CH. VIIl] 



SOLENOSTELY 



141 



Fig. 1 30. Loxsoma Cunninghamii. Diagram sliowing the 

 form of the vascular system aj a node of the rhizome. 

 ji- = solenostele; // = leaf-trace departing; /§'= leaf-gap. 

 The arrow points toward the apex of the rhizome. 

 (After Gwynne-Vaughan.) 



parenchymatous pith, and itself enveloped in cortex (Fig. 130). The middle 

 zone of the stelar ring itself is occupied by a band of tracheides, as a rule 

 continuous, with groups of proto- 

 xylem embedded at intervals 

 among the metaxylem. This is 

 surrounded on either side — or 

 sometimes only on the outer 

 side — by phloem. The former 

 is described as the avipJiiphloic, 

 the latter as the ectophloic con- 

 dition. Investing these tissues is 

 the parenchymatous pericycle, 

 of one or more layers, and finally 

 there is a complete covering of 

 endodermis within the tube and 

 outside it (Fig. 133). The evolutionary relation of this solenostelic state to 

 the primitive protostele is best studied in the ontogeny of Ferns that are 

 themselves solenostelic, but are closely related to such as remain protostelic 

 even in the adult. The best examples are found in the genus Gleiche7iia. 



The section Mertensia of the genus Gleichenia includes species that are 

 protostelic throughout the adult plant {G.flabellata, linearis, etc.), and others 

 in which the adult stem is sole- 

 nostelic {G. pectinatci). All of 

 them have creeping stems with 

 long internodes, a fact which 

 makes them particularlysuitable 

 for the study of the origin of sole- 

 nostely. It will be shown later 

 that the soral condition indi- 

 cates G. pectinata as a relatively 

 advanced species. Thus it is 

 not a mere assumption that the 

 solenostely is an advanced state, 

 but it is backed by evidence 

 drawn from other characteristics 

 as well. Cut transversely through 

 an internode of the long creeping 

 rhizome of G.flabellata, the stele 

 is seen to be cylindrical, with a 

 solid xylem-core composed of 

 tracheides and parenchyma, the 

 numerous protoxylems being immersed within its margin (Fig. 131). Cut 



Fig. 131. Transverse section of protostele of the rhizome 

 of Glekhenia flabellata, R. Br., from a section ;by 

 Gwynne-Vaughan, showing a leaf-trace being given 

 off. ( X 30.) 



