important to note. In a stele ^'ews of sporangium of Hemitdia, showing 



„ , , . the complete vertical annulus and the trans- 



usually several protoxylem regions ^^^^^ 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 



Figs. 372-376. — Types of annulus; 

 372, sporangium of Lygodium with apical 



of vessels is called the protoxylem, 

 and its position with reference to 



