298 STUDIES IN FOSSIL BOTANY 



and a broad peripheral zone of cortex, traversed ' 

 throughout by innumerable adventitious roots, seen in 

 approximately transverse section in our figure. In this 

 case the limit between the two regions is sharply 

 defined, by an almost continuous band of sclerenchyma. 

 The immense development of the peripheral zone of 

 roots is a most characteristic feature of the genus 

 Psaronius. Most authors^ have regarded the zone in 

 which the roots are embedded as forming part of the 

 cortex of the stem, as in the Marattiaceae, in which the 

 adventitious roots arise near the apex of the stem, and 

 grow down through the cortical tissues for a long 

 distance before they become free. 1 There are two 

 difficulties in applying this interpretation to Psaronius. 

 The connection between the roots and the surrounding 

 tissue is much more intimate here even than in 

 Marattiaceae, and the root-zone is never traversed by 

 the leaf-traces, as it should be if it belonged to the 

 primary cortex. It has therefore been suggested that 

 the embedding tissue in Psaronius may be formed by 

 the compacted hairs of the roots themselves ; in many 

 cases the filamentous character of the tissue lends 

 support to this explanation. On the other hand, the 

 evident continuity between the tissue in question and 

 the true cortex appears decisive in favour of the older 

 view. Stenzel's explanation that the embedding tissue 

 is a secondary zone, developed pari passu with the 

 growth of the roots after the leaves had fallen, appears 

 the most probable, the radial arrangement of the 

 elements, often observable, agreeing well with a secondary 



1 See the well-known longitudinal section of the stem of Angiopteris 

 evecta in Sachs's Text-book and other manuals. 



