464 MOSSES AND FERNS chap. 



vascular bundles are ridges upon the surface of the internodes 

 and foliar sheaths, due to greater growth at these points, 

 as a result of which a regular series of cortical lacunae (vallecu- 

 lar canals) is formed, alternating with them (Fig. 266, C), 

 and lying just outside of the cortical zone containing the vascu- 

 lar bundles. In some of the small species of Equisetum, as in 

 the primary shoot, the central lacuna is absent. 



A cross-section of the fully-developed stem of E. telmateia 

 (Fig. 266, C) shows this very regular arrangement of the vas- 

 cular bundles and lacunae. In addition to the large cortical 

 ones, each vascular bundle has, on the inner side, a large air- 

 space, which like the other is formed by the tearing apart of the 

 tissues of the bundle. In this way the primary tracheids are 

 torn apart and often destroyed, so that all that remains of them 

 are the isolated thickened rings adhering to the sides of the 

 canal. The bundle is strictly collateral in structure, and very 

 much resembles that of many grasses and other simple Mon- 

 ocotyledons. The phloem is composed of sieve-tubes, which, 

 according to Russow (i), have only horizontal sieve-plates, 

 and no lateral ones as in the Ferns. These are mingled with 

 cambiform cells. In the species in question there is in addition 

 a zone of bast fibres at the outer limit of the phloem. 



Surrounding the whole circle of bundles in E. tchnateia, 

 E. arvense, and several other species, there is a common endo- 

 dermis (Fig. 270, en). In others the arrangement is different 

 (Pfitzer (i) ; Van Tieghem (6)). Thus in E. limosum, each 

 separate bundle has its own endodermis ; in E. hiemale there is 

 a common inner as well as an outer endodermis in the aerial 

 stems, while the bundles of the rhizome are like those of E. limo- 

 sum. Inside the endodermis lies the single pericycle. 



There has been some controversy as to the nature of the vas- 

 cular system in Equisetum. Van Tieghem (6, 8) describes the 

 stem of Equisetum as "astelic" ; Strasburger ((11), vol. 3) 

 considers it as monostelic. Jeffrey has attempted to reduce the 

 structures to his ''siphonostelic" type, i. e., he would compare 

 the complex of vascular bundles to the cylindrical stele of the 

 Ferns and Lycopods. The spaces between the vascular strands 

 of the internodes he considers as ''gaps" comparable to the foliar 

 gaps in the stele of the Ferns, or the ramular gaps in the stele 

 of the Lycopods. He is, moreover, of the opinion that the solid 

 stele C'protostele") found in the fossil Sphenophyllales is the 



