PTERIDOPHYTA 211 



xylem is generally made up of scalariform or of pitted tracheids. Scalari- 

 form tracheids, in which the bands of thickening reseml)le the rungs of a 

 ladder, are most common in pteridophytes and pitted tracheids in 

 spermatophytes. Primary xylem, consisting of all wood differentiated 

 directly from embryonic cells of a terminal meristem, includes both proto- 

 xylem and metaxylem. Secondary xylem is wood that arises through the 

 activity of a cambium. It occurs in only a few living pteridophytes, but 

 is characteristic of nearly all spermatophytes except the monocotyledon- 

 ous angiosperms. 



Types of Steles. The vascular tissues lie within the stele,^ which in 

 roots and stems generally forms a central core. This is surrounded by a 

 cylindrical region, the cortex, outside of which lies a layer of cells constitut- 

 ing the epidermis. The innermost layer of the cortex is the endodermis. 

 Four main types of steles occur in vascular plants and all of them are 

 found in pteridophytes. These are as follows: 



(1) The protostele is the simplest and most primitive type. Here the 

 xylem forms a solid central strand surrounded by phloem, no pith being 

 present (Figs. 176, 188, 200, 235, and 311). (2) The amphiphloic siphono- 

 stele has the xylem in the form of a hollow cylinder enclosing pith, with 

 phloem both inside and outside the xylem (Figs. 236 and 249). (3) The 

 ectophloic siphonostele also has the xylem surrounding pith, but there is no 

 internal phloem (Figs. 221, 237, 269, 285, 294, and 315). (4) The dictyo- 

 stele is the most advanced type. Here the vascular cylinder is broken up 

 into a network of separate strands that, as seen in cross section, may be 

 either arranged in a circle or scattered (Figs. 238 and 316). 



The arrangement of xylem and phloem with reference to each other fol- 

 lows four general types, three of which occur in pteridophytes. (1) In 

 the radial arrangement, which is most primitive, the xylem and phloem 

 are in separate strands and occupy alternating radii (Figs. 171, 176, 220, 

 227, and 311). The xylem may or may not meet in the center. (2) In 

 the amphicribral arrangement the phloem completely surrounds the xylem 

 (Figs. 188 and 239). (3) In the collateral arrangement the xylem and 

 phloem lie side by side on the same radius, with the phloem external to the 

 xylem (Figs. 221, 294, and 315). Where the phloem occurs both outside 

 and inside the xylem, the arrangement is bicollateral. (4) In the amphiva- 

 sal arrangement the xylem surrounds the phloem. This is an advanced 

 type occurring only among the monocotyledonous angiosperms (Fig. 317). 



In practically all vascular plants the root is an exarch radial protostele. 

 Stems display a great variety of vascular structure. Those of lycopods 

 are much like roots and so are very primitive. The stems of ferns display 

 all four stelar types, the amphiphloic siphonostele and the dictyostele 



* Practically all stems have a single stele, and thus are sometimes designated as 

 monostelic. Polystelic stems, containing more than one stele, are very rare (see p. 231). 



