PTERIDOPHYTES 
157 
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 protostele 
(see p. 125), recognized as being the most primitive type (as in Gleichenia, 
fig. 378); (2) the amphiphloic siphonostele, 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 
of vessels is called the protoxylem, Lygodium in position; some indusia shown 
, . . . . , , (after SADEBECK); 374. sporangia of A neimia. 
and its position with reference to showing apica , ann ' ulu / and the verticalde . 
the subsequent xylem (metaxylem) hiscence (after SADEBECK); 375, 376. two 
is important to note. In a Stele views of sporangium of Hemitdia, showing 
the complete vertical annulus and the trans- 
usually several protoxylem regions yerse de j iscence (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 
