XX] 



RECENT FERNS 



313 



may be described as the result of the replacement of some of the 

 axial conducting tracheae by parenchyma or other non- vascular 

 tissue consequent on an increase in diameter of the whole atele 

 and the concentration of the true conducting elements towards 

 the periphery^. 



The occurrence of the internal cylinder of phloem, pericycle, 

 and endodermis in a solenostele is rendered intelligible by a 

 study of fern seedlings and by a comparative examination of 

 transitional types connecting protosteles and solenosteles through 

 medullated protosteles and steles of the Lindsaya type. A 



Fio. 240. Cyathea Imrayana. (From Tansley after de Bary. ) (Solerenohyma 

 represented by black bands.) 



further stage in stelar evolution is illustrated by what is termed 

 the dictyostele, the arrangement of vascular tissue characteristic 

 of Nephrodium Filix-mas, Cyathea (fig. 240), Polypodium vulgare 

 and many other common ferns. 



If a solenostele is interrupted by leaf-gaps at intervals 

 sufficiently close to cause overlapping, a transverse section at 

 any part of the stele will show apparently separate curved 

 bands of concentrically arranged xylem and phloem, which on 

 dissection are seen to represent parts of a continuous lattice- 



^ For an account of the probable methods by which this has been effected 

 and of the factors concerned, see Tansley (08). 



