xxiv] 



ANATOMY 



199 



Certain species of § Dicranopteris, viz. G. linearis and pectinata, dififer from 

 the rest in soral construction, and the latter species stands apart also in the 

 absence of scales, stiff hairs being present. Nevertheless G. linearis conforms 

 to the rest of the genus in being protostelic: but G. pectinata shows a 

 structural advance on any other species, in being solenostelic in the adult 

 state. Its structure is illustrated diagrammatically by Fig. 481, which 

 demonstrates that it is a solenostelic Gleichenia, with slightly fluted xylem. 

 The shallow pocket seen in G. jiabellata if extended from node to node 

 would give the structure present in G. pectinata. It becomes therefore a 

 point of some importance to see how the transition from protostely to 



Fig. 481. Diagram illustrating the solenostelic structure and attachment of the leaf-trace in 

 G/eichetiia pectinata. The transverse sections show the structure corresponding to the several points 

 indicated. (After Boodle.) 



solenostely is brought about in the individual life. A full account of the 

 change has been given in Vol. I, pp. 142-4, where Fig. 134 suggests the 

 ontogenetic steps which may be held as illustrating not only an individual 

 advance, but probably also a phyletic departure from the primitive protostely 

 of the Family. 



On the other hand, it has been shown in Vol. I, p. 131 how Platyaonia 

 has made a similar advance, but less complete. Here the whole habit of the 

 plant is different. The leaves of this small xerophyte are closely crowded 

 on the shortened rhizome, and are of very different sizes: the pinnate fertile 

 leaves far exceeding the simpler sterile leaves, with which they alternate 

 in successive zones. These facts alter the balance of the requirements for 



