164 THE MORPHOLOGY OF PTERIDOPHYTES 



solenostele. Pteris also has a continuous sorus near the 

 margin of the lamina (i.e. the sorus is superficial in origin) 

 and the margin becomes inrolled to protect it (Fig. 25O). 

 In some species, e.g. Pteris cretica (Fig. 25P), the soral 

 region is somewhat expanded and indicates the way in 

 which the acrostichoid condition might have evolved. Indeed, 

 Bower suggested a close relationship between Pteris and 

 Acrostichum. Chromosome counts support this view, but 

 they also suggest that Pteris is wrongly classed with 

 Pteridium. The haploid numbers are, for Pteridium fifty- 

 two, for diflferent species of Pteris twenty-nine and 120 and 

 for Acrostichum thirty. The fact that in most of the Adianta- 

 ceae n = 29 or 30 suggests a possible affinity between Pteris 

 (and Acrostichum) and this family. It should be noted that 

 Lygodium, too, has a haploid number n = 29 or 30. Holttum 

 believes that another acrostichoid genus Stenochlaena is 

 closely related to Acrostichum, but a chromosome number 

 lying somewhere between seventy and eighty casts some 

 doubt on this. Bower placed it near Blechnum but, for the 

 time being, Stenochlaena should perhaps remain unplaced. 



Onocleoideae 



Holttum leaves this subfamily unplaced in his classification, 

 while Bower thought that it shows some affinities with the 

 Cyatheaceae and with the Blechnoideae. It contains two 

 genera, Matteuccia (two species) and Onoclea (monotypic). 

 Both are markedly dimorphic, with specially modified 

 fertile fronds. The fertile pinnae are narrow and the margins 

 are tightly inrolled so that protection of the sorus is derived 

 more from them than from the indusium, which is thin and 

 papery (Fig. 25K). Both are dictyostelic and covered with 

 scales. Matteuccia has open venation and Onoclea reticulate. 



Dryopteridoideae 



The ferns in this subfamily have a short stout stem which is 

 more or less erect, covered with scales and dictyostelic. 



