ANATOMY 647 



supported by the structure of the individual plant when young. In both 

 cases a prevalence of a cylindrical protostelic state, with comparatively 

 slight disturbance of the axial system on departure of the leaf-traces, would 

 be expected if the shoot were primitively strobiloid. Further, the leaf-trace 

 would be relatively simple. 



Leaving aside for the moment the Marattiaceae, which are anatomi- 

 cally a peculiarly specialised series in themselves, a comparison of the 

 early types of Ferns points clearly to origin from a protostelic state with 

 a leaf-trace consisting of a single strand, which comes off with but slight 

 local disturbance from the periphery of the stele. This, with certain 

 variants, is the typical condition in the Botryopterideae : a near approach 

 to it is found also in the earliest Osmundaceae, though those of later 

 epochs depart from the simple type by elaboration of the stele, as 

 described above (p. 539). The close correspondence of the Hymeno- 

 phyllaceae with certain of the Botryopterideae stamps their structure as 

 relatively primitive also, though it shows some variants upon the simple 

 protostelic state. Lygodium also, recognised as the most primitive genus 

 of the Schizaeaceae, is protostelic, and the same is the case with the 

 simpler species of Gleichenia ; in fact, those early stocks of Ferns which 

 are recognised by comparison of other characters, as well as by their 

 geological history, as forming the phyletic basis of the Leptosporangiate 

 series, show the protostelic structure, or a condition very little removed 

 from it. 



The Ferns thus mentioned are all included in the Simplices, except the 

 Hymenophyllaceae. These are exceptional among the Gradatae in showing 

 a protostelic structure of the axis : most of the Gradatae have a more 

 elaborate stem-structure, which may be held to be derivative from the pro- 

 tostele, just as the basipetal sorus is probably derivative from the type of 

 the Simplices. The probable steps towards a solenostelic state are illustrated 

 in Lindsaya and Deitnstaedtia, and suggested also in Gleichenia ; but the 

 solenostelic structure is typically seen in Dipteris and Loxsoma, as well as 

 in the Dennstaedtiinae. Here at each leaf-insertion the vascular tube opens 

 by a foliar gap. Where the internodes are long and the gap itself short, 

 as in the rhizomic species, the structure is easily intelligible. It is but a 

 slight step to the dictyostelic type, as seen in Ferns with short axis and 

 overlapping leaf-gaps : the transition is illustrated in the J)cnns,taedtia- 

 Davallia series, and has probably occurred also in the Alsophila-Cy>i/li<a 

 series, and elsewhere. It seems probable that the progression from a 

 protostelic to a solenostelic or dictyostelic state has been effected in 

 several distinct phyletic lines, while the dictyostelic, with or without internal 

 accessory strands, is the most elaborate system of all. 



It usually accompanies an advanced soral condition : that this is, how- 

 ever, no obligatory parallelism is shown by the comparison of Matonia with 

 Dipteris. The latter retains a simple solenostelic structure of the axis, 

 though its sori have progressed to the condition of the Mixtae : the 



