DENNSTAEDTIINAE 



60 1 



Dennstaedtia ( Dicksonia) rubiginosa. Diagram of 

 the vascular system of the rhizome including a node, 

 and the base of a leaf-trace. /. s//. and i.s. as in Fig. 

 333 B. /= lacunae in the solenostele not related to 

 the departure of a leaf-trace. The upper surface of 

 the rhizome would face the observer. (After Gwynne- 

 Vaughan.) 



This simple vascular structure holds for most species, with minor modifica- 

 tions of form of the leaf-gap, and of insertion of the supply for lateral axes. 

 There is, however, a further com- 

 plication in Dennstaedtia adiantoides 

 and rubiginosa : in the former a 

 local thickening appears internally 

 at the margins of the leaf-gaps in 

 the ordinary stelar cylinder : this 

 becomes extended to form an in- 

 wardly projecting ridge, continuous 

 from one leaf-gap to another (Fig. 

 333 B). In D. rubiginosa this ridge 



is represented by a separate strand, FIG. 333 c. 



which still maintains its connection 

 with each leaf-gap-margin, but may 

 divide into several distinct rods 

 (Fig- 333 c )- This peculiarity is 

 of importance for comparison with 

 what is seen in the stems of the Pterideae on the one hand, and of the 

 Cyatheae on the other, while it also has its bearings in the elucidation of 

 the complex structure already seen in the Matonineae. 



These facts of vascular anatomy, coupled with those relating to the 

 sorus appear to indicate for the Uennstaedtiinae a very interesting inter- 

 mediate position. On the one hand their confirmed solenostely is evidence 

 of a relatively primitive state, derived probably from a protostelic source ; 

 but it shows only slight indications of passing onwards to the more 

 advanced state of dictyostely. Moreover, the constant condition of the 

 leaf-trace as an undivided, ribbon-like strand is a clear index of their 

 primitive position. Similarly, the uniform occurrence of hairs and the 

 absence of ramenta points to the simpler rather than to the more advanced 

 Filicales. 



On the other hand, the soral conditions are characteristically those 

 of the Gradatae : the basipetal sequence of sporangia is quite as marked 

 as in Dicksouia or Loxscma, with which they share the basal Indusium, 

 here, as in Dicksonia, two-lipped. But in certain species occasional 

 departures from the strict basipetal sequence occur : these are most 

 prominent in Dennstaedtia rubiginosa, a species which shows also the 

 vascular strands within the solenostele. With the loss of basipetal succession 

 and the advent of the elongated stalk the sporangium loses its regularity 

 of orientation and its markedly oblique annulus. But this is what might be 

 expected, since there is no longer any mechanical reason for the regularity. 

 In both of these characters, of anatomical structure and of sorus, the 

 Dennstaedtiiriae approach the Pterideae. On the other hand their relation 

 to Dicksonia and to Loxsoma is clear : it is based primarily upon habit 



